PERFORMANCE OF A NONNUCLEAR RESONANT-FREQUENCY CAPACITANCE PROBE - III - ERROR ANALYSIS

All techniques used to measure soil water content have errors associat ed with them. These errors are further propagated when soil volumetric water contents are numerically integrated over a number of depth inte rvals to estimate the soil water storage for the entire profile. Previ ous studies had field-tested the performance of a commercial capacitan ce probe (Troxler Sentry 200-AP). The probe was calibrated and the mea sured data on crop water use were compared with that reported previous ly in literature using other instruments. However, the errors associat ed with the field estimates of soil water storage and crop water use m easured with the Sentry 200-AP capacitance probe were not evaluated. T he objective of this study was to analyze these associated errors. Equ ations were derived to evaluate the errors incurred in calculating the volumetric water content using the fitted field calibration curve for the Sentry 200-AP capacitance probe and in the estimates of the crop water use. This study suggests that the error in making in situ measur ements of soil water content in heterogeneous soil systems using the S entry 200-AP capacitance probe depends on the degree of saturation at the point of measurement in the soil profile. The Sentry 200-AP capaci tance probe produces errors of > 5% in measuring soil volumetric water contents of > 35% in heterogeneous soil systems.