Perfect Irrigation Scheduling System Based on Soil Electrical Resistivity

Abstract

Iran has an arid to semiarid climate with precipitation occurring in the rainy seasons. Producers often use tensiometers for irrigation scheduling because they provide direct measurement of soil moisture status and are easily managed. Electrical resistivity is an effective predictor of various soil hydraulic properties, including water content, degree of saturation, and hydraulic conductivity. An irrigation scheduling system was developed based on soil electrical resistivity. The system consists of a sensor to monitor soil electrical resistivity. Sensors were made from two galvanized sheets buried at 5, 10, 15, and 20 cm in soil. The sensors were in series with a multimeter (20 kohm). The equipment was controlled by a custom-written computer program that requires setting a high tension set point (when irrigation is turned on) and a low tension set point. Sensors at the different depths provided an understanding of how electrical resistivity changes in response to irrigation. Immediately following irrigation the electrical valve automatically closed when the wetting front in the soil passed the sensor, resulting in a value near 100 (maximum resistivity that can be recorded with the system), indicating that the soil was saturated. When the sensor record 100 the common tensiometer reading was 0, indicating a negative correlation between the systems. Moisture changes more rapidly near the soil surface than at lower depths. Irrigation of vegetables can be controlled by measuring soil resistivity and calibrating the system for each crop.