Spatial Range of Representativity Complements Physical Sphere of Influence of Soil Water Content Sensor in Spatially Variable Field Soils.

Calibrating electro-magnetic soil water content sensors at the field scale is difficult inasmuch as field soils are spatially variable. Moreover, obtaining calibration samples in the vicinity within the physical sphere of influence is prohibitive as it would destroy the sampling location for subsequent measurements. The objective of this study was to evaluate a soil water capacitance probe calibration based on the spatial range of representativity. In a farmer’s field in western Kentucky, soil water capacitance probe access tubes were installed at 45 locations at regular distances of 10 m along a transect to a depth of 0.8 m. Capacitance probe readings of scaled frequency were taken at eight 10-cm depth increments in each of the 45 access tubes. Soil samples utilized for calibration were sampled outside the physical sphere of influence of the capacitance probe access tube. Different data stratification scenarios were studied based on soil clay content among other criteria. Calibration functions consisted of a two-parameter power function between volumetric soil water content and scaled frequency. The two parameters (constant and exponent) were spatially correlated. Their spatial processes along the 450-m transect could be described with autoregressive state-space models. Spatial processes of soil clay content and processes of coefficients in adjacent soil layers supported this spatial estimation. The spatial range of representativity complements small physical spheres of influence. Space-time fields of soil water content along this transect are presented for different soil depth compartments during a three-year field study with periods of bare soil conditions and growing crops.