PNNL

Abstract

Determination of large number of soil hydraulic parameters of heterogeneous soils remains a challenge since inverting for too many parameters can lead to the non-uniqueness of parameter values and may need very long simulation time, for example, months or more. In this research, a combined parameter scaling and inverse technique (CPSIT) was proposed to upscale hydraulic parameters from local scale to field scale. The CPSIT approach includes two steps. Step 1, the number of parameters to be estimated at field scale (FS) is reduced by applying parameter scaling [Zhang et al., 2003]. A heterogeneous soil is treated as a composition of multiple equivalent homogeneous media (EHM) and a reference EHM is chosen. Each parameter is assigned a scaling factor that is defined as the ratio of the parameter of an EHM to the corresponding parameter of the reference EHM. These scaling factors are determined using local-scale parameter values. Step 2, the FS parameters of the reference EHM are determined using the inverse technique and well-designed field experiments. Then, the FS parameters of each of the EHMs is determined by multiplying the FS parameter value by the corresponding scaling factor. The advantages of the CPSIT approach are that the number of parameters to be inverted is reduced by a factor of the number of EHMs (M) and the simulation time is reduced by a factor of about the square of M. The CPSIT approach was tested by upscaling the hydraulic parameter using a field injection experiment in a heterogeneous soil at the Hanford’s Sisson and Lu site, which has the horizontal dimension of 16 m and vertical dimension of 18 m. The results show that, when the CPSIT upscaled parameters were used to simulated flow, the mean squared residual was reduced by 86% relative to that when the local-scale parameters were used.