Upscaling of soil hydraulic properties for steady state evaporation and infiltration

Abstract

Estimation of effective/average soil hydraulic properties for large land areas is an outstanding issue in hydrologic modeling. The goal of this study is to provide flow‐specific rules and guidelines for upscaling soil hydraulic properties in an areally heterogeneous field. In this study, we examined the impact of areal heterogeneity of soil hydraulic parameters on soil ensemble behavior for steady state evaporation and infiltration. The specific objectives of this study are (1) to address the impact of averaging methods of shape parameters and parameter correlation on ensemble behavior of steady state flow in an areally heterogeneous field and (2) to investigate the effectiveness of the “average parameters” in terms of the degree of correlation between hydraulic property parameters for the steady state evaporation and infiltration in unsaturated soil. Using an analytical solution of Richards' equation, the ensemble characteristics and flow dynamics based on average hydraulic property parameters are studied for evaporation and infiltration. Using various flow and average scenarios, we illustrated the resulting differences among the various averaging schemes. For vertical evaporation and infiltration the use of a geometric mean value for the shape parameter α of Gardner‐Russo model and Brooks‐Corey model and arithmetic mean value for the saturated hydraulic conductivity Ks simulates the ensemble flow behavior the best. The efficacy of the “average parameters” depends on the flow condition and the degree of correlation between the hydraulic property parameters. With the α and Ks parameters perfectly correlated, the “average parameters” were found to be generally most effective. The correlation between the hydraulic conductivity Ks and the parameter α results in an ensemble soil behavior more like a sand.