Modelling runoff generation and connectivity for semi-arid hillslopes and small catchments
The processes relating to runoff generation in a semi-arid environment at the hillslope scale are poorly understood. This research considers the amount and origin of water reaching the channel during a storm event using a combination of field experiments and computer simulation techniques. From the field experiments, it was found that the key controls on runoff generation at the point scale are the surface cover of rock fragments, vegetation cover, slope gradient and surface roughness. The effect of land management was found to be greater than geology. The simulation modelling work investigated the controls on runoff generation at the hillslope and small catchment scales. It was found that the storm characteristics are far more important than surface properties in determining the amount of discharge from a slope. The temporal fragmentation of the rainfall was found to control the distribution of flow path lengths and hence the amount of discharge leaving a slope. The key surface controls on the form of the discharge hydrograph are slope length, slope gradient and the hydrological properties at the base of the slope. The origin of runoff was investigated using autonomous software agents able to trace the flow of water through a catchment. This technique is able to give a unique picture of the origin of runoff within a catchment. It was shown that the spatial pattern of the origin of runoff is complex and varies significantly between catchments. This research has shown that there are two key themes in determining the amount of runoff reaching the channel network: the interplay between the distribution of flow path lengths generated during a storm and the distributions of flow lengths to the channel as a function of the landscape. The second theme relates to the importance of the spatial structure of hydrological areas within the landscape.