Determining Soil Hydraulic Properties from One‐step Outflow Experiments by Parameter Estimation: I. Theory and Numerical Studies

Abstract

The numerical feasibility of determining water retention and hydraulic conductivity functions simultaneously from one‐step pressure outflow experiments on soil cores by a parameter estimation method is evaluated. Soil hydraulic properties are assumed to be represented by van Genuchten's closed‐form expressions involving three unknown parameters: residual moisture content θ, and coefficients α and n. These parameters are evaluated by nonlinear least‐squares fitting of predicted to observed cumulative outflow with time. Numerical experiments were performed for two hypothetical soils to evaluate limitations of the method imposed by constraints of uniqueness and sensitivity to error. Results indicate that an accurate solution of the parameter identification problem may be obtained when (i) input data include cumulative outflow volumes with time corresponding to at least half of the final outflow and additionally the final outflow volume; (ii) final cumulative outflow corresponds to a sufficiently large fraction (e.g., >0.5) of the total water between saturated and residual water contents; (iii) experimental error in outflow measurements is low; and (iv) initial parameter estimates are reasonably close to their true values.