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After dry periods, we on the island of Oahu are often asked to conserve water. While water use restrictions are not always mandatory, they sometimes are. We are all aware of the drought years in the early 1980s when water use for turfgrass areas was cut by 50 percent or more.
Undoubtedly, as development on Oahu increases, water use for turf and landscape plantings will become more and more restricted. We must become more efficient in our use of water if we are to maintain attractive landscapes.
METHODS. There are several methods of determining when and how much to water plants. Innovations in irrigation systems include sophisticated environmental monitoring equipment, tensiometer-controlled irrigation scheduling, and other techniq
ues that make irrigation decisions easier.
Even with these sophisticated systems, there usually has to be some input from the manager on water scheduling and amount to water.
WATER BUDGET SYSTEM. One of the easiest methods is the water budget system, or simply replacing the water used by plants. As in a household budget, the important things to know about a water budget are the inputs and outputs.
Quite simply, water inputs are irrigation and/or rainfall. Outputs are evaporation, runoff, leaching out of the root zone, and transpiration by plants.
We try to irrigate so that runoff and leaching losses are minimized and evaporation and transpiration are the main water losses. These are usually called "evapotranspiration." Actually, once turf fully covers the soil, transpiration is the only significant water loss.
If we consider the plant root zone to be a reservoir that stores water and makes it available to plants, then we need to know something about the:
Table 1. Approximate maximum rooting depth of some warm-season turgrasses.
Species Maximum rooting depth
(in.)
Common bermudagrass 48-60
Hybrid bermudagrasses 60-98
Zoysiagrasses 24-48
St. Augustinegrass 32-48
Carpetgrass 12-18
(From Handrek and Black, 1984)
The best way to determine the rooting depth is to dig a hole and visually inspect the depth of the root system. It is usually not practical to irrigate to a root depth greater than about 18 inches. In most soils, water can move upward by capillary ac
tion. If you overestimate the depth of the root system slightly, the water will not likely be wasted.
The rate at which water enters the soil ("water infiltration rate") differs greatly depending mainly on soil texture. Water infiltration rates will be further reduced by compaction (especially, on clay soils), thatch, soil layers, or steep slopes.
Table 2 shows approximate water infiltration rates for different types of soil.
Table 2. Approximate amount of available water retained and water
infiltration rate for various Hawaiian soils.
Soil Available water Water infiltration rate
(in./foot of soil) (inches per hour)
Sands 0.6-1.0 6.3-20.0
Sandy loams 1.0-1.7 2.0-6.3
Loams 2.0-6.3 1.7-1.9
Silt loams 2.0-6.3 1.8-2.5
Clays 0.06-0.63 1.7-2.0
(From USDA Soil Survey, 1972.)
Water infiltration rate is important in determining how fast irrigation water can be applied or how effective rainfall is. If sprinklers are applying water at 0.5 inch per hour and the water infiltration rate is 0.25 inch per hour, then half the wate
r applied will run off.Equally significant, if rainfall occurs at a rate exceeding the water infiltration rate of the soil, then the effective rainfall is equal only to the water infiltration rate of the soil.
Soils store different amounts of water, depending largely on soil texture. Clays hold relatively large amounts of water, and sands hold less. Most of the water in sands, however, is available to plants, while much of the water in clay soils is held s o tightly that plants cannot absorb it.
Table 2 gives approximate amounts of water held in different types of soil.
The available water storage capacity of the soil and the rooting depth of the plants growing in it together determine the amount of water the plant can use before the reservoir has to be replenished.
The rate at which water is used by plants depends upon several factors. Water use rate differs for different plants. The amount of:
On sunny days in Hawaii, turfgrasses with unlimited available water will use between 0.1 and 0.4 inch of water per day. As the water content of soils drops, plants use water at a much slower rate.
Warm season turfgrasses are especially well adapted at efficient use of water. It is seldom desirable to maintain turfgrasses in a lush state of growth.
Water use rates can be estimated from evaporation rates. If you have a Class A weather pan on or near the area to be watered, water use rate of warm season turfgrasses is approximately 50 percent of Class A pan evaporation.
An approximation of Class A pan evaporation can be obtained by using a 1-gallon (3-pound) coffee can with the outside covered in aluminum foil and a coarse screen over the top to keep birds and other animals out. Place the evaporator in the sunlight. Fill to about 2/3 full and measure the amount of water evaporated daily in inches.
Turfgrasses in Hawaii will require about 50 percent of the amount evaporated daily. Rainfall (unless it is more than the evaporator can hold) will be automatically compensated for.
The following example illustrates the use of a water budget method of determining how often and how much water to apply. Suppose you have a hybrid bermudagrass lawn on a loam soil. You have found that the turf has an effective rooting depth of 18 inc hes. You have constructed a homemade evaporator.
How much water should you allow to evaporate before watering the lawn, and how much water will be required to recharge the soil reservoir? Assume that you will water when 50 percent of the available water has been used.
A. Calculate the size of the reservoir.
Loam soils hold about 1.3 to 2.0 inches of available water per foot of depth. We will use the intermediate figure of 1.75 inches.
1.75 in. x 1.5 ft. root zone to be irrigated = 2.6 in. of available water.
B. Determine how much water can be used from the reservoir before
irrigating. (This is also the amount of water you will try to replace.)
Irrigate when approximately 50 percent of the available water has been used.
2.6 in. x 0.5 = 1.3 in.
Irrigate when 1.3 in. of water has been used.
C. Calculate the amount of water that can be lost from the
evaporation can before irrigating.
Water use rate of warm season grasses is approximately 50 percent of pan evaporation.
1.3 in./0.5 = 2.6 in.
2.6 in. of water can be evaporated before irrigating.
D. Irrigation systems never apply water in a perfectly uniform
manner.
Some water also evaporates before it reaches the soil, especially if water is being applied on hot, windy days.
Assume that efficiency of irrigation is approximately 80 percent.
1.3 in./0.8 = 1.6 in.
Therefore, apply approximately 1.6 in. of water when 2.6 in. has been lost from the evaporator.
While these figures may be rough approximations, they should be accurate enough to serve as a basis for sound watering practices.
Watering deeply and infrequently will encourage deep rooting of all plants. As shown in Figure 1, frequent watering, which prevents even moderate moisture stress from developing, results in much higher rates of water use than infrequent watering in w hich moderate soil moisture tensions are allowed to develop.
Handreck, K. A. and N. D. Black. 1984. Growing media for ornamental plants and turf. New South Wales University Press. P. O. Box 1, Kensington, NSW Australia, 2033.
USDA. 1972. Soil survey of the islands of Kauai, Oahu, Maui, Molokai, and Lanai, State of Hawaii. U. S. Department of Agriculture Soil Conservation Service, in cooperation with the University of Hawai. 232 pp.