A simple model of saturation excess runoff generation based on geomorphology, steady state soil moisture

A simple conceptual model of saturation excess runoff generation, the Hydro geomorphic Steady State model (HGSS) is developed using widely observed geo morphic relationships: the saturation excess threshold, the area-slope rela tionship, and the cumulative area diagram. HGSS provides theoretical insigh t into the link between runoff generation by saturation excess and the geom orphology that governs it. Current models for saturation excess runoff gene ration require detailed analysis of catchment digital elevation maps (DEMs) , and their prediction accuracy depends on the DEM grid resolution and the analysis techniques used on the DEM data. HGSS can predict the saturated ar ea at steady state without site-specific DEM analysis. In its simplest form , HGSS requires only two parameters, each a combination of geomorphology pa rameters, together with soil and recharge properties. Effectively, HGSS pre dicts an antecedent recharge-dependent runoff coefficient. A scaling soluti on indicates that the percentage of the catchment saturated decreases with the square root of catchment area over a broad range of catchment areas and that the proportion of the catchment saturated has a log-log linear relati onship with recharge. HGSS is tested by comparing its predictions with two DEM rainfall-runoff models, the steady state TOPMODEL and a kinematic wave saturation excess model DISTFW-EXT, with HGSS predictions matching for stea dy state conditions.