Operational Characteristics of the Watermark Model 200 Soil Water Potential Sensor for Irrigation Management

Abstract

The static and dynamic response characteristics of the Watermark model 200 soil water potential sensor were evaluated using the pressure plate method and greenhouse experiments. The sensor had a nearly linear resistance versus water potential relationship within the 0 to –200 kPa (0 to –29 psi) range. At saturation, sensor resistance was approximately 1 kW, and at –103 kPa (–15 psi) resistance was approximately 23 kW, with a coefficient of variation of 11% (49 sensors). Using three to six sensors at each location and depth should yield estimates of soil water potential within 10% of the actual value with a 90% confidence level. The dynamic response was good during typical soil water drying cycles following complete rewetting, but the sensors did not respond fully to rapid drying or partial rewetting of the soil. Response was improved, without affecting the basic calibration, when a finer textured material with greater unsaturated hydraulic conductivity was used in the transmission portion of the sensor.