Comparing the reliability of two soil moisture probes for high clay content NSW soils

The accurate measurement of soil moisture can be a time-consuming task. Soil moisture is highly variable, and it can be difficult to capture spatially and temporally. While remote sensing has become a popular and ever-improving tool, on-ground measurement is required for both calibration, validation, and evaluation of any emerging technology. Many applications commonly require or use a relative value of soil moisture to assess the temporal persistency of moisture regimes across catchments or agricultural landscapes. This paper compares on-ground indirect soil moisture measurements from two commonly available manufacturer-calibrated in-situ measurement devices against gravimetric soil moisture data. The devices used are a Delta T Theta Probe and a Campbell Scientific CS659 while the gravimetric soil moisture data are from soil cores collected in 2014, 2015 and 2018 (308 samples) with a range of soil moisture states from wilting point to field capacity. The gravimetric and probe measurements returned R values of approximately 0.8 for 2014 and 2015. A decrease in correlation (to approximately 0.3 and 0.5) was observed for the 2018 data. The manufacturer-calibrated probe measurements did not provide a 1:1 relationship with the gravimetric soil moisture data and require calibration for bias correction. However, results show that either the Theta Probe or CS659 are reliable and comparable to the gravimetric results in most conditions. Best results will be obtained by using appropriate techniques and knowing the limitations of devices. The calibration equations developed in this study are useful for others for both understanding and measurement of soil moisture.