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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 Hydrogeomorphic Steady State model (HGSS) is developed using widely observed geomorphic relationships: the saturation excess threshold, the area‐slope relationship, and the cumulative area diagram. HGSS provides theoretical insight into the link between runoff generation by saturation excess and the geomorphology that governs it. Current models for saturation excess runoff generation 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 area at steady state without site‐specific DEM analysis. In its simplest form, HGSS requires only two parameters, each a combination of geomorphology parameters, together with soil and recharge properties. Effectively, HGSS predicts an antecedent recharge‐dependent runoff coefficient. A scaling solution 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 relationship 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 steady state conditions.