Field calibration of an impedance soil water probe for the shallow seedbed across field topography

Impedance soil water probes enable frequent and non‐destructive determination of soil water status in situations where gravimetric soil sampling is too demanding of time and sampling space. The ThetaProbe is an impedance soil water probe requiring calibration for local soil conditions, because measurement accuracy can be affected by properties of the soil. Often, only a single calibration is performed for an experimental site. An experiment investigating the seedbed to 75‐mm depth across a field topography with variable soil properties was examined to determine which soil properties affected the calibration of the ThetaProbe, and if soil‐specific calibration was required to derive suitable estimates of the water status in the experiment. Experimental factors examined included hillslope aspect, hillslope position, crop residue and soil depth. Soil properties, other than volumetric water content, significantly affecting the probe measurements were bulk density, electrical conductivity and temperature. The probe underestimated soil water at very low water contents, and overestimated soil water at contents greater than 11 m³ m^?3, compared with gravimetric measurements. A single calibration, not corrected for hillslope position at a water content of 20 m³ m^?3, overestimated water content by 0.02 m³ m^?3 in the summit hillslope position and underestimated water content by 0.04 m³ m^?3 in the toeslope position. A single calibration, not corrected for soil depth at a water content of 20 m³ m^?3, overestimated water content by 0.02 m³ m^?3 in the 0‐ to 25‐mm soil layer and underestimated water content by 0.03 m³ m^?3 in the 50‐ to 75‐mm layer. The complexity of microsites in a shallow seedbed requires soil‐specific calibration in field experiments containing heterogeneous soil properties.