Streamflow generation

During the past 10 years under the impetus of the International Hydrological Decade there has been a burst of research activity aimed at obtaining better insight into the mechanisms of streamflow generation. The research has focused on an examination of the ways in which water moves from hillslopes into small stream channels during and between rainfall events in upstream tributary basins. The research has been of two types: field measurements in representative experimental drainage basins and theoretical studies using mathematical models of hydrologic processes, carried out with the aid of a digital computer. The field studies have identified the sources of lateral inflow to streams as overland flow, subsurface storm flow, and groundwater flow. They have also proven that most overland flow is generated either on small upland partial areas that are more or less fixed in size and that are controlled by the distribution of soil types or on expanding and contracting ‘variable source areas’ that are adjacent to streams and that are controlled by the topographic and hydrogeologic configuration of the hillslopes. They have shown qualitatively that the relative importance of the different processes depends on such controls as climate, geology, topography, soil characteristics, vegetation, and land use. This variety of field conditions coupled with the complexity of the processes themselves has hindered the growth of generalized interpretations based solely on the field evidence, and hydrologists have increasingly turned to mathematical modeling to isolate the various components of the system. Physically based mathematical models, in the form of boundary value problems based on the partial differential equations of flow, are now available for all the processes involved in streamflow generation. The concepts of open channel hydraulics form the basis for models of flow in a stream channel and for overland flow. The concept of saturated and unsaturated porous media flow has been brought to bear on models of infiltration, subsurface storm flow, and groundwater flow. Applications of these models at field sites and for series of hypothetical cases have identified the physical parameters that control the various observed mechanisms of downslope water movement and for some environments have determined the necessary conditions for the dominance of one mechanism over the others.