QUANTIFYING THE EFFECTS OF SUBSURFACE HETEROGENEITY ON HILLSLOPE RUNOFF USING A STOCHASTIC APPROACH

We use the fully integrated model Parflow to evaluate the role of heterogeneity on hillslope runoff production and baseflow. Simulations were generated with idealized, high-resolution hillslopes configured with and without baseflow. Heterogeneous, correlated random fields were used to create spatial variability in hydraulic conductivity. Ensembles of multiple realizations were used to determine an average surface water outflow for a given hillslope as a function of rainfall rate and degree of subsurface heterogeneity. Outflow from slopes without baseflow show typical hydrograph patterns which agree with previous studies of Hortonian runoff generation. However, baseflow cases show a much different runoff behavior, with very little sensitivity to the variance of hydraulic conductivity. A technique is presented that explicitly interrogates individual realizations to perform accurate hydrograph separation between overland and subsurface flow. This technique allows determination of the contributions from variance of overland and baseflow for each case. This hydrograph separation technique is used to show that the variance of heterogeneity has a role in determining the proportion of overland and subsurface flow, even when effective hillslope behavior might be reached. This method is also used to evaluate to other hydrograph separation techniques and demonstrates that the recursive filter class of techniques can accurately proportion flow for some cases.