25 October 2014

Regionalization of Subsurface Stormflow Parameters of Hydrologic Models: Up-scaling from physically based numerical simulations at hillslope scale


Due to the high infiltration capacities of soils and their spatial organization, subsurface stormflow is a dominant streamflow generation mechanism, especially in steep forested regions. However; its contribution is poorly represented in current generation of land surface hydrological models (LSMs) and catchment-scale rainfall-runoff models. The limited physical basis of common parameterizations precludes a priori estimation (i.e. without calibration), which is a major drawback for use in global models. A team of scientists from University of Illinois and Department of Energy Pacific Northwest National Laboratory explored the methods to derive physically based storage-discharge relations for use as parameterizations of subsurface stormflow in land surface models without the need to resolve the flows at smaller scales explicitly. This study approaches the goal through a two-step model procedure upscaling from hillslope to catchment scale. Theoretical simulation results produced storage-discharge relationship in terms of soil hydraulic properties, topographic slope and their heterogeneities, which were consistent with results of previous studies. Yet this theoretical derivation has a systematic inconsistency with the empirical relationship derived from a large number of catchments. This divergence could be explained, to first order, by climate in the form of climatic aridity index, pointing to a possible co-dependence of climate, soils, vegetation and topographic properties. This result suggests that subsurface flow parameterization must account for both the physics of flow and the co-dependence of soil and topographic properties with climate, including possibly the mediating role of vegetation.