Parameter estimation of hydraulic properties from one‐step outflow data

Abstract

The one‐step outflow method was presented by Kool et al. (1985 a, b ) as a fast procedure to estimate unsaturated soil hydraulic properties. Many investigators, however, encountered difficulties obtaining a unique solution to the problem. The purpose of this paper is to evaluate some of the problems related to this method by looking at response surfaces and to make recommendations for improvements in the methodology. The one‐step outflow procedure was investigated using synthetic data sets generated for 7.6‐cm‐diameter by 7.6‐cm‐high soil cores. A finite difference solution of Richards' equation was used to generate the synthetic data sets. Response surfaces were calculated for the van Genuchten parameters α, n , and K s for various combinations of matric potential, water content, and outflow data from initially saturated soil cores subjected to a step change in pressure at one of its boundaries. The results clearly show that with outflow data alone, the parameter estimation procedure will have difficulty converging. However, if matric potential data are available for a point in the core at some distance from the outflow boundary, parameter estimation sensitivity is greatly improved. The procedure used was found to be most helpful for determining which auxiliary variables should be measured for parameter estimation from outflow experiments.