Soil moisture monitoring for climate research: Evaluation of a low-cost sensor in the framework of the Swiss Soil Moisture Experiment (SwissSMEX) campaign

Abstract

[1] Soil moisture measurements are essential to understand land surface–atmosphere interactions. In this paper we evaluate the performance of the low-cost 10HS capacitance sensor (Decagon Devices, United States) using laboratory and field measurements. Measurements with 10HS sensors of volumetric water content (VWC, Vol.%), integrated absolute soil moisture (millimeters) over the measured soil column, and the loss of soil moisture (millimeters) for rainless days are compared with corresponding measurements from gravimetric samples and time domain reflectometry (TDR) sensors. The field measurements were performed at two sites with different soil texture in Switzerland, and they cover more than a year of parallel measurements in several depths down to 120 cm. For low VWC, both sensor types present good agreement for laboratory and field measurements. Nevertheless, the measurement accuracy of the 10HS sensor reading (millivolts) considerably decreases with increasing VWC: the 10HS sensors tend to become insensitive to variations of VWC above 40 Vol.%. The field measurements reveal a soil type dependency of the 10HS sensor performance, and thus limited applicability of laboratory calibrations. However, with site-specific exponential calibration functions derived from parallel 10HS and TDR measurements, the error of the 10HS compared to the TDR measurements can be decreased for soil moisture contents up to 30 Vol.%, and the day-to-day variability of soil moisture is captured. We conclude that the 10HS sensor is appropriate for setting up dense soil moisture networks when focusing on medium to low VWC and using an established site-specific calibration function. This measurement range is appropriate for several applications in climate research, but the identified performance limitations should be considered in investigations focusing on humid conditions and absolute soil moisture.