A new practical aid to regional hydrogeologic planning: the Runoff Coefficient Map

The principal aim of this research is to solve problems of water-budget estimation in particularly indeterminate conditions, of the kind that are frequently encountered in the field, in which absence or insufficiency of hydrographic instrumentation prevents the adequate evaluation of runoff. To do this we have developed techniques that are based on the method of Kennessey (1930), in which the runoff coefficient of a watershed in estimated using three physiographic variables (slope angle, permeability, and vegetation cover) and a parameter that delines the climatic conditions of the lacal area. Our comparison of the Kennessey method with instrumental records from a wide variety of instrumented drainage basins in centrai Italy shows that the method provides reliable estimates of runoff. Our analysis al so indicates the best criteria tor its application, as shown by the examples and results presented in this article. This research enables us to present a new application of the method, namely the runoff coefficient map, which, in addition to its specific usefulness far the evaluation of runoff, has three advantages. First, it supplies a mare realistic vision of the local distribution of runoff/rainfali ratios; second, it allows one to overcome the spatial constraints imposed by the narrow frame of reference represented by the watershed; and third, it makes an important contribution to the solution of other hydrogeological problems, such as the evaluation of the amount of water required far adequate recharge of the intake areas of deep aquifers ar the maintenance of geothermal processes. Hence we propose this mapping technique as a valuable practical aid to regional hydrogeologic planning.