A parameter scaling concept for estimating field-scale hydraulic functions of layered soils

Abstract

Predicting flow and transport in unsaturated porous media is often hampered by limited data and the uncertainties in constitutive property information at the appropriate spatial scales. Some studies have used inverse flow modeling for parameter estimation to overcome these limitations. However, determination of the soil hydraulic parameters of layered soils remains a challenge since inverting for too many parameters can lead to the nonuniqueness of parameter values. Here we propose a parameter scaling method that reduces the number of parameters to be estimated. First, parameter scaling factors are determined using local-scale parameter values. After assigning scaling factors to the corresponding soil textures in the field, the reference hydraulic parameter values at the field scale can be estimated through inverse modeling of well-designed field experiments. Finally, parameters for individual textures are obtained through inverse scaling of the reference values. The number of unknown variables is reduced by a factor equal to the number of textures (M) and the simulation time is reduced by the square of the number of textures (M 2). The proposed method was tested using two infiltration-drainage experiments in layered soils. The STOMP numerical simulator was combined with the inverse modeling program, UCODE, to estimate the hydraulic parameters. Simulation errors were significantly reduced after applying parameter scaling and inverse modeling. When compared to the use of local-scale parameters, parameter scaling reduced the sum of squared weighted residual by 93-96%.