Towards a catchment‐scale model of subsurface runoff generation based on synthesis of small‐scale process‐based modelling and field studies

Methodologies for developing a macro‐scale model of subsurface stormflow generation on a steep forested catchment in Japan are addressed. Field studies on this catchment have indicated that subsurface flow, consisting mainly of ‘old’ water displaced by ‘new’ rain water, dominates the storm response. Detailed field measurements on the catchment allowed simple, catchment‐scale relationships to be developed between the volume of saturated groundwater, on the one hand, and discharge, and surface and subsurface saturated areas, on the other. Attempts are made to find linkages between these empirical relationships and physically‐based descriptions of hydrological processes operating at smaller scales. A distributed model based on the saturated‐unsaturated groundwater flow on steep catchments was developed and tested with field data collected on this catchment. Derivation of catchment‐scale relationships can be carried out by a straightforward integration of the distributed model output. An alternative disaggregation‐aggregation approach is presented, whereby the catchment is divided into a number of hillslope flow strips. By applying the distributed model on each flow strip, under steady‐state conditions, it is possible to infer the spatial variability of groundwater volume in the various flow strips. This variability is related to a measure of hillslope topography and geometry. Knowing the catchment topography and geometry for any catchment, it is then possible to derive useful catchment‐scale relationships which are applicable under quasi‐steady‐state conditions. Hydrological relationships derived in this manner are compared with corresponding empirical relationships. The methodologies presented are effective for understanding the linkages between catchment scale response and small‐scale flow processes.