Equations to compensate for the temperature effect on readings from dielectric Decagon MPS-2 and MPS-6 water potential sensors in soils

Dielectric sensors use electrical permittivity as a proxy for water content. They determine permittivity by using sensor-type-specific techniques and calibration functions, and relate it to a volumetric water content. Water potential sensors then translate the water content into a potential based on the sensor-specific moisture characteristic curve. Dielectric readings are affected by temperature, which may distort hydraulically-induced changes in soil water content. Methods for the removal of spurious temperature effects are lacking for dielectric water potential sensors. With this study, we aimed to fill this knowledge gap for the dielectric Decagon MPS-2 and MPS-6 water potential sensors. We first determined the temperature effect on MPS readings with laboratory experiments in which temperature was cycled between 4 and 26 degrees C in different soil types. We then fitted single empirical equations that compensated for the temperature effect on MPS readings. Finally, we validated temperature-compensated MPS soil water potentials, and therefore the equations, in a multi-year field study in two forest soils where hourly data from three sensor models were available, i.e., from MPS-2, MPS-6, and a heat capacity sensor (ecoTech pF-Meter) that is not sensitive to temperature effects. The temperature fluctuation experiments showed that the strongest temperature effects on MPS readings occur under dry conditions and that the MPS sensors themselves are largely responsible for these effects. Likewise, the field-based validation showed that the MPS readings were highly affected by temperature under dry conditions. Applying a temperature compensation to these readings, using the equations from the temperature fluctuation experiments, resulted in strong correlations near the 1:1 line between data from the MPS and pF-Meter sensors. Therefore, we recommend using the equations to remove spurious temperature effects from MPS-2 and MPS-6 readings in non-saline soils with water potentials between -100 and -2000 kPa (at 22 degrees C) and temperatures between 4 and 26 degrees C.