Surface water hydrology—runoff generation and basin structure

This review addresses the problem of developing a theory of the relationship between runoff generation and basin structure. It is suggested that the current generation of physically based models of basin hydrology cannot predict runoff generation in the field with any adequacy because they are not good descriptors of runoff processes, except under some special circumstances. Some of the complex factors contributing to runoff generation that lead to this conclusion are discussed. Special emphasis is placed on the problems posed by the spatial variability of soil characteristics to the application of physically based models. The way in which basin structure changes with spatial scale is then considered. The ordered nature of drainage basins suggests that it may be possible to develop some parsimonious theory relating basin structure to runoff generation. Finally, the importance of time scale is stressed. Changes in basin structure over time are frequently ignored in hydrological analysis. Over long periods of time these changes are on the average gradual, but over the time scale of years there may be important periods of change induced by extreme events. Such an event may affect runoff generation and the frequency characteristics of events that follow it, particularly in small basins. All these complicating factors affect our ability to predict the response of an ungauged basin using physically based models, one of the ultimate tests of physical hydrology.