Remote and retroactive soil moisture probe calibration

Calibrating soil moisture probes to site-specific conditions requires multiple lab measurements of soil moisture through time, that in most situations is financially and logistically unfeasible across a probe network. In this study we test a pedotransfer function (PTF) based calibration method, introduced by others, referred to in this study as PTF scaling. The method is based on scaling soil moisture measurements to the bounds of PTF-estimated crop lower limit (CLL) and drained upper limit (DUL). We introduce three experiments to test the viability of PTF scaling for calibrating soil moisture probes. In Experiment A we PTF scale and conventionally calibrate soil moisture measurements from six sites across Muttama, NSW, Australia, and compare them to lab measured soil moisture. We find PTF scaling to be more accurate than the conventional calibration at 4 out of 6 sites. In Experiment B, we compared PTF scaled soil moisture measurements to conventionally calibrated measurements and found that many sites could be PTF scaled to an accuracy under 5 %VWC, depending on PTF choice and land use. Lastly, Experiment C was conducted to understand the sensitivity of this method to changes in long-term minimum and maximum soil moisture. At most sites the PTF scaling typically stabilised after 2-3 years, given the site had experienced an above average rainfall event. We find PTF scaling to be an appropriate substitution for conventional calibration methods under optimised PTF choice, scaling percentile, and minimum probe record length.