ESTIMATING SOIL WATER CONTENT USING SITE-SPECIFIC CALIBRATION OF CAPACITANCE MEASUREMENTS FROM SENTEK ENVIROSCAN SYSTEMS

Newly developed sensors, including the EnviroSCAN capacitance system have the potential to monitor and estimate soil moisture content continuously at various depths. A simple site calibration of the sensors is required to obtain accurate soil water content because these sensors are shipped with a default (uncalibrated) equation to the user. Therefore, our research objectives were: 1) to calibrate the EnviroSCAN capacitance system versus soil moisture values estimated by a neutron probe calibrated with gravimetrically measured water contents, then 2) statistically compare the calibrated soil water content results with those estimated by the uncalibrated equation using three years of field data. Both the EnviroSCAN capacitance and the neutron probe were installed in a Warden silt loam soil planted to alfalfa. The average water contents of the soil profile estimated by the EnviroSCAN capacitance was used to develop a site specific calibration equation by comparing the sensors’ scaled frequencies for 1998 with the soil water content of the neutron probe. The site calibration equation was then statistically validated using both 1999 and 2000 soil water contents. The statistical analyses indicated that discrepancies existed between soil water contents of the site calibration equation and those estimated by the uncalibrated equation. For instance, the RRMSE values of soil water content produced from the calibrated equation were 7%, 41%, and 40%, compared with uncalibrated RRMSE values of 68%, 59%, and 66%, for 1998, 1999, and 2000, respectively. These results support that the site calibration equation was found to give more accurate estimates of individual values (low RRMSE) of volumetric soil water content compared to those obtained from the uncalibrated equation.