Vertical Flow Aggregation in the Vadose Zone with Spatial- and Cross-Correlated Hydraulic Properties

Knowledge of hydrological data at or near the soil surface of "large" areas — a field, catchment or watershed — is of considerable interest for the management of surface and subsurface water resources and environmental research dealing with mass transfer at the earth-atmosphere interface. The gap between the local scale for conceptual modeling and observations, and the much larger application and validation scale is bridged with aggregation procedures. Especially for the vadose zone, aggregation is subject to uncertainty due to cross- and auto-correlation of hydraulic properties and nonlinear flow. This study compares a priori aggregation of hydraulic properties with a benchmark a posteriori aggregation of simulation results. Monte Carlo simulations of evaporation were conducted for a synthetic watershed. First, probability density functions (pdf's) and cross-correlations of hydraulic parameters were established from a set of 126 retention curves and saturated hydraulic conductivities from the UNSODA database. Second, random fields of two hydraulic parameters were generated with imposed auto- and cross-correlation. Third, evaporation was simulated using a stream tube model. Fourth, a posteriori and a priori aggregation were employed to estimate water content and moisture fluxes for different field scales and to quantify the behavior of field-scale mean and variance. A posteriori aggregation yielded the correct mean and a realistic decrease in variance with increasing field scale. On the other hand, a priori aggregation resulted in erratic predictions of the mean and variance of the evaporation flux at the field scale. The cross-correlation between hydraulic parameters enhanced clusters with low or high water contents and evaporation fluxes.