Neutron probe calibration in a vertically stratified vadose zone

A procedure is described for neutron probe calibration in deep multilayer vadose zones. Calibration equations relating neutron count ratios (CR) to soil water content were developed for the upper 2.5 m of layered soil profiles using soil texture, water content, and neutron probe data. These equations were extended down to a depth of 10 m by relying on neutron probe data to delineate soil texture zones at depth. Data from two constant-flux field infiltration experiments were used to verify the calibration procedure. The water balance for each of nine soil profiles within the infiltrated area was computed using up to four separate calibration equations for each soil profile. The use of two or more texture-based calibration equations greatly improved the agreement between water applied at the surface and water measured within each of the nine layered profiles. Best results were obtained using four such equations, three corresponding to distinct soil horizons and one to interfaces between them (soil texture boundaries). The calibration procedure proposed here is applicable to deep soil profiles when soil texture and bulk density data are only available for the upper soil profile.