In this example, adjusted GTD is equal to 53.4
Now, if we multiply the adjusted GTD by the number of trees (e.g., 1,000) it will give us the total daily system demand in gallons (53,400 gal/day). If we divide the 53,400 by the flow rate (i.e. 95 gpm), we get the required daily system operation time of 562 minutes or 9.4 hours.
SOIL MOISTURE MONITORING
The measurement of soil moisture is an important aspect of irrigation scheduling. This measurement verifies that the proper amount of water is being applied. Here are a few common methods used for soil moisture measurement.
Soil feel test . A shovel, soil auger, or push tube can be used to obtain soil samples. Samples should be taken from a depth of 8 to 9 inches to the bottom of the effective rootzone. Squeeze the soil between the thumb and index finger to form a ribbon. Soil type and the grower's experience will indicate the relative amount of moisture in the soil.
Tensiometers . Tensiometers give a direct reading of the soil moisture tension existing in the rootzone. These instruments should be placed in strategic locations within the plant root- and wetted zone. Depending on the plant's maximum rooting depth and soil type, more than one tensiometer may be required per site. For example, one might be placed at a depth of 12 inches and another at a depth of 30 inches. These should be located just at the inner edge of the normal wetted area.
Tensiometers require frequent supervision and must be correctly installed and serviced in order to obtain reliable results. Some growers feel that graphing daily readings is a good way to track moisture status. Generally, for optimum results the tensiometers should read in the 10 to 30 centibar range.
Electrical resistance blocks . These instruments measure available soil moisture percentage. A calibration curve is required to determine the soil moisture percentage based on soil type and ohm resistance. Electrical resistance blocks should be installed at locations similar to those discussed for tensiometers.
In a field watered via micro-irrigation, the main objective of soil moisture monitoring is not to determine the moisture content of the soil. This can become too difficult because of the spatial variability of moisture content. Rather, use of soil moisture monitoring equipment is primarily to evaluate the size of the wetted area and secondarily to determine the moisture content within the wetted area.
Regardless of the device chosen to monitor the wetted zone-soil feel test, tensiometers, electrical resistance blocks, or others-the principle is the same. If the wetted area is expanding, the system is applying more water than the crop is using. If the wetted area is contracting, the crop is using more water than the system is applying. By monitoring the wetted area frequently, adjustments can be made and the system operation fine tuned.
Finally, the first irrigation should be run early enough in the season to allow for repairs and system maintenance. Each spring, the system should be thoroughly flushed out, including mainlines, laterals, drip lines, and filter stations. In the first irrigation, the system should be operated long enough to fill the soil profile. Subsequent irrigations should be scheduled to maintain this wetted profile throughout the growing season.
The scheduling of irrigations should be updated regularly. Many micro-irrigation systems are operated daily. During periods of sufficient rainfall, the plants may not require an irrigation. However, if salinity is a problem, it may be critical to run the system during rainy periods to prevent salts which accumulate at the edge of the normal wetted zone from being pushed back into the rootzone. We suggest that you contact your local farm advisor or irrigation dealer on specific problems in your area.