A combined parameter scaling and inverse technique to upscale the unsaturated hydraulic parameters for heterogeneous soils

Abstract

Determining a large number of soil hydraulic parameters for heterogeneous soils remains a challenge because inverting for too many parameters can lead to parameter values that are nonunique. Furthermore, such inversions may need very long simulation times, for example, months or more when inverting field‐scale problems. In this research, a combined parameter scaling and inverse technique (CPSIT) is proposed to upscale hydraulic parameters from the local scale to the field scale. The CPSIT approach includes two steps: (1) parameter scaling and (2) inverse modeling. In step 1 the number of parameters to be estimated at field scale (FS) is reduced by applying parameter scaling whereby a heterogeneous soil is treated as a composition of multiple equivalent homogeneous media (EHMs). In step 2 the FS parameters for the reference EHM are determined using the inverse technique and observations from well‐designed field experiments. The advantages of the CPSIT approach are that the number of parameters to be inverted is reduced by a factor of the number (M) of EHMs, and the simulation time is reduced by a factor of about M2. The CPSIT approach was tested by upscaling the hydraulic parameters using a field injection experiment at the Hanford Site. Results show that when the CPSIT upscaled parameters were used to simulated flow, the mean squared residual was reduced by 83.2% relative to that when the local‐scale parameters were used.