GRID MAPPING OF RIVER DISCHARGE

This paper summarises methods for creating maps of river discharge, an d evaluates them using river flow data from Europe. Most published map s are isoline maps, but choropleth maps are more appropriate for showi ng the spatial distribution of river flow. Five approaches to producin g grid choropleth maps can be identified. The first averages the data from all the small catchments in a grid cell; this overestimates runof f. The second involves statistical interpolation around small catchmen ts, but it,is difficult to account for ah the details of spatial varia bility in a statistical interpolation. The third method uses empirical relationships to estimate runoff from gridded climate data; the resul ts, however, depend on the climate data used. The fourth approach uses a water balance model to estimate runoff from gridded climate data; i t also is dependent on the input climate data. A final approach produc es a weighted cell average based on catchment runoff maps, and this wa s shown to produce reasonable estimates of gridded runoff.