The water requirement of a crop is often measured in terms of "crop coefficient," which describes the crop's water demand compared to the amount of evaporation from an open pan of water placed in the orchard. Research in Hawaii has shown that young, nonbearing coffee trees require 60 percent of the amount of water normally lost to evaporation from an open pan, whereas bearing trees over two years old demand 75-80% of pan evaporation. Overirrigating and leaks in the drip tube can lead to waterlogged conditions resulting in poor root development, root injury, poor yields, and-if uncorrected-death of the plants.
Poor Drainage
Figure 28. Trees on the right, have poor growth, laterals died, due to
poor drainage in a high rainfall area, probably rock close to the surface
resulting in standing water on the surface or just beneath.
Symptoms of extreme overirrigation may include some wilting; frequently,
leaves may have dead margins; later, laterals and the top of the vertical
die, with the dead leaves remaining on the tree. The symptoms may appear
to be damage from black twig borer, but this cause can be eliminated if no
entry holes are found. It is easier to overirrigate on heavy clay soils,
which hold more water and drain slowly. However, overirrigation can occur
on a'a lava soils if there is a solid rock "pan" below the surface that
allows water to stand. The presence of standing water under a tree half an
hour after irrigation is shut off is a sure sign of overirrigation and a
signal to take steps to correct the problem.
Applying irrigation based on calculated water demand is both cost-effective and important to crop health. Devices for monitoring crop water needs include soil tensiometers, open evaporation pans, and evaporation blocks; irrigation consultants usually know what works best in a given situation. Coffee is irrigated when the tensiometer reads 50 centibars, and water is generally applied for 1-48 hours to ensure deep wetting.
When using an evaporation pan, one needs to take rainfall into account. A simple though approximate system includes a ruler that measures 1/16- or 1/32-inch increments and a 12-inch deep by 24-inch diameter pan half filled with water and covered with wide-mesh screen to prevent animals from bathing in or drinking the water. The water level is checked once a week; only if there is less water than there was a week ago is irrigation needed. To determine the amount of water to apply, you need to know how much water your system can deliver, the age of the trees, and the percentage of land covered by trees.
For example, assume your orchard is planted 10 x 5 ft, your trees are bearing age (expect a 75 percent crop coefficient or water use compared to pan evaporation), and the lateral branches are touching in the row and have grown into the alley, which is now 4 ft wide; therefore, coverage is 60 percent (5 x 6 ft = 30 ft2; 30 ft2 / 50 ft2 spacing = 60 percent). In the past week the water level dropped 1/2 inch. The amount of water needed is determined as [1/2 inch (due to evaporation) x 0.6 (60% coverage) x 0.75 (75% crop coefficient) = 0.22 acre inches], or about 1/4 inch over each acre of orchard. An acre inch is 27,154 gallons. Given that there are 870 trees per acre and 0.22 acre inch of water is applied, each tree receives about 7 gallons of water to replace the water used in the previous week. This calculation assumes 100 percent efficiency of application. No irrigation system is 100 percent efficient, but each system has an expected efficiency. If the efficiency of the system is 80%, then the amount to be applied in this example is 0.275 acre inches (0.22 / 0.8), or 8.75 gallons per tree (7 / 0.8 = 8.75).
Growers who irrigate may also wish to fertigate (fertilize through the drip irrigation system). Only fertilizers that completely dissolve in water can be used, and these formulations are often more expensive. If you are using the municipal water system, backflow-prevention valves are required to prevent fertilizer contamination of the water. An example of a fertilizer schedule for bearing coffee, proposed by Osgood and Chang (1994), is given on p. 15.
