Creation of, application of, and animated discussion of mathematical models of watershed processes have been favorite pastimes of hydrologists for many decades. The original impetus for the development of watershed models was engineering design---the need to estimate properties of the hydrograph to be expected from a given sequence of storms. In recent years, the main driving force for development and refinement of watershed models has shifted. In addition to long-standing issues such as flood forecasting, the hydrological community now is involved very much in interdisciplinary scientific issues. Some of these critical and emerging areas in hydrological science were summarized by a committee of the National Research Council ( NAS 1991; Eagleson 1991). Examples pertinent to watershed science include the need for improved understanding of: (1) how the flow of water through and storage of water within a catchment affects the chemical composition of water in streams, soils, and underlying rocks (e.g., see Turner et al. 1990); (2) how the biophysical mechanisms that determine evapotranspiration from catchments can be parameterized (e.g., Band 1993); and (3) how space and time scales are linked in processes that convert rainfall and snowmelt into runoff (e.g., Gupta et al. 1986). An ingredient common to these and other examples of ``emerging areas'' is that spatial and temporal variability in processes are of critical importance.
The challenges faced in watershed modelling in recent years have reflected the need to deal with spatial variability and scaling and the need to consider explicitly linkages among hydrology, geochemistry, environmental biology, meteorology, and climatology. Some of the most important advances in watershed modelling over the last quadrennium have involved approaches that employ geographic information systems (GIS), remotely sensed data (e.g., see Engman, this issue), and environmental tracers, including linkages with geochemical models (e.g., see Hooper, this issue). This review of U.S. contributions focuses on these aspects of work on watershed modelling.
