The hydro - geomorphic modelling of saturation excess runoff generation

The steady-state and dynamic response of saturation excess runoff generation is explored. The main objective is to develop simple models incorporating catchment geomorphology which do not require site specific DEM analysis. Current soil moisture storage based models for this runoff generation require detailed analysis of DEM and accuracy of predictions depend on DEM grid resolution as well as the techniques used to extract the drainage path information. These models (eg. TOPMODEL) also assure steady-state conditions or no lateral soil moisture redistribution for predicting the dynamic response of saturated area. A simple conceptual model for prediction of saturated area at steady-state conditions (when catchment soil moisture is not changing) is developed using catchment geomorphology. This model does not require site specific DEM analysis. The model predictions match those of a kinematic wave model developed as part of this research. The kinematic wave model was used to study the behaviour of soil moisture and saturated area for transient conditions, when catchment soil moisture is changing. Results show that the assumptions used in current storage based models lead to errors in predicted saturated area and soil moisture for transient conditions. It is shown that the distribution of saturated area and soil moisture for transient conditions is different from that at steady-state, even though both cases have equal saturated area. The relationship between catchment average soil moisture and saturated area is also shown to be hysteretic. Current storage based models ignore this hysteresis. Using the results of the above study, a conceptual model to predict the dynamic response of the saturated area is developed. This model is an parsimonious extension for transient conditions of the steady-state model. Flow convergence factor is incorporated to model flow convergence and divergence in two dimensional catchments. A simple method to determine flow convergence factor is presented based on catchment geomorphology. The conceptual model predicted saturated areas for catchments match satisfactorily those from the kinematic wave model. The kinematic wave model was used to study more advanced catchment geomorphology statistics that are required to better describe the flow convergence factor. In addition to the geomorphology that describe steady-state conditions, geomorphology that account for flow convergence and divergence and spatially varying subsurface velocity are required to better describe the dynamic response of saturated area. While the topographic index can predict the steady-state saturated area it cannot alone describe the dynamic response of saturated area.