Irrigation Decisions Simplified with Electronics and Soil Water Sensors'

Two simple, inexpensive systems use electrical resistance mea-surements to provide useful, immediate information to assist deci-sions made on irrigation water application. in one system a micro-processor-based circuit coupled to a progranunable calculator provides an on-site estimate of the time until the next irrigation will be re-quired, based on field data and an operator-supplied parameter. The second system simply signals the arrival of the wetting front at any location in the soil by giving a visual indication, such as raising a mechanical flag. The microprocessor-based circuit measures and stores the resistance of four gypsum blocks once a day. The program in the portable calculator accesses this information and uses it to extrapolate the soil drying rate to predict the number of days until the next irrigation. By restricting the microprocessor circuit to data acquisition only and putting all number-handling routines into the calculator program, the cost and complexity of the microprocessor circuit is minimized, whereas maximizing the programming flexi-bility. This makes it feasible to install a number of these devices at different locations, all serviced by the same portable calculator. The water infiltration circuit intermittently scans eight sets of stainless steel electrodes to locate the soil wetting front during ir-rigation. When the resistance across the electrodes decreases, sig-'sling the arrival of the front, the circuit trips a spring-loaded Bag. This provides a visible sign that the wetting front has reached that point in the soil. The equipment worked well. When irrigation was required in six or fewer days, the microprocessor/calculator system made correct predictions 85 % of the time. An example of how easily any irrigation scheduling method may he converted to the micro-processor/calculator system is presented. Additional Index Words: water management, irrigation automa-tion