Comparison of four soil moisture sensor types under field conditions in Switzerland

Many environmental and hydrological applications require knowledge about soil moisture. Its measurement accuracy is known to depend on the sensor technique, which is sensitive to soil characteristics such as texture, temperature, bulk density and salinity. However, the calibration functions provided by instrument manufacturers are generally developed under laboratory conditions, and their accuracy for field applications is rarely investigated, in particular over long time periods and in comparison with other sensors types. In this paper, four side-by-side profile soil moisture measurements down to 110 cm using three low-cost sensors and one high-accuracy and high-cost time domain reflectometry (TDR) sensor are compared over a 2-year period at a clay loam site in Switzerland. The low-cost instruments include the (1) 10HS (Decagon Devices, United States), (2) CS616 (Campbell Scientific, United States), and (3) SISOMOP (SMG University of Karlsruhe, Germany) sensors, which are evaluated against the (4) TDR-based TRIME-IT/-EZ (IMKO GmbH, Germany) sensors. For the comparison, the calibration functions provided by the manufacturers are applied for each sensor type. The sensors are evaluated based on daily data regarding their representation of the volumetric water content (VWC) and its anomalies, as well as the respective temperature dependency of the measurements. Furthermore, for each sensor type the actual evapotranspiration is estimated using the soil water balance approach and compared with measurements from a weighing lysimeter. It is shown that the root mean square difference (RMSD) of VWC for the low-cost sensors compared to the TDR measurements are up to 0.3 m3/m3, with highest values in near-surface layers. However, the RMSD for the VWC anomalies are lower compared to those for absolute values. We conclude that under the studied conditions none of the evaluated low-cost sensors has a level of performance consistent with the respective manufacturer specifications. Hence the derivation of site-specific calibration functions is vital for the interpretation of measurements with low-cost soil moisture sensors. Furthermore, some weaknesses of the tested low-cost sensors such as the lack of sensitivity in certain soil moisture regimes or spurious dependency on soil temperature, imply intrinsic issues with the measurements derived with this type of instruments. This is particularly critical for a number of environmental and hydrological applications, including the assessment of remote sensing measurements.