Joint Meeting of the Second International Soil Sensing Technology Conference, the Soil Physics Technical Committee Annual Meeting, and the ASA Sensor-based Water Management Community
January 3-7, 2012
Honolulu, Hawaii
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Measuring area-average soil moisture using cosmic rays
Marek Zreda 1 , Chris Zweck 1 , Trenton Franz 1 , Rafael Rosolem 1 , Jim Shuttleworth 1 , Xubin Zeng 2 and Ty Ferre 1
1 Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721, USA (marek@hwr.arizona.edu) 2 Department of Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, USA
Area-average soil moisture at the horizontal scale of hectometers and vertical scale of decimeters can be measured using cosmic-ray fast neutron intensity in air above the soil surface. Hydrogen is very efficient at moderating (or removing) fast neutrons, and therefore emission of neutrons from soil and their concentration in the atmosphere are inversely correlated with the soil moisture content. Other sources of hydrogen at and near the earth's surface contribute to moderating fast neutrons, and their influence on the neutron signal have to be understood. Fortunately, the most important pools of hydrogen are either included in the local calibration (surface water, built-up environment, lattice water, biomass), or can be handled on the case-by-case basis using ancillary data or/and the neutron signal (ponded water, snow pack, intercepted precipitation, atmospheric water vapor). Calibration is conducted at each site by measuring soil moisture on numerous soil samples collected within the footprint, computing the area-average soil moisture, and linking it to the neutron intensity measured at the time of sampling. The cosmic-ray method is implemented in the COsmic-ray Soil Moisture Observing System (COSMOS) that provides continental-scale soil moisture data in the USA, mostly for atmospheric science use. Other current and potential applications include calibration and validation of microwave remote sensing satellites (the current SMOS mission and future SMAP mission); monitoring soil moisture in agricultural settings; providing data for intelligent irrigation systems; measuring biomass and its seasonal variations; determining soil-moisture memory; determining area-average soil hydraulic properties; and measuring snow-water equivalent. Acknowledgements: Research on the cosmic-ray method was supported by the National Science Foundation, the Army Research Office and the Packard Foundation. COSMOS is supported by the Atmospheric and Geospace Sciences Division of the National Science Foundation (grant ATM-0838491).
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Calibration and Validation of a Mobile Cosmic-ray Soil Moisture Observing System Tyson E. Ochsner 1 , Marek Zreda 2 , Jingnuo Dong 1 , Chris B. Zou 3 , and Michael H. Cosh 4 1 Dep. of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 2 Dep. of Hydrology and Water Resources, University of Arizona, Tucson, AZ 3 Dep. of Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK 4 Hydrology and Remote Sensing Laboratory, USDA Agricultural Research Service, Beltsville, MD
The Cosmic-ray Soil Moisture Observing System (COSMOS) has recently been developed as a powerful new tool for monitoring soil moisture based on detection of ambient neutrons generated by cosmic-rays ( Desilets et al., 2010 ; Zreda et al., 2008 ) . A stationary, above-ground COSMOS probe can be used to infer average soil moisture in a footprint with ~600 m diameter and is sensitive to depths from 10 to 80 cm ( Zreda et al., 2008 ) . A vehicle-mounted, roving COSMOS probe was used by Desilets et al. (2010) to produce a soil moisture survey along a transect on the island of Hawaii on 29 January 2010, as a proof of concept, and maps of soil moisture have been produced over a 40 km by 40 km area in Oklahoma (Zreda et al., 2011) and over the entire Island of Hawaii (unpublished). Results to date suggest that the COSMOS rover may be well-suited for field campaigns to calibrate and validate satellite-based remote sensing of soil moisture (e.g. SMOS, AMSR-E, SMAP). However, it has not yet been demonstrated that the COSMOS rover can be accurately calibrated with the soil moisture in the top few centimeters, the depth detectable by satellite techniques. Furthermore, COMSOS rover soil moisture surveys have not been validated with large scale, ground-truth data. Therefore, our objectives are to calibrate the COSMOS rover to surface soil moisture and to validate the rover's performance across a range of soil types and soil moisture conditions. Three soil moisture surveys were completed for a 10 km x 16 km region with diverse soil types around the Marena, Oklahoma In Situ Sensor Testbed (MOISST) in June 2011 using the COSMOS rover described by Desilets et al. ( 2010 ) . Field-scale, ground-truth measurements of 0-6 cm volumetric water content were collected simultaneously in five fields in the survey domain using ThetaProbes (ML2x, Delta-T Devices) calibrated by soil sampling. The rover was calibrated to ground-truth measurements on 3 June, under dry surface conditions, and validated against ground-truth data on 6 June and 13 June, with dry and wet soil surface conditions, respectively. This presentation will include the neutron maps and soil moisture maps produced with the rover and a statistical evaluation of the calibration and validation quality.
References: Zreda, M., X. Zeng, J. Shuttleworth, C. Zweck, T. Ferre, T. Franz, R. Rosolem, D. Desilets, S. Desilets, and G. Womack. 2011. Cosmic-ray neutrons, an innovative method for measuring area-average soil moisture. GEWEX News, 21(3), 6-10.
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Microwave Satellite and Field Measurements of soil moisture in Kuwait Desert Hala K Al-Jassar and K. S. Rao Physics department, Kuwait University, Kuwait
Soil moisture is an important parameter for the climate modeling of the desert, though the available soil moisture is very low. To start with, a model is developed to retrieve soil moisture suitable to Kuwait desert conditions. The model was applied with AMSR-E multi-frequency data and validated with field experiments. As a follow up of our studies, at present, KISR (Kuwait Institute for Scientific Research) test site located at Sulaibiya ( 10 x 5 km) is taken up for detailed investigation in collaboration with MIT-USA sponsored by KFAS (Kuwait Foundation for Advancement of Science). Regular field experiments are conducted to measure the soil moisture at about 20 locations spread over the study area. The sampling is done at 0-10 cm and 10-20 cm. The satellite derived soil moisture seems to be higher than the field measured up to depths of 20 cm. Soil moisture at depths of 1 m (MIT measurements) is being studied now and compared with the satellite derived values. It appears that the satellite sensors are responding to depths more than 1 m in Kuwait desert area. Our further studies are aimed at monitoring soil moisture up to a depth of 2 m. Presently we are in the process of procuring suitable soil moisture probes.
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Comparing methods for quantifying soil moisture in the southern Sierra Nevada, California.
M. Meadows1, P. Hartsough2, J. Hopmans2, A. Malazian2, B. Kerkez3, R. Bales4, A. Kelly5, M. Goulden5 1 Sierra Nevada Research Institute, UC Merced 2 Dept., Land, Air and Water Resources, UC Davis 3 Civil engineering department at UC Berkeley 4 Sierra Nevada Research Institute, UC Merced 5 Department of Earth System Science, University of California, Irvine, California, USA
Soil moisture was monitored at two elevations (1,200 and 2,000m) in the southern Sierra Nevada using cosmic-ray, time domain reflectometry (TDR), dielectric, neutron probe, and gravimetric or volumetric sampling techniques. These techniques are compared to develop a better understanding shallow (0-50 cm) soil moisture distribution, and to determine the feasibility of decoupling vegetation moisture storage from soil moisture storage within the cosmic-ray signal. Multiple embedded sensors (TDR and dielectric) were deployed across varying soil depths, aspects, elevations, and canopy covers to capture spatial and temporal variations of soil volumetric water content within the spatial range (~34 ha) of COsmic-ray Soil Moisture Observing Systems (COSMOS). Soil samples were collected within the COSMOS footprint for calibration and comparison during the COSMOS installation, June 2011. Throughout a spring-fall drying season, area average volumetric water content observed by COSMOS were compared to real-time, in situ observations of soil moisture using TDR and dielectric sensors, and with measurements of soil moisture taken periodically during surveys within the COSMOS footprint. Surveys of soil moisture in the upper 40 cm of soil were made along transects around the COSMOS with handheld TDR and gravimetric sampling techniques. A neutron probe was also used to measure soil moisture at 14 locations within the COSMSO footprint. Results show that the COSMOS and the embedded sensor networks effectively observed trends of snow disappearance and soil drainage throughout the summer and fall. Survey and neutron probe data show that soil moisture measured with all of the in situ monitoring systems were well coupled to collected soil samples. The COSMOS does well tracking diurnal and seasonal trends in the near surface soil profile, however misses some of the finer scale trends observed within the embedded sensor network and surveys, which allow soil moisture availability mapping. This research site is part of the NSF-supported Southern Sierra Critical Zone Observatory, which is co-located within the Kings River Experimental Watersheds, a U.S. Forest Service integrated watershed research site in the Southern Sierra Nevada
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Watershed Scale Calibration of Different Soil Water Content Measuring Sensors
Farhat Abbas 1 , Samira Fares 2 , and Ali Fares 2*
1 Department of Environmental Sciences, GC University Faisalabad 38000, Pakistan 2 University of Hawaii-Manoa, College of Tropical Agriculture and Human Resources, Honolulu, HI
Soil water content sensors have been calibrated under laboratory, plot, and field scales. However, few calibrations have been conducted for sensors at the watershed scale. The current work reports on the results of a watershed scale calibration of three different soil water content monitoring sensors: EC-20 (Decagon Devices, Inc), ML2x (Delta-T-Devices), and SM200 (Delta-T-Devices). We also evaluated the effect of soil spatial variability on the performance of these sensors in a forested watershed. The watershed scale calibrations of the selected sensors were performed for two depths at six monitoring locations across the upper Makaha valley sub-watershed. The soil moisture sensors were subjected to different water contents ranging between field capacity (~ 0.23 cm 3 cm –3 ) and saturation (~ 0.59 cm 3 cm –3 ). Soil samples were taken and used to determine actual soil water content following the thermo-gravimetric method. These samples were also used to determine their bulk density and total porosity. The watershed soil properties significantly varied ( P < 0.05) vertically as a function of the monitoring depths (20 and 80 cm) and horizontally across the watershed. There was a positive linear and an inverse linear correlation between the soil water content at the time of soil sampling and the bulk density ( ? b), and between the soil water content at the time of sampling and the total porosity ( ? t), respectively. The use of the default calibration equations for the three sensors resulted in under-estimations of the actual water content. Calibration equations were determined for each depth, location and across depths and locations. The individual and the watershed-scale field calibrations were more accurate than their corresponding default calibrations; thus, for accurate soil water content monitoring site specific calibrations of these sensors are needed.
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Application of observation operators for field scale soil moisture averages and variances in agricultural landscapes Eunjin Han 1 , Venkatesh Merwade 2 , Gary C. Heathman 3 and Michael H. Cosh 4 1 Graduate Research Assistant, School of Civil Engineering, Purdue University, West Lafayette, IN. e-mail: ehan@purdue.edu 2 Assistant Professor, School of Civil Engineering, Purdue University, West Lafayette, IN; e-mail: vmerwade@purdue.edu 3 Soil Scientist, USDA-ARS, National Soil Erosion Research Laboratory, West Lafayette, IN; Corresponding author; email: Gary.Heathman@ars.usda.gov 4 Soil Scientist, USDA-ARS, Remote Sensing and Hydrology Laboratory, Beltsville, MD: email: Michael.Cosh@ars.usda.gov
Soil moisture is a key variable in understanding hydrologic processes and energy fluxes at the land surface. In spite of developing technologies for in-situ soil moisture measurements and increased availability of remotely sensed soil moisture data, scaling issues between soil moisture observations and the proper linkage of soil moisture estimates across different scales of observations and model predictions remain important areas of research for the calibration and validation of current and upcoming space-borne surface soil moisture retrievals, as well as successful application of data assimilation techniques. This study aims to link two different scales of soil moisture estimates by upscaling point soil moisture measurements to field average in representing field-scale agricultural watersheds (~ 2 ha) located within the Upper Cedar Creek Watershed in northeastern Indiana. Several statistical methods, mainly focusing on cumulative distribution function (CDF) matching, are tested to upscale point measurements to the field average soil moisture. These transforming equations are termed observation operators. This study also tests the temporal and spatial (horizontal and vertical) transferability of the observation operators. Results indicate that the observation operators were not transferable in time, but were spatially transferable. The CDF matching method shows the best results for all data sets: correlation coefficients are close to 0.99, RMSE is improved more than an order of magnitude from the original data set and bias is successfully reduced to zero. The CDF matching method successfully estimated the dynamic variations of STDEVs for all data sets. Correlation coefficients between the observed and predicted STDEVs are higher than 0.9, except for the certain 5cm measurements in one field. Thus, the CDF matching approach is also shown to be an effective method to deduce soil moisture variability from single point measurements.
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Field Calibration of Soil Water Capacitance Probe and Space-Time Field of Soil Water Storage in a Farmer's Field
Susmitha Nambuthiri, R. Jason Walton, Ole Wendroth Department of Plant & Soil Sciences, University of Kentucky, Lexington, KY
Appropriate calibration of field moisture sensors such as the capacitance probe is essential for obtaining meaningful soil water content measurements. Different options exist regarding the strategy of field calibration. The objectives of this study were i) to evaluate different calibration methods for 45 locations along a slope transect in a farmer's field in Western Kentucky, ii) to identify whether calibration should be based on location, soil depth or textural composition, iii) and to analyze a space-time field of soil water storage for temporal stability and its spatial relationship to crop yield. In a field, diviner capacitance probe access tubes were installed along a transect of 45 locations to enable soil water content measurements at eight 10-cm-depth increments to a depth of 80 cm. Dry bulk density field measurements obtained from a Giddings soil auger were smoothed in depth and horizontal direction using a vertical 2-D kriging scheme. For calibration, soil samples were taken at five different times and soil moisture levels within 1.3 m radial distance from each of the 45 access tubes. The calibration approaches were a) site- and depth-specific, b) site-specific, c) depth-specific, and d) according to the clay content class. The most promising calibration approaches were the site- and depth-specific (a) and the depth-specific (c). Calibration results will be presented in detail. The difference between physical and statistical sphere of influence are explained. In the second part of this contribution, the space-time field of soil water storage will be analyzed for temporal stability of spatial patterns. Co-spectral analysis is used to quantify common variation scales of spatial crop yield distribution, soil water storage at characteristic times, soil texture, and landscape topography. Field-scale soil water storage measurements are applied in autoregressive state-space analysis to predict spatial crop yield variation. One of the results of this spatial analysis will be the quantification of measurement error of soil water content. Measurement of field-calibrated soil water content with the capacitance probe contributes to better understanding of field soil water and crop-related processes.
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Determining Water Retention in Seasonally Frozen Soils Using Hydra Impedance Sensors
Thijs Kelleners
Department of Ecosystem Science & Management, University of Wyoming, Laramie, WY
The soil freezing characteristic, defined as the relationship between freezing soil temperature and unfrozen water content, can be used to determine the soil water retention curve in-situ. The objective of this study was to investigate whether freezing characteristics measured with Hydra impedance sensors result in accurate depth-wise soil water retention curves. Hydra sensors measuring the complex permittivity and soil temperature were installed at five depths at five sites in southeastern Wyoming and monitored for approximately two years. A dielectric mixing model was calibrated using unfrozen soil data to predict liquid water content and ice content in frozen soils. Total water potential in the frozen soils was calculated from the Hydra sensor soil temperature measurements using the Clapeyron equation. Soil water pressure heads were calculated from total soil water head by subtracting the osmotic head. Comparison of the resulting Hydra sensor water retention data with depth-wise laboratory retention data showed mixed results (coefficient of determination 0 = R 2 = 0.94). The best results were obtained for the shallowest sensors at the five sites (0.74 = R 2 = 0.93) because of the more significant and more prolonged soil freezing at these depths, resulting in relatively wide ranges for the calculated soil water pressure heads. Fitted curves for the Hydra sensor water retention data yielded unreliable parameters because of insufficient information on the wet end of the water retention curves.
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A Novel Method to Determine The Volume of Influence for Soil Moisture Sensors
Yurui Sun 1 , Jin Cai 1 , Wenyi Sheng 1 , Qiang Cheng 1 , Xuzhang Xue 2 1 College of Information and Electrical Engineering, China Agricultural University, 100083 Beijing, China 2 National Research Center of Intelligent Equipment for Agriculture, 100097 Beijing, China
Fringe effect sensors have been widely used for soil moisture measurements. A distinct advantage is that the probe structure facilitates working at different depths in the field. However, there is a major concern regarding the volume of influence surrounding the probe. For this purpose, both a theoretical approach and experimental methods were considered from previous studies. Because the fringing field appears as a circular distribution across two electrodes, it is challenging to find an analytical solution from known electromagnetic equations. The difficulty of the experimental methods is how to prepare soil samples with different moisture contents. Thus, a clever and simple method has been needed for many years. This study presents an experimental method based on a confirmed similarity between soil drying-wetting and freezing-thawing transitions in terms of soil dielectric properties. Two types of fringe-effect sensors were tested. The experimental results of the volume of influence of these sensors seem convincing.
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Colloid-Facilitated Radionuclide Transport under Field Conditions at Hanford Washington Z. Fred Zhang 1 , Chris E. Strickland 1 , Markus Flury 2 , Ziru Liu 2 , Glendon W. Gee 1 , James B. Harsh 2 , Ray Clayton 1 1 Pacific Northwest National Laboratory 2 Washington State University
A large amount of radiological waste is located in the vadose zone at the Hanford Site in southeastern Washington State. Research at Hanford suggests that colloids can be mobilized and potentially transport radionuclides deep into the vadose zone under transient flow. Our goal is to quantify in situ colloid mobilization and colloid-facilitated radionuclide transport of sediments at Hanford under field conditions. We conducted experiments on colloid transport in several 762-cm deep lysimeters, which have been either irrigated with synthetic rainwater to represent the wet, very wet and Chinook (i.e., rapid snowmelt) conditions or non-irrigated to represent natural conditions at Hanford. The ground surface within the irrigated lysimeter is covered to prevent evaporation. Fiberglass wicks were installed at depths of 30.48,.60.96, 121.92 and 213.36 cm to collect soil solution for colloid analysis. The wicks were installed horizontally and sit on rigid holders, each of which is one half of a split 1.27-cm-diameter PVC pipe. One end of the wick was placed near the center of the lysimeter; the exposed portion was kept within a flexible plastic tubing outside of the lysimeter to create a hanging water column with the end of the wick in a sampling bottle. Soil cores were taken for colloid analysis at the top -foot of the soil profile after about one and a half years. Additionally, the lysimeters are equipped with neutron access tubes to measure soil water content and tensiometers to measure pressure head. We applied synthesized 200-nm size EuOHCO 3 colloids to the lysimeters and have characterized their surface and colloid properties. Different-sized polystyrene colloids were also applied as tracer colloids. Solution was collected regularly from the wick samplers and was analyzed for Eu concentrations and particle counts with electron microscopy and energy-dispersive x-ray analysis. The Eu colloids were detected in the deepest wick sampler (213.36 cm depth) only two and a half months after application of the wicks, while the bulk mass of Eu was still located close to the soil surface. Generally the irrigated lysimeters showed larger Eu concentrations in the wick outflow than the lysimeter exposed to natural precipitation. The results indicate that transient infiltration events can cause colloid mobilization and translocation and that colloid-facilitated transport is a likely mechanism for radionuclide transport in near-surface sediments.
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Application of Soil Sensors to Deep Vadose Zone as a Tool for Groundwater Protection and Optimization of Remediation Strategies Ofer Dahan Zuckerberg Institute for Water Research (ZIWR), Ben-Gurion University of the Negev, Sede Boker Campus 84990, Israel.
Water content and chemical composition of pore-water in sediments are commonly measured in studies aiming to evaluate processes that take place in unsaturated sediments. Qualitative data on these parameters is required in all aspects of agronomy, vadose zone hydrology, groundwater recharge, soil remediation, and cleanup operations of contaminated vadose zone. Over the past several years, significant development in soils sensors was achieved and commercial probes were introduced to the market. Most of the commercial instruments relay on two main approaches: (a) measurements of the wet soil permittivity, as an indication for water content, and (b) application of tensiometers (porous ceramic cups) to allow measurements of the soil water pressure or collect pore water samples for chemical analysis. However these instruments are often very limited in depth application and designed for shallow soil horizons. Data from the shallow soil horizon is most essential for agricultural purpose. Nevertheless, the erratic nature of the soil water content and pore water chemical composition in the upper layers make their application in studies on deep percolation and groundwater recharge very challenging. Recently, a vadose-zone monitoring system (VMS) was developed to allow continuous monitoring of the hydrological and chemical properties of percolating water in the deep vadose zone. The VMS is designed for installation across the entire vadose zone, from land surface to the water table (tens of meters). The system is composed of flexible time-domain reflectometry (FTDR) probes for real-time tracking of water content profiles, and vadose-zone sampling ports (VSPs) for frequent sampling of sediment pore-water at multiple depths. It is installed through uncased slanted boreholes using a flexible sleeve that allows attachment of the monitoring devices to the borehole walls. Up-to-date, the system has been successfully implemented in several studies on water flow and contaminant transport in various hydrological and geological setups. These include floodwater infiltration in arid environments, land use impact on groundwater quality, and control of remediation process in contaminated vadose zone. The data which is collected by the VMS allows direct measurements of flow velocities and fluxes in the vadose zone while continuously monitoring the chemical evolution of the percolating water. While real time information on the hydrological and chemical properties of the percolating water in the vadose is essential to prevent groundwater contamination, it is also vital for any remediation actions. Remediation of polluted vadose zone often involves manipulation of the hydrological, and biochemical conditions of the sediment in order to improve pollutant attenuation. Accordingly, real time data on the sediment water content and chemical composition of the percolating water across the vadose zone may be used to select remediation strategies and determine their efficiency.
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Automated Moisture Measurements: The Basis for Mining Industry Operational and Reclamation Decisions Mark Phillip 2 , Mike O'Kane 3 , Qing Song 3 , Tyler Birkham 4 and Lindsay Tallon 5 2 O'Kane Consultants USA Inc., Anaconda, MT 59711 3 O'Kane Consultants Inc., Calgary, Alberta T2E 7K6, Canada, 4 O'Kane Consultants Inc., Cranbrook, British Columbia V1C 1C1, Canada, 5 University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A8, Canada,
Soil moisture measurements provide critical information to the mining industry. Soil science and geotechnical engineering principles were first applied approximately 20 years ago by the mining industry in pursuit of store and release or evapotranspirative covers that would minimize infiltration of meteoric waters into mine waste. The development of automated soil moisture sensors has provided the mining industry with an efficient means to track the performance of reclamation cover systems and tailings consolidation, understanding how changes in moisture storage affect the greater water balance. Soil moisture measurements are currently being used to understand 2D flow within a sloped cover system, moisture content variations within waste dumps and watersheds and the settlement of suspended tailings. This paper discusses the application and need for moisture content readings in the mining industry to make both operational and reclamation decisions.
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CALIBRATION OF FREQUENCY DOMAIN REFLECTOMETRY (FDR) EQUIPMENT IN A RED-YELLOW ARGISOL IN SOUTHERN REGION OF BRAZIL Viviane Santos Silva Terra ¹, Carlos Reisser Júnior ², Luis Carlos Timm 3 , Claudia Fernanda Almeida Teixeira³, Lauricio Martini Madaloz 4 ¹ PhD Student in PPG, Family Agricultural Production Systems, UFPel-Brazil ² Researcher, Embrapa Temperate Climate -Brazil ³ Professor, Federal University of Pelotas-UFPel-Brazil 4 Agricultural Engineer
Methods to determine soil water content are of great importance in modern agriculture. Methods for the determination of soil water content can be classified into direct methods that allow a direct determination of soil water content, and indirect methods (e.g. TDR and capacitance) that allow an estimation of soil water content based on measurements of specific soil properties. Diviner® 2000 is a capacitance soil moisture sensor that measures soil water content through a non-metallic access tube. This device requires site specific calibration for accurate measurement of soil water content. Sensor calibration is costly, time consuming and very difficult in wet/rainy condition. One possible approach to simplify the calibration process is the use of an indirect method for the determination of soil water content. The objective of this study was to compare two calibration approaches of this capacitance soil moisture sensor. The study was performed in a field with Red-Yellow Argisol. Tensiometers were installed next to the device's access tubes. The soil water content was determined in an 10 cm depth increment up to 70 cm below the soil surface by the gravimetric method from soil samples collected around another access tube, using the same soil water content. The soil water content was directly determined by the gravimetric method and estimated based on the reading of tensiometer. Water content values estimated by the capacitance sensor correlated very well (R2 =0.99) with the responding values measured by the gravimetric method. On the other hand, the correlation between water content values estimated based on the tensiometer and the capacitance readings was only good (R2 values between 0.84-0.88). The quality of these correlation relationships was better at the surface depth where there is higher sand content, than at lower soil depth where there is high clay content. Based on the results of this study, one could say that due to indirect calibration of this capacitance soil moisture sensor using tensiometer could be used for soils with high sand content, but less precise calibration will be obtained in soils with high clay contents.
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Calibration of temperature dependence of dielectric probes using time series of field data Tadaomi SAITO 1) *, Haruyuki FUJIMAKI 2) , Hiroshi YASUDA 2) , Koji INOSAKO 1) and Mitsuhiro INOUE 2) 1) Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori, 680-8553, Japan 2) Arid Land Research Center, Tottori University, 1390 Hamasaka, Tottori, Tottori 680-0001, Japan
Dielectric probes have been widely used for non-destructive determination of volumetric soil water content. Since the output of such sensors is affected by soil temperature, the calibration for the effect is indispensable for accurate determination of soil water content. A new calibration method of temperature dependence of dielectric probes using time series of field data which requires no laboratory experiments was proposed in this study. In general, empirical temperature calibration equations are derived from laboratory experiments using responses of probe outputs to temperature variations under constant water content conditions. In contrast, daily fluctuations of probe output with daily fluctuations of soil temperature from field observation data were used to derive calibration equations in this study. The research field was located in Liudaogou Basin in Shaanxi Province in the Loess Plateau, China. The average annual precipitation for th is region is about 400 mm and categorized as semi-arid. Soil water content and temperature had been previously monitored with ECH 2 O EC-5 and EC-20 probes and temperature sensors at various depths . The ECH 2 O probes employ the capacitance method and are well known as low-cost, commercially available soil water content sensors. Hourly outputs from the EC-5, EC-20 and temperature sensors at 5-cm depth were analyzed to derive calibration equations. Data taken on the days that meet the following conditions were previously removed from the analysis: i) rainy days, ii) minimum soil temperature below 0 °C and iii) daily temperature difference less than 10 °C. Under the assumption that actual daily water content was constant at a daily average value for each day, a slope value was obtained by dividing the difference in daily probe outputs by difference in daily minimum and maximum soil temperatures for each day. Temperature calibration equations were derived from the relationships between the daily average water contents and the slope values. As a result, the derived calibration equations from field data were in good agreement with the calibration equations derived from the laboratory experiments for both EC-5 and EC-20 probes. The calibration equations were applied to the field data and they validly reduced the temperature effect s in daily and seasonal time-scale data .
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Field Calibration Issues for Frequency Domain Reflectometry Soil Moisture Probes E.R. Ojo, P.R. Bullock and O.O. Akinremi Dept. of Soil Science, 362 Ellis Building, University of Manitoba, R3T 2N2
Frequency domain reflectometers (FDR) are instruments that can be deployed for in situ soil moisture measurements. When they are monitored with a data logger using data telemetry, they can provide high frequency updates on soil moisture status, which are critical for many applications related to agriculture and hydrology. They can also assist with development and testing of other methods for soil moisture determination, such as models and remote sensing provided that they are calibrated to an acceptable level of absolute accuracy. The Steven's hydra probe was used to establish a network of 13 soil moisture monitoring site-years in central and eastern Manitoba, Canada in 2009 and 2010 for the purpose of testing soil moisture models. All the probes were within the acceptable range when tested in water. Prior to their deployment in the field, each hydra probe was tested using a laboratory calibration technique to ensure that they performed uniformly under controlled conditions. At each field monitoring site, four hydra probes were installed to monitor moisture levels at 4 depths (5, 20, 50 and 100 cm). Volumetric soil moisture readings at specified times at the same depths were used to verify the soil moisture readings from the hydra probes obtained using the real dielectric value. Both the laboratory and field calibrations showed that the root mean square error of the default factory calibration increased with increasing clay content of the soil. The default factory calibration was reasonably accurate for soils with less than 20% clay content but it requires adjustment prior to field use for soils with higher clay content. Keywords: Frequency domain reflectometer, FDR, soil moisture, calibration.
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Watershed Analysis Using Online WEPP: Mashel River, Nisqually River Basin J.Q. Wu and S. Dun Biological Systems Engineering, Puyallup Research and Extension Center, Washington State University, Puyallup, WA 98371-4998
The Mashel River is a major tributary of the Nisqually River Basin and primary home to native anadromous fish. The Nisqually River originates from Mt. Rainier and flows northwest 126 km into South Puget Sound. The hydrologic regime of the Mashel River has been altered by over one and a half centuries of settlement and development. Timber harvesting, road constructions, agricultural production and associated channel alterations, and urban development and growth have all led to elevated sediment and nutrients loading, reduced streamflow, and increased stream temperature. Impairment of stream water severely degrades the aquatic habitat and affects salmonids' reproduction. In this study, we conducted watershed analysis for the Mashel River sub-basin using the online WEPP, a newly developed online GIS interface for watershed hydrologic and erosion assessment. The online WEPP GIS interface uses the OpenLayers and MapServer GIS software with base image data from Google Maps. WEPP inputs for watershed applications, including DEM and land cover and soil maps, are automatically retrieved from web servers. Daily climate inputs are generated from the long-term climate parameters of the nearest weather station using CLIGEN, a stochastic weather generator. Water balance was simulated respectively for current and historical landuses. Streamflow as driven by climatic extremes was evaluated under the current management condition with stormwater diversion vs. pre-development condition without stormwater diversion. Based these analyses, we proposed alternative stormwater management plans.
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THE NASA SOIL MOISTURE ACTIVE PASSIVE (SMAP) MISSION: Overview and Relevance to In Situ Soil Moisture Sensing
Peggy O'Neill 1 , Dara Entekhabi 2 , Eni Njoku 3 , and Kent Kellogg 3
1 Hydrological Sciences Laboratory / Code 617, NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA
2 Dept. of Civil & Environmental Engineering, Massachusetts Institute of Technology Cambridge, MA 02139 USA, 3 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109 USA,
The National Research Council's (NRC) Decadal Survey, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond , was released in 2007 after a two - year study commissioned by NASA, NOAA, and USGS to provide consensus recommendations to guide the agencies' space-based Earth observation programs in the coming decade [1]. Many factors involving engineering maturity, scientific advances, and societal benefits of potential missions were considered as part of the NRC evaluation process. Five of the six NRC Earth science discipline panels (water resources & the hydrologic cycle; climate; weather; human health & security; and land use, ecosystems, & biodiversity) cited numerous science and applications needs that could be wholly or partially met by a mission devoted to measuring surface soil moisture and its freeze/thaw state. Based on the NRC recommendations and on its own evaluation of technical readiness, NASA selected the Soil Moisture Active Passive (SMAP) mission to be the first of the Decadal Survey missions to be developed, with a target launch date of October, 2014. This mission is a joint effort of NASA's Jet Propulsion Laboratory (JPL) and Goddard Space Flight Center (GSFC), with project management responsibilities at JPL.
SMAP will be launched into a sun-synchronous dawn-dusk orbit, and its wide swath of 1000 km will enable global mapping of surface soil moisture and freeze/thaw every 2-3 days. The SMAP instrument design incorporates an L-band radar (3 km spatial resolution) and an L band radiometer (40 km spatial resolution) sharing a single 6-meter rotating mesh antenna to provide measurements of soil moisture and landscape freeze / thaw state [2]. These observations will (1) improve our understanding of linkages between the Earth's water, energy, and carbon cycles, (2) benefit many application areas including numerical weather and climate prediction, flood and drought monitoring, agricultural productivity, human health, and national security, (3) help to address priority questions on climate change, and (4) potentially provide continuity with brightness temperature and soil moisture measurements from ESA's SMOS (Soil Moisture Ocean Salinity) and NASA's Aquarius missions. At the L band frequencies utilized by SMAP (1.26 and 1.41 GHz), the SMAP radar and radiometer are sensitive to moisture conditions in a soil layer approximately 5 cm deep on average. By coupling these surface layer measurements with appropriate models and data assimilation, SMAP will also routinely produce estimates of the root zone soil moisture (~1 m deep). In order to validate the surface and root zone soil moisture products derived from the SMAP microwave measurements, a variety of ground measurements are desired over the SMAP spatial (3, 9, and 36 km) and temporal resolutions (every 2-3 days). These measurements will be used both for direct comparison with SMAP-retrieved soil moisture (5 cm and root zone soil moisture measurements) and for verification of parameterizations in the SMAP retrieval algorithms (soil temperature, vegetation water content, surface roughness). SMAP calibration / validation activities will be focused on the acquisition of SMAP-relevant ground data from distributed instrument networks and from SMAP core validation sites. REFERENCES [1] National Research Council, “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond,” pp. 400, 2007. [2] Entekhabi, D., E, Njoku, P. O'Neill, K. Kellogg, plus 19 others, “The Soil Moisture Active Passive (SMAP) Mission,” Proceedings of the IEEE , Vol. 98, No. 5, May, 2010.
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The NOAA CREST Microwave Observation Unit for the Monitoring of Soil Moisture and Snow in Northeastern U.S.
Reza Khanbilvardi, Marouane Temimi, Kyle C. McDonald, Tarendra Lakhankar, Hamidreza Norouzi NOAA-CREST Institute, City University of New York, New York City, NY
NOAA CREST has established, in partnership with federal agencies and other academic institutions, a microwave observation unit for the monitoring of key soil surface parameters such as soil moisture, surface emissivity, snow depth, snow water equivalent, amongst others. Microwave observations are carried out through the use of tower-based radiometers that measure surface microwave brightness temperature at three different frequencies, namely the L band and the 37 and 89 GHz. The L band was deployed to duplicate measures from current sensors like SMOS and lay the ground work for future missions such as Aquarius and SMAP, whereas the higher frequencies (i.e. 37 and 89 GHz) were selected because of their higher sensitivity to snow proprieties. Microwave observations are taken in two sites; the first is the region of Millbrook NY and the second is Caribou, Maine. The latter is hosted by the NOAA NWS Weather Forecast Office whereas the former is collocated with a NOAA Climate Research Network (CRN) site that is hosted by the Cary Institute of Ecosystems. Both sites comprise a set of in situ sensors to measure several soil parameters. Soil moisture Stevens Hydra probes were deployed in six sites in Millbrook and Caribou. Measurements are taken regularly at three different depths, 2.5, 5 and 10 cm. The same measurement is duplicated at each site for quality control purposes. Soil temperature is also taken at the same depths. In Millbrook, the collocated CRN site provides quality controlled measures of precipitation and soil moisture at deeper soil layers. The objective of this research is to make use of observations from the NOAA CREST Microwave Observation Unit in conjunction with in situ measures of soil temperature and moisture to study the differences in diurnal cycles between microwave and thermal temperatures and the effect of those differences on the retrieval of surface parameters like soil moisture and emissivity. A time lag between skin temperature and microwave temperature diurnal cycles was noticed in literature which requires further study. The phase and amplitude of the diurnal cycles of the thermal temperature and the microwave temperature are examined and compared to in situ observations of soil moisture as liquid water affects the penetration of the microwave signal and its agreement with skin temperature. Ultimately, a relationship will be established between phase lag and soil moisture observations.
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EFFECT OF SOIL MOISTURE ON POLARIMETRIC-INTERFEROMETRIC REPEAT PASS OBSERVATIONS BY UAVSAR DURING 2010 CANADIAN SOIL MOISTURE CAMPAIGN
Scott Hensley * , Thierry Michel * , Jakob Van Zyl * , Ron Muellerschoen * , Bruce Chapman * , Shadi Oveisgharan * , Ziad S. Haddad * , Tom Jackson † , Iliana Mladenova †
* Jet Propulsion Laboratory, California Institute of Technology, Pasadena, † USDA Agricultural Research Center, Hydrology and Remote Sensing Laboratory, Beltsville, MD 20705
Soil Moisture Active Passive (SMAP), a proposed mission in support of the Earth Science Decadal Survey, conducted a field campaign in June 2010 to support algorithm development. As part of the experiment in situ soil moisture measurements were made over a two week period in which multiple UAVSAR flights were conducted. Repeat-pass polarimetric-interferometric data generated from these flights were analyzed to see if phase changes could be correlated with soil moisture changes. Also, we compared the data to that predicted by simple surface scattering models and showed moderate agreement with the Oh model [3]. Repeat pass radar interferometry is routinely used to measure surface deformation with millimeter accuracy [4]. It has been observed and noted by several investigators that soil moisture changes can impact interferometric measurements [2], [1], [5] and [6]. Because interferometric phase measurements are extremely sensitive to small changes in the surface it is plausible to hope that an algorithm could be developed to convert the phase changes to quantitative soil moisture changes. As a first step in that direction, we examine polarimetric interferometric repeat pass measurements by UAVSAR conducted in Canada during June 2010 in support of NASA's proposed Soil Moisture Active Passive mission and compare them to scattering models currently used as the basis for soil moisture retrieval algorithms. [1] Brian Barret, Ned Dwyer, and Padraig Whelan. Comparative Analysis of Soil Moisture retrieval using C- and L- band Inteferferometric SAR, In 3rd Annual Irish Earth Observation Symposium, November 2009. [2] Scott Hensley, Howard Zebker, Bruce Chapman, Jakob Van Zyl, Cath- leen Jones, Thierry Michel, and Alex Fore. A Polarimetric And Interferometric Exploration Of Soil Moisture Using The UAVSAR Instrument, In IGARSS 2009, July 2009. [3] Yisok Oh, Kamal Sarabandi, and Fawwaz T. Ulaby. Semi-empirical model of the ensemble-averaged differential mueller matrix for microwave backscattering from bare soil surfaces. IEEE Trans. Geosci. Remote Sensing, 40:13481355, 2002. [4] P. A. Rosen, S. Hensley, I. R. Joughin, F. K. Li, S. N Madsen, E. Rodriquez, and R. M. Goldstein. Synthetic Aperture Radar Interferometry. Proc. IEEE, 88:333–382, 2000. [5] U. Wegmuller and C. L. Werner. Retrieval of vegetation parameters with sar interferometry. IEEE Trans. Geosci. Rem. Sens., 35:18–24, 1997. [7] Ting Zhang, Qiming Zhing, Ying Li, and Yun Xiang. STUDY ON RELATION BETWEEN INSAR COHERENCE AND SOIL MOISTURE. In The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, volume XXXVII of Part B7, 2008. [6] Ting Zhang, Qiming Zhing, Ying Li, and Yun Xiang. Study On Relation Between InSAR Coherence And Soil Moisture, In The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, volume XXXVII of Part B7, 2008. This research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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Optimal Control Algorithm for Precision Irrigation Based on a Wireless Dual-Sensor Control System Yandong Zhao 1 Yurui Sun 2 1 School of Technology, Beijing Forestry University, 100083 Beijing, China 2 College of Information and Electrical Engineering, China Agricultural University, 100083 Beijing, China
Efficient use of agricultural water resources has received increasing attention in arid and semiarid areas. On the other hand, precision irrigation can be regarded as a case application of optimal control theory. This study provides an application note based on a developed dual-sensor, which can monitor soil water dynamics (SWD) within the root zone, i.e., 10-35 cm at two monitored depths. Using this sensor technique together with wireless data transmission, a simple irrigation control system with an optimal control algorithm was developed and tested within a grass field in Beijing City from May of 2010 to September of 2010. The novel system reduced water consumption by 30 percent over from the conventional control method (Bang-bang control). Thus, both the developed dual-sensor and the method of optimal control decision are two highlights of this study.
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Heat Pulse Soil Moisture Sensor for Field Applications
Tamir Kamai1, Gerard J. Kluitenberg2, John H. Knight3, and Jan W. Hopmans1 1 Dept., Land, Air and Water Resources, UC Davis 2 Kansas State University Manhattan, KS 3 Queensland University of Technology, Australia.
Despite the vast importance of real-time water content data, measurement tools are limited. Among others, the heat-pulse sensor is useful for measuring both thermal properties and water content. Measurements with this sensor are soil independent and data acquisition requirements are relatively simple. However, so far, applications of this sensor to field settings were limited because of its delicate design that is required to meet the assumptions of the line-source heat transfer model. In this study we present a sensor with large-diameter rigid probes together with a heat transfer model that accounts for the finite diameter and finite heat capacity of the probes. We tested sensors and analyzed data of different sizes for various soil types, to determine optimum sensor design . With our new sensor, results show excellent agreement between predicted and heat-pulse-estimated soil thermal properties and water content. We believe the proposed field design is well suited for field applications of soil moisture, with an accuracy of about 2 volume percent.
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Heat Pulse Soil Moisture Sensor for Field Applications
Tamir Kamai1, Gerard J. Kluitenberg2, John H. Knight3, and Jan W. Hopmans1 1 Dept., Land, Air and Water Resources, UC Davis 2 Kansas State University Manhattan, KS 3 Queensland University of Technology, Australia.
Despite the vast importance of real-time water content data, measurement tools are limited. Among others, the heat-pulse sensor is useful for measuring both thermal properties and water content. Measurements with this sensor are soil independent and data acquisition requirements are relatively simple. However, so far, applications of this sensor to field settings were limited because of its delicate design that is required to meet the assumptions of the line-source heat transfer model. In this study, we present a sensor with large-diameter rigid probes together with a heat transfer model that accounts for the finite diameter and finite heat capacity of the probes. We tested sensors and analyzed data of different sizes for various soil types, to determine optimum sensor design . With our new sensor, results show excellent agreement between predicted and heat-pulse-estimated soil thermal properties and water content. We believe the proposed field design is well suited for field applications of soil moisture, with an accuracy of about 2% volume.
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Potential for Soil Moisture Estimation Using Spatially Distributed Temperature Sensing W.W. Miller, Scott Tyler, Lucas Williamson and Christine Hatch 1 . 1 Professor of Soils & Hydrology, Professor of Hydrologic Sciences, Graduate Student Hydrologic Sciences, and Post Doctoral Research Scientist, University of Nevada, Reno
The search for salvageable water commonly focuses on irrigated agriculture because evapotranspiration (ET) is a major consumer. Effective agricultural water reallocation is predicated in part on the ability to use less water in the production process. Distributed Temperature Sensing (DTS) along optical fibers allows for the nearly continuous measurement of temperature over length scales appropriate for agricultural soil water management. This new technology allows for the continuous measurement of temperature along a fiber optic cable for distances of up to 4 km with a spatial resolution of ~1 meter. The availability of such fine spatial (typically 1-2 m) and temporal (typically 10-60 s) resolutions can provide insights into local-scale process that were impossible to resolve with traditional monitoring instruments, and we hypothesize that a functional relationship between DTS and changing SMC can be developed that will facilitate more efficient irrigation scheduling and WUE. We have developed an effective plow system for installing a set of three optical fibers at 5, 10 and 15 cm depths within the soil profile over lengths of up to 1 km, and data are presented from an experimental site in the Walker Basin, Yerington, Nevada. By measuring the time rate of temperature change with depth at each spatial location, the phase lag and amplitude damping of the diurnal temperature signal can be used to estimate the soil thermal diffusivity during periods when infiltration is limited. Passive temperature measurements were taken at a 1 m interval along the length of the cable. Soil samples co-located with the fiber optic cables were used in the laboratory to derive a relationship between water content and thermal conductivity, as well as to determine the bulk soil heat capacity. By combining these measurements, efforts were made to predict soil moisture on spatial and temporal scales. Although soil moisture estimates using passive temperature sensing have not proven sufficiently sensitive, use of an active temperature pulse and heat dissipation protocol appears promising.
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Spectroscopic and Thermal Constraints on Moisture Darkening
Christopher Small Lamont Doherty Earth Observatory Columbia University, Cornell University
The phenomenon of moisture darkening is widely observed for a variety of media but there is no consensus on the physical process(es) responsible. At least two models have been proposed (see Philpot et al, this volume) but the mechanisms on which the models are based are not mutually exclusive. It is reasonable to expect that the manifestation of the process may depend on the physical properties of the media being darkened by the moisture. We attempt to derive observational constraints on the process of moisture darkening using simultaneous spectroscopic and thermal measurements.
The process of moisture darkening in soils is of particular importance because of its impact on albedo and therefore mass and energy flux over large areas of the Earth surface. However, the compositional and textural heterogeneity of most soils poses a variety of challenges to understanding the process – and to interpretation of experimental results. To circumvent some of the complications, we focus on compositionally homogeneous sediments – primarily sands. To understand the effect of varying moisture content, we conduct drying experiments in which we measure visible-infrared reflectance (350-2500 nm) and thermal emission (8-14 µm) of sand as it progresses from saturated to dry.
Despite variations related to grain size and mineralogic composition, we observe several consistencies in our experiments. Spectroscopically, dehydration brightening increases linearly through most of the drying process with an abrupt increase in brightness at the end of the process. The linear increase in brightness does not appear to be accompanied by a change in surface temperature – although the abrupt increase does. The two phase brightness increase and temperature change suggests that the dehydration process undergoes a transition from steady state to volume limited evaporation. The implication is that remote sensing of soil moisture may depend critically on the phase of the drying process being imaged.
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Preliminary model of the spectral reflectance of wetted soils William Philpot Cornell University
Charles Bachmann Naval Research Laboratory
Christopher Small Lamont-Doherty Earth Observatory
Soils darken when wet with little visible color change. Several explanations for the darkening have been suggested based on at least two, very distinct theoretical hypotheses. Ångström (1925) attributed the reduction in the visible to multiple internal reflections within the film of water coating soil particles. A number of authors have expanded on this idea, modifying Ångström's formula to account for light that is not totally internally reflected and incorporating the effect of spectral absorption by the water itself, a necessity when considering spectral reflectance through the short wave infrared (SWIR). An alternative explanation attributes the darkening to an increase in forward scattering in wetted soil. As with the case of internal reflection, this increases the interaction of light with soil and results in increased probability of absorption by the soil. As with the internal reflectance model, this could account for the decrease in reflectance, but it would still be necessary to extend the model to account for absorption by water in order to account for spectral variations, especially those in the short wave infrared. It seems clear that the darkening of wetted soil could be attributed to multiple mechanisms. What is not clear is which of those mechanisms is the most appropriate or important. Indeed, both fundamental explanations could well be significant. Another uncertainty is how and when absorption by water becomes important and exactly how that enters into the process. There are still other issues that have not been clearly addressed. For example, the absorption spectrum of pure water is known, but pore water contains dissolved material and the water itself is partially bound to the soil. Both factors may alter the effective absorption by water. In this paper we explore some of the implications of the different mechanisms for describing spectral reflectance of wetted soils.
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Validation of the Agricultural Irrigation and Nutrient Decision Support Model Kelly T. Morgan
University of Florida, Department of Soil and Water Science, Southwest Florida Research and Education Center, Immokalee, FL 34142
This report is to document the in-season real-time soil moisture simulation of a crop by the Agricultural Irrigation and Nutrient Decision Support Model. The model as developed simulates plant growth, and water and nutrient uptake using the Cropgro sub-model of the Decision Support System for Agrotechnology Transfer (DSSAT) that estimates growth, yield and water and nutrient uptake of many common row crops including tomato and pepper. Currently, all crop models in DSSAT simulate water and nutrient movement in the soil and uptake by the plant as a 1D simulation assuming irrigation water and fertilizer nutrients are applied to 100% of the area simulated. Likewise, DSSAT assumes that water and nutrients are taken up from 100% of the simulated area. These assumptions tend to average water and nutrient use over a broad area, and for a large area (>100 acres) has been found to provide reasonable water and nutrient use and crop growth and yield simulation. However, for low volume irrigation management of large fields, particularly those fields adjacent to water quality sensitive areas (e.g. wetlands or urban communities), a 1D approach is not adequate to evaluate crop yields or agricultural impacts. Therefore, a 2D water and nutrient movement and uptake model was developed. The area under the plastic mulch bed and furrows between rows were defined as cells with specific dimensions. Water and nutrient movement from irrigation and fertilizer sources in vertical and horizontal directions are estimated based on soil characteristics, plant uptake parameters for specific plant age, and environmental factors (i.e. evapotranspiration). Water movement was calibrated in 2010 using soil water content sensor data collected from a commercial tomato crop. The influence of water table height on increased soil water content and upward nutrient movement was added to the model in 2011. A bell pepper crop was grown in south central Florida on Eugallie fine sand was planted on February 24, 2011 with harvest starting 78 days after planting. Soil water content using capacitance soil moisture sensors at 10, 20, 40 and 70 cm depths were compared to model estimated soil water content at the same depths. Actual and simulated soil water content values agreed within a relatively narrow range with the exception of days 31 to 36 after planting when a heavy rain caused the model to overestimate soil water content. The model values decreased to a reasonable value after drainage of excess water. Total fruit yield was simulated for the 2011 growing season at 38,640 kg ha -1 fruit. Final crop yield for the 40 ha field was 39,200 kg ha -1 equaling a 1.4% underestimation by the model. The model has not been calibrated for yield and should not be expected to be 100% accurate but a 12% reduction is well within expected ranges for most crop models.
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Spatio-temporal monitoring of water and nutrient dynamics within the grapevines root-zone scale
Fuentes S. 1 , De Bei R. 1 , Buss P. 2 , Dalton M. 2
1 The University of Adelaide. Plant Research Centre. PMB1, Glen Osmond, SA, 5064. Australia. 2 Sentek Pty Ltd. 77 Magill Rd, Stepney, SA, 5069. Australia.
In Australia, high percentage of fresh water (70%) is used for horticultural irrigation and from this, 33% is used for viticulture only. Water scarcity in Australia has pressed the viticulture industry and research institutions to find new techniques to save water. Furthermore, current climate change scenarios have forecasted reductions of rain events and increments of climatic anomalies, such as heat waves in the growing period. South Australia is one of the main viticulture regions in Australia, which will most likely be adversely affected by climate change, making the need to improve efficiency of water delivery and uptake by grapevines a priority. Regulated deficit irrigation (RDI) and partial root-zone drying (PRD) have been developed to mainly save water by maximizing water use efficiency (WUE) of grapevines, reduce vigor, improve quality and maintain yield. Implementing RDI and PRD consists in the temporal and spatial manipulation of soil wetting patterns within the root-zone of grapevines, which triggers hydraulic and hormonal signals from roots-to-shoots to manipulate the physiology of vines. These techniques have produced narrower vine water stress thresholds and the necessity to monitor soil wetting patterns (SWP) within the root-zone scale. A novel analysis methodology has been developed to visualize the 2D and 3D wetting and nutrient patterns under pressurized irrigation systems using an array of probes close to the root system and programming techniques using MATLAB® 2011b. Results using this new tool have demonstrated that accurate SWP dimensions can be produced in real time compared to modeling techniques and SWP visualizations using a Perspex box. Also, this technique allows the visualization and quantification of the gradient of soil moisture and salinity within the wetting and nutrient patterns, which is not possible using the validation techniques previously mentioned. Furthermore, grapevine water and nutrient uptake can be visualized and correlated to other techniques, such as sap flow sensors and infrared thermography. The spatial and temporal resolution that this new technique offers will allow making more in depth studies of soil wetting and nutrient patterns dimensions and dynamics within the root-zone. Therefore, using this visualization technique, precision irrigation and fertigation can be achieved in field conditions to optimize irrigation and nutrient applications for crops under pressurized irrigation systems.
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RELATIONSHIPS BETWEEN MICROCLIMATE, VEGETATION AND SOIL TEMPERATURE, AND INITIATION OF GROWING SEASON DURING SEASONAL FREEZE/THAW TRANSITIONS IN A BOREAL FOREST
Kyle C. McDonald1 , Reiner Zimmermann2, Jessica Potter3, Erika Podest3 Marouane Temimi 1, Tarendra Lakhankar 1, Reza Khanbilvardi 1
1 CREST Institute, City College of New York, New York City, NY 2University of Hohenheim, Stuttgart D-70599 Germany 3 Jet Propulsion Laboratory, California Institute of Technology
Characterization of seasonal freeze-thaw transitions in northern ecosystems is key to better understanding land-atmosphere carbon exchange and the cycling of water, carbon, and energy in the high latitudes. In Arctic/boreal soils, aggregate soil moisture in spring and autumn is largely influenced by the soil's freeze/thaw state. During seasonal transitions, spatio-temporal variability in soil freeze/thaw state can vary with such parameters as snow and vegetation cover, topography, and slope aspect. In this study, we utilize in situ measurements of soil temperature profiles, vegetation stem temperature, vegetation sap flux, and stand-level climate to investigate the relationships between soil freeze/thaw and moisture vegetation biophysical activity, and climate, during springtime thaw along a north-south transect extending from Alaska's North Slope, though the central interior. Tree stem temperature is measured with thermistors implanted in the stems of several trees. Trees monitored include a variety of species representative of growth conditions extending from well-drained to poorly-drained boggy soil conditions. Soil temperature profiles are monitored for each growth condition/species situation. Xylem sap flux is monitored using a constant energy input method. Withincanopy air temperature is also measured in the forest stand. Measurements are made every 10 minutes, with 2-hour averages stored on a data logger. Air temperature, relative humidity, and solar radiation are also available from weather stations. We examine relationships between landscape component temperatures and initiation of vegetation growing season, using xylem sap flux as the growing season indicator, for thirteen years of in situ data collections (~1994-2006). The soil thaw horizon is compared to vegetation thaw timing, and both are compared to the initiation of sap flow in the trees. Correspondence between vegetation thaw, soil thaw, and sap flux initiation is compared across tree species and growth conditions. We examine within site variations and seasonal timing along the north-south transect. Particular emphasis is placed on the soil state as related to snow and vegetation condition. Portions of this work were carried out at the Jet Propulsion Lab, California Institute of Technology, under contract to the National Aeronautics and Space Administration .
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New Irrigation Scheduling Software combining ETo and soil water dynamics to improve the measurements of daily crop water use, Kc factors and the onset of crop water stress of a flood irrigated alfalfa crop Buss, P., Stevens, S. and Dalton, M. Sentek Pty Ltd , 77 Magill Rd , Stepney, SA 5069 The measurement of soil moisture or water budgeting based on Evapotranspiration (ETo) and crop factors (Kc) are important irrigation scheduling methods used in commercial agriculture around the world. A shortcoming of the water budgeting method is that the calculation of crop Evapotranspiration (ETc) occurs often under standard conditions. Significant bias can be introduced on the Kc factor, if no influence is being considered of phenological crop stage, duration of the cropping cycle, or impact from soil water or salinity stress, crop density, pest and disease loads, weed infestation and cultural activities. Using data sets from flood irrigated alfalfa in Australia shows how in situ measurements of daily crop water use can be utilized to correct daily Kc values for non-standard conditions and that the combination of ETo and daily crop water use (DWU) can lead to the timely detection of the onset of water stress. Algorithms based on soil water dynamics are used to refine the daytime water use of the crop in millimetres (mm) of the active rootzone. This is then referenced to daily ETo values to generate daily Kc values and seasonal Kc curves incorporating day–to –day changes in environmental conditions. The use of an Evapotranspiration Stress Index (ETSI) using ETo and daily soil water data to more accurately detect the onset, severity and duration of crop water stress caused either by waterlogging or drought stress is novel. This new software is designed as an improved irrigation scheduling tool, combining old and new measurement elements to minimize the number of crop stress days in any soil, crop or climate. This also shows that two traditionally rivalling methods for irrigation scheduling can be used to mutual benefit if they are integrated into one approach. Key Words: ETo, Kc, crop stress, algorithm, software, irrigation
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Improved Performance and Accuracy of Three New Soil Moisture Sensors in a Hawaiian Oxisol Ali Fares 1 , Mohammad Safeeq 2 , Samira Fares 1 , and Ripendra Awal 1* 1 University of Hawaii-Manoa, College of Tropical Agriculture, Honolulu, HI 96822. 2 Department of Geosciences, Oregon State University, Corvallis, Oregon.
Capacitance based soil water content measuring devices are well known for their sensitivity to different soil physical and chemical properties, i.e., temperature, bulk density, organic matter content, and salinity. Limited research work has been conducted on the effect of these parameters under weathered tropical soils, e.g. Hawaiian soils; thus, there is a need to determine the effects of these soil properties on these sensors in weathered soils. In this study, we evaluated i) the effect of sensor to sensor variation and temperature on three relatively new soil moisture sensors (ECH 2 O TE-5, TM-5, and EC-5) in a Hawaiian Oxisol ( Isohyperthermic, Rhodic Eutrustox ) and; ii) analytical algorithms for temperature correction on the measurements of the sensors. Readings of the three sensors used show positive linear response to temperature that significantly increased with increasing temperature. The temperature effect increased exponentially with increasing water content. TE-5 readings are more sensitive to temperature than those of TM-5 and EC-5. The sensitivity of TM-5 and EC-5 readings varied as a function of water content. They decreased with increased soil water content where their highest sensitivity was observed at 10% water content. ANOVA results show that temperature, and probe-probe variations were significant (P<0.05) for all the three sensors. However, the hysteresis effect was only significant for TM-5 and EC-5 sensors. The temperature correction algorithm was an exponential model for TE-5, and a linear model for TM-5 and EC-5 sensors. Their standard error of estimates was 1.49, 0.185, and 0.143 mV o C -1 for TE-5, TM-5, and EC-5, respectively. Reductions in slope magnitudes for TE-5, TM-5, and EC-5 sensors, after implementing the temperature correction algorithm, were 23-155%, 84-155%, and 87-168%, respectively. TM-5 and EC-5 are less sensitive to temperature than TE-5 and other earlier generations of ECH 2 O sensors (i.e. EC-20, and EC-10).
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Comparison of Commonly-Applied Electromagnetic Sensors for Soil Water Content and Electrical Conductivity Measurements in Mineral, Organic and Saline Soils Carlos M.P. Vaz1, Scott B. Jones2, Mercer Meding3 and Markus Tuller3 1 Embrapa Agricultural Instrumentation, São Carlos, SP, Brazil, 13560-970 2 Utah State University, Department of Plants, Soils and Climate, Logan, UT 84322-4820 3 The University of Arizona, Department of Soil, Water and Environmental Science, Tucson, AZ 85721
An increasing number of electromagnetic sensors (EM) are being developed and deployed for soil water content and electrical conductivity measurements in agricultural, hydrological and ecosystem studies. Our objectives were to observe and compare the effects of soil mineral composition, organic matter and salt concentration on EM sensor response. Ten well characterized source soils spanning the entire textural range were utilized to test the influence of clay and organic matter content, iron oxides, ferromagnetic minerals and salinity on water content and EC measurements. Systems and sensors tested included TDR 100, CS616, WET-2, Hydra, Theta, SM300, 5TE and 10HS, which are based on different measurement principles ranging from time domain reflectometry to capacitance and impedance methods, each with different oscillation frequencies. Soil samples were measured at eight different water contents with nearly constant bulk density. The influence of the soil texture, organic matter and electrical conductivity were evaluated for each sensor and the validity of factory supplied-calibration relationships was tested. Consistent with previous studies, soils higher in clay content, organic matter and electrical conductivity resulted in higher root mean squared error (RMSE), when sensor-measured volumetric water contents were compared with water contents determined via oven-drying. There is also a trend for reduced RMSE with increasing measurement frequency, though this result is inconsistent for some sensors when comparing sensor studies using liquid dielectrics.
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Life Under Your Feet: Monitoring the Soil Ecosystem with Wireless Sensor Network Katalin Szlavecz, Andreas Terzis, Alexander Szalay Johns Hopkins University, Baltimore MD
Soil biological activity is determined by a complex interaction of physical, chemical and biological factors. Temperature and moisture, both varying spatio-temporally, are among the most significant abiotic factors, in addition to substrate availability. Soil organic matter and soil biota respond to environmental changes, and these responses are different for gradual, long-term change, such as monotonic global warming, than they are for changing patterns of episodic conditions, such as an increase in frequency of extreme events. To obtain more continuous data and thus capture “hot spots-hot moments” we have built and deployed several wireless sensor networks for soil monitoring. The wireless sensor network is built on top of the open source TinyOS operating system, and uses a self-routing network for the wireless connections. The wireless “motes” connected to the sensors are normally asleep, saving power, they only wake up for turning on the sensors and do a burst of data collection, then go to sleep again. At regular intervals they wake up and upload the data to the relay nodes. These, in turn are connected to the gateways, typically a relatively low power PC, connected to the internet. The data management system is using Microsoft SQL Server as the underlying database, while the preprocessing and transformation of the data is done using a set of Linux tools. The system collects temperature, moisture and CO 2 concentration data belowground. Over the past five years we have deployed this system in an urban residential area (Cub Hill testbed: 53 sampling locations in urban forest and lawn habitats), in a mid-Atlantic deciduous forest (Smithsonian Environmental Research Center, MD: 37 sampling locations), in an agricultural field (USDA Beltsville Agricultural Research Center Farming System Project: 22 sampling location in no-till crop fields), in a high altitude desert (Atacama Desert, Chile: 3 sampling locations), and in the Amazon basin (Yasuni National Park, Ecuador: 12 sampling locations). In this presentation I will summarize our experiences in these diverse environments, and talk about the new generation of motes we are currently building.
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Through a geophysical lens: Case studies of field-scale soil-moisture measurements using electric and electromagnetic methods Heinse, R., M. Wessel and M. Corrao Measuring and monitoring near-surface soil moisture is a vital component of agricultural, ecological and hydrological resource management and modeling, and key to understanding critical zone processes. Soil textural and structural patterns coupled with position in the landscape, subsurface flow paths and plant community structures tend to produce clustered high spatial variability signatures of soil moisture distributions that are difficult to quantify by means of highly accurate point-scale sensors or remotely sensed water contents alone. Near-surface geophysical techniques are well positioned to fill this scale gap, but their use generally requires the solution of an inverse problem to estimate water content. In this paper we examine several case studies from forest, range, and agricultural lands in which inverse solutions to near-surface geophysical data were used to map spatial and temporal patterns of soil moisture. Because the spatial and temporal resolution of geophysical data is a complex function of contrasts in physical state variables, survey designs and choice of inversion methods, results are often target specific. We examine approaches and pitfalls of using ground penetrating radar and electrical resistivity imaging data to extract different soil properties and processes, such as soil texture and soil moisture change. Enabling the coupling with accessible a-priori and posterior information, the use of geophysical methods can greatly enhance our understanding of transient soil-moisture patterns, and of the links with critical-zone processes.
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Evaluation of a Frequency Phase Shift Soil Moisture and Electrical Conductivity Sensor
Kosuke Noborio, and Takehiro Kubo
School of Agriculture, Meiji University; Kawasaki, Kanagawa 214-8571, Japan
For soil water and solute transport research, time domain reflectometry (TDR) has been commonly used since its introduction to soil research in 1980 by Topp and his colleagues. Although TDR seems to be very versatile for laboratory and field experiments, it requires modest user skills for accurate measurements and the instrument itself is still expensive. A new soil moisture and electrical conductivity (EC) sensor was recently developed by a Japanese manufacturer, A.R.P. Co. Ltd., at reduced cost. The new sensor independently measures soil-water content, -bulk electrical conductivity, and -temperature using frequency phase-shift techniques. We evaluated the new sensor for measurements of water content, EC, and temperature using three different sensors in variably saturated soils moistened with saline water of various EC levels and in saline water of various EC levels. There was little difference among them for water content measurement except near saturation. For sand and volcanic ash soil, water contents between air-dry and near saturation were linearly related to sensor output in mV, but the slopes were different for sand and volcanic ash soil. Polynomial curves, however, provided better calibration for the full range of water content between air-dry and saturation. In saline water up to 54 mS m-1, the mV output in the EC measurement mode was linearly related to solution EC. Although slopes were quite similar among the three sensors, zero mV intercepts differed. We concluded that the new sensor could be used for soil-water content and -EC measurements with appropriate calibration.
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Using Soil Moisture Observations as Ground Truth for Remote Sensing
Alan Robock and Thomas W. Collow
Department of Environmental Sciences Rutgers University New Brunswick, NJ 08901-8551 USA Soil moisture is an important variable in the climate system. An observational data set of actual in situ soil moisture measurements is crucial as ground truth for remote sensing. While satellites can only detect soil moisture in the top few cm of soil, their global spatial coverage has the potential to provide useful information for initializing weather forecasts, monitoring droughts and floods, and studying climate change. The European Space Agency launched the Soil Moisture Ocean Salinity (SMOS) satellite in November 2009, which is the most advanced soil moisture satellite to date, with an L-band microwave radiometer. Using SMOS soil moisture retrievals for 2010 processed using algorithm V4.00, we evaluated SMOS retrievals by comparing them to in-situ soil moisture observations for the top 5 cm at several stations in the Great Plains of the U.S. A major issue with comparing the satellite data with in-situ data is that a SMOS footprint is about 40 km across and we compare to point observations. To address this issue, we chose locations in Oklahoma that have 10 to 25 different in-situ observations within each SMOS footprint. The SMOS retrievals have a dry bias when compared to the average of all of the in-situ stations in a footprint. There are large differences between the in-situ observations, even for probes only a few meters apart. Observations from different sensors within a SMOS footprint differ from each other by a larger amount than they differ from the SMOS retrieval. Soil moisture values from the Oklahoma Mesonet appear to be higher than all the other in-situ observations. Removing the mean and normalizing the data brings the in-situ observations into better agreement with each other and with SMOS but they still contain substantial differences. Agricultural Research Service Micronet regions in Oklahoma had highly varying values of soil moisture despite being in close proximity to one another, but when averaged and compared to SMOS they had less of a bias than the other regions. Further north in the Great Plains, SMOS retrievals of top 5 cm soil moisture from descending orbits were consistently about 5% by volume wetter than ascending retrievals, due to radio frequency interference from air defense radars in Canada.
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In Situ Soil Moisture Networks in the Development and Validation of the Soil Moisture Active Passive (SMAP) Satellite
Tom Jackson and Michael Cosh USDA ARS Hydrology and Remote Sensing Lab Andreas Colliander Jet Propulsion Laboratory Pasadena, CA Tyson Oschner Oklahoma State University Stillwater, OK
The Soil Moisture Active Passive (SMAP) mission has developed a calibration and validation (Cal/Val) plan that describes the approach to be used in assessing the quality and accuracy of the SMAP data products. Two major objectives of Cal/Val are improving the retrieval algorithms and providing a final validated data set that meets the mission requirements. For soil moisture, the primary mission requirement is to provide estimates of soil moisture in the top 5 cm of soil with an error no greater than 0.04 m3/m3 at a 9 km spatial resolution and at intervals of 3 days. Ground-based or in situ observations are one of the resources that will be employed in validation. Establishing, calibrating, and maintaining a robust and globally distributed network of in situ instrumentation will be essential to the success of the SMAP Cal/Val program. During the pre-launch phase of the satellite project, two specific programs were initiated to establish the infrastructure necessary for validation using in situ observations; core validation sites (CVS) and an in situ sensor testbed. The CVS program was developed to provide high quality in situ observations of the specific data that SMAP requires for validation. It was based on an unfunded announcement of opportunity by NASA, which included design guidance. Beyond data quality, these agreements assure short latency delivery of the observations to the validation team. Over thirty groups distributed around the world responded to the opportunity. Unfortunately, unlike many standard meteorological measurements, in situ soil moisture measurements available from operational and research groups have no established standards. If SMAP is to fully exploit these in validation, a method for integrating them must be established. As a first step, SMAP established the Marena In Situ Sensor Testbed (MOISST) in Oklahoma. MOISST will be used to test and calibrate various soil moisture probes from different manufacturers as well as installation options. Details and initial results on these prelaunch activities will be presented. Additional participation opportunities in SMAP validation are available. USDA is an equal opportunity provider and employer.
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US Climate Reference Network Soil Moisture Measurement Approach Michael A. Palecki a and Jesse E. Bell b a NOAA's National Climatic Data Center, Asheville, NC b Cooperative Institute for Climate and Satellites, North Carolina State University, Raleigh, NC
During summer 2011, installation of Stevens Hydra Probe II instruments to observe soil moisture and temperature was completed at 114 US Climate Reference Network (USCRN) stations across the conterminous states. At each site, three sets of probes were placed in three independent plots about three meters from the instrument tower base. At most sites, the probes were installed at depths of 5, 10, 20, 50, and 100 cm; sites with shallow soils were installed at the 5 and 10 cm depths only. The continuity of the record is enhanced by triplicate observations at each depth, allowing valid observations to continue even if an instrument fails underground. The redundancy of measurements at each depth also creates a unique opportunity to understand the spatial representativeness of individual soil probe measurements, and the nature of local soil moisture/temperature variance. This will be especially important when validating remotely sensed or modeled soil moisture/temperature. Lessons learned with regards to system engineering, soil sampling, and network maintenance will be discussed, along with plans for field calibration of the soil moisture measurements over time. The nature of within-site variability to date will also be examined, providing some insight into the representativeness of individual in situ measurements of soil moisture within a small area, and how this varies from place-to-place.
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Quality characterisation of the in-situ soil moisture observations within the International Soil Moisture Network
GRUBER Alexander 1 , XAVER Angelika 1 , DORIGO Wouter 1 , DRUSCH Matthias 2
1 Institute of Photogrammetry and Remote Sensing, Vienna University of Technology, Vienna, Austria. 2 European Space Agency, ESTEC, Noordwijk, The Netherlands.
The International Soil Moisture Network (ISMN; http://www.ipf.tuwien.ac.at/insitu/ ) is a platform for collecting and redistributing in situ soil moisture measurements from existing soil moisture networks. Incoming data is harmonized in terms of units and sampling intervals as well as in the provided metadata before being stored in the database. Furthermore, a basic flagging scheme is used to describe the data quality without manipulating it, which will be extended in the near future. A web interface allows the user to easily query and download the data. Special care has been taken to make downloads compliant with international data and metadata standards. In a short time the ISMN has evolved into one of the most prominent platforms for exchanging in situ soil moisture data containing more than 700 stations from 24 networks. Nevertheless, the data implemented in the ISMN is very inhomogeneous in terms of the observation depths, reaching from a few centimetres up to several meters, used measurement techniques (e.g. TDR/FDR probes, capacitance probes), and sensor calibrations. This results in differences in the characteristics and quality of the measurements and complicates a straightforward global comparison of the in-situ measurements with satellite products or land surface models. In the presentation we will show novel methods to describe the characteristics of the different networks, stations and datasets and propose a quality control procedure for enhanced flagging of the individual measurements. The suggested quality indicators aim to provide a valuable support for selecting and weighting the individual datasets and measurements in the validation process.
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A homogeneous equivalent model for the soil dielectric permittivity spectrum
Paolo Castiglione Soil Sensors Laboratory 402 N Grand Avenue, Bozeman, MT
We present a novel mixing model for the soil dielectric permittivity based on the Differential Effective Medium (DEM) approximation first introduced by Bruggeman. The soil is represented as a dilute suspension of solid particles in an air background. The particles are uniform, randomly oriented ellipsoids. The solution of the DEM approximation for ellipsoidal inclusions represents a novel contribution of the author. The water phase is introduced as uniform layers coating each solid particle. For a given texture, the thickness of the water shells is determined by the water content of the equivalent medium. An interesting characteristic of the proposed representation is the ability to model the separate contribution of free and bound water. More specifically, the dielectric properties of the each molecular layer of water vary with the distance from the particle surface. Water at a distance greater than 10 molecular layers exhibits free-water properties, while water in proximity of the solid surface is bound, and therefore exhibits lower static permittivity values as well as relaxation times. The parameters for the permittivity of the bound water derive from independent studies, and are obtained through measurements based on magnetic resonance techniques. Notwithstanding the limitations of the DEM approach, the insights offered by the proposed model are remarkable. For example, the interfacial (Maxwell-Wagner) polarization is clearly identified, and the effects of texture and salinity on low frequency relaxation phenomena can be easily investigated. Moreover, the temperature effects observed with TDR and capacitance techniques can be predicted and accounted for. While most of our findings are only qualitative, the proposed model accurately reproduces the relationship between TDR-measured dielectric permittivity and water content experimentally found by Clark Topp in 1980 for coarse soils, and sheds new light on the effects of texture on TDR and capacitance measurements.
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Can a Dielectric Soil Moisture Sensor Really Measure Volumetric Water Content to Within 2% VWC in All Soils? Douglas R. Cobos 1,2 , Colin S. Campbell 1,2 , Lauren L. Bissey 1 1 Decagon Devices, Inc., Pullman, WA, USA 2 Dept. of Crop and Soil Sciences, Washington State University, Pullman, WA, USA
Dielectric soil moisture sensors use various techniques to measure the dielectric permittivity of the soil, which is then converted to volumetric water content (VWC) through the use of a theoretical or, most often, empirical transfer function. Even if the sensor measures permittivity with perfect accuracy, error will be introduced into the VWC measurement by the transfer function. We make use of a generalized dielectric mixing model to examine the magnitude of errors in estimated VWC that can be expected from using a single transfer function across a wide range of soils. The results of the analysis make a strong argument for developing soil-specific transfer functions when accuracy of better than about ±4% VWC is desired.
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Assessing Soil Moisture Sensor Accuracy and Repeatability Using Dielectric Standards
Colin S. Campbell 1,2,* , Douglas R. Cobos 1,2 , and Gaylon S. Campbell 1
1 Decagon Devices, Inc., Pullman, WA, USA 2 Dept. of Crop and Soil Sciences, Washington State University, Pullman, WA, USA
One of the first questions asked about any soil moisture sensor relates to its accuracy. Although most manufactures' websites openly state a water content accuracy for each sensor, several studies have shown that volumetric water content (VWC) accuracy (or the accuracy of VWC estimate) is dependent on many other soil properties like bulk density, mineralogy, organic matter content, etc., making the accuracy statement impossible to verify or validate. For the subset of sensors that use an electromagnetic field to measure dielectric permittivity, several papers have demonstrated that sensor accuracy (defined as the accuracy of the dielectric) can be established in a range of standard solutions with known dielectric properties. However, these papers differ on what solutions to use for the analysis and at what concentrations. We propose a range of 8 dielectric standards with particular emphasis on the dielectric range of mineral soil that verify and validate the performance of water content sensors and establish accuracy. In addition, the standards allow the benchmarking of sensors over time to ensure that each new sensor that is produced does not differ from the first. Because of the serious health risks associated with the regular use of these dielectric standards, we also establish a range of 5 secondary standards that can quickly verify sensor performance with slightly less accuracy but without special equipment for handling the solutions. The use of these primary and secondary standards provides a way to establish dielectric accuracies for all EM sensors in a reproducible and repeatable manner.
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In-Situ Measurement of Soil Complex Dielectric Permittivity from 10 MHz to 1 GHz in the Shallow Subsurface
Ryan E. North , US Army Engineer Research and Development Center Chi-Chih Chen , The Ohio State University, Electroscience Laboratory Jason R. McKenna , US Army Engineer Research and Development Center
Knowledge of the depth-dependent, complex dielectric permittivity from 10 MHz to 1 GHz is important in understanding the effectiveness of ground penetrating radar (GPR) as well as a general knowledge of the moisture distribution in the shallow subsurface. GPR is a highly site dependent geophysical method applicable to a range of problems with a primary limitation being a function of moisture content and therefore dielectric permittivity. In this presentation we will demonstrate the use of an off-the-shelf portable vector network analyzer and software combination along with a custom soil probe used to measure the in-situ complex dielectric permittivity of the shallow subsurface. In-situ measurements with this new probe produced excellent agreement with commercial time-domain reflectometry and frequency-domain reflectometry measurements as well as laboratory measurements of the soil moisture content and complex dielectric permittivity over the same frequency range. In-situ measurements were performed at several locations in the continental United States as part of a program to select soil proxy sites for overseas locations where the potential applicability of ground penetrating radar as a detection method was unknown. This method has great potential as we add more site data to our geostatistical models of geologic regions based on extensive field and laboratory measurements. Approved for public release; distribution is unlimited. Permission to publish was granted by Director, Geotechnical & Structures Laboratory
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Models for vector network analyzer to determine the dielectric permittivity of sand Xiufu Shuai Department of Geography, University of Hawaii at Manoa
Vector network analyzer (VNA) has been used to determine the spectra of soil dielectric permittivity. The relevant models to describe the propagation of electromagnetic wave along a transmission line include the reflection scattering model ( S 11) and the transmission scattering model ( S 21). The objectives of this study were: (1) to develop the S 21 model; and (2) to use the S 11 and S 21models to describe the reflection and transmission of electromagnetic wave along a multi-section transmission line. Two experiments were carried out to test the S 21 model. The first experiment was to fill the Hewlett Packard 805C waveguide with one section of sand for the measurement of the complex dielectric permittivity of sand. The second experiment was to fill the HP50C waveguide with two separated sections of sand to form a five-section transmission line. Results showed that the measured complex permittivity of sand followed Debye equation. The measured magnitudes of S11 and S21 for both the 3-section and 5-section transmission lines were constituted of multiple periodic peaks and valleys. The developed S 21 model successfully described the 3-section transmission line in the frequency up to 2.7 GHz. Both the S 11 and S 21 models successfully described the 5-section transmission line in the frequency range of 30 MHz–1.5 GHz.
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Waveguide-Integrated Generation and Time Domain Analysis of EM Waveforms Result in Accurate, Low Cost Volumetric Water Content Measurement Scott Anderson Acclima Inc., 2260 East Commercial Street, Meridian, Idaho
Since the 1980's much research has been done to correlate soil moisture with the bulk permittivity of the soil. Sufficiently accurate models exist today that can be relied on to convert accurate permittivity measurements into volumetric water content data for all soil types. The accurate measurement of soil permittivity using low cost instrumentation has been a substantial barrier to the overall objective of measuring soil moisture. Propagation of EM signals through the soil and careful time domain signal analysis on the propagated waveform provide an accurate measurement of bulk permittivity that is largely independent of soil electrical conductivity, a factor that has plagued other methods of permittivity determination. Time Domain Reflectometer systems have been used successfully to make high-confidence soil permittivity readings under normally-encountered EC conditions. The disadvantages of TDR systems are cost and inconvenience. A new approach to TDR is discussed wherein the step function generator and waveform digitizer are mounted directly on the incident end of the waveguide, thus eliminating the bandwidth-limiting coax cable and the separately-packaged TDR set. This reduces cost by more than an order of magnitude. It also preserves the high frequency waveform detail by eliminating the bandwidth-constraining coax cable. Step rise times as low as 35ps are possible. The time resolution of the digitized waveform can be 5ps or less. By placing the digitizer at the distal end of the waveguide a Time Domain Transmissometer is created that has the accuracy and EC independence advantages of a TDR but can handle higher EC loading because of the elimination of the reflection loss. The TDR version measures EC by reading the steady state waveform amplitude. The TDT version measures EC by determining the propagated waveform attenuation. Both types of sensors can be installed permanently using low-cost conventional wired communications or wireless techniques and are sufficiently efficient for battery operation.
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