Estimation of in situ unsaturated soil hydraulic functions from scaled cumulative drainage data

Simulation of water flow and transport processes in soils rely on field representative soil hydraulic functions. The linear variability concept in combination with the inverse technique was used to estimate in situ soil hydraulic properties in a 32‐ha field. Measured cumulative drainage curves were scaled yielding scaling factors. Subsequently, the drainage and moisture content distribution of the scaled reference profile were input to a numerical model to optimize the soil water retention and hydraulic conductivity curves for the reference soil profile by inverse solution of the scaled Richards equation. Field hydraulic functions for each location were computed from the reference curves and scaling factors. In addition, undisturbed soil cores taken from 0.3‐m and 0.6‐m depths at 44 locations were used to determine soil texture, and soil water retention and hydraulic conductivity curves in the laboratory using the multistep outflow technique. These hydraulic functions were scaled using the simultaneous scaling technique. The reference field hydraulic functions compared well with those determined from the soil cores taken from the 0.6‐m depth. In situ saturated hydraulic conductivity variability was one order of magnitude less than that of the soil cores.