Performance Assessment of Soil Moisture Sensors for irrigation scheduling in a Sandy Clay Loam Soil under a Tropical Setting

The deployment of soil moisture sensors in crop cultivation offers a promising avenue for achieving precise water application to optimize irrigation water use. However, the limited understanding of soil moisture sensor performance has impeded their widespread integration into irrigated agriculture. This study evaluated the field performance of different soil moisture sensors, one measuring soil matric potential (i.e., Watermark sensor) and the other which is an electromagnetic-based sensor (i.e., ML3 Thetaprobe sensor) on a sandy clay loam Soil under irrigated and rainfed conditions. The sensors were installed at soil depths of 20, 40, and 60 cm within the root zones of thriving eggplants. Over the entire growth cycle of three months, daily soil moisture measurements were recorded from the two sensor types for both irrigated and rainfed conditions. Before deployment, the sensors were calibrated against the standard gravimetric method. The performance of the sensors was evaluated using metrics such as Root Mean Square Error (RMSE), Mean Bias Error (MBE), and the Index of Agreement as well as observing their daily trends and response to irrigation events. The findings showed that the ML3 Thetaprobe sensor exhibits greater sensitivity to soil moisture fluctuations than the Watermark sensor under irrigated and rainfed conditions. In addition, the Watermark sensor exhibited consistently lower soil moisture readings at 20, 40, and 60 cm soil depths in comparison to the ML3 Theta Probe under irrigated and rainfed conditions. In conclusion, there is need for careful consideration when choosing sensors for soil moisture measurement for accurate measurement. While the Watermark sensor may provide valuable information under certain conditions, its sensitivity to soil saturation and its slow response time must be taken into account. Depending on the specific application and soil conditions, the choice of sensor should be made with a deep understanding of its strengths and limitations.