New insights about the climatic and geologic control on the probability distribution of floods

An application of the theoretically based distribution of floods recently derived by Iacobellis and Fiorentino [2000] was carried out with the aim of analyzing the climatic and geologic control on the distribution itself. In particular, 20 basins in a wide area of southern Italy were considered. These basins were classified by the use of a climatic index I depending an the average annual rainfall and on the potential evapotranspiration. This index takes positive values in humid climates and negative values in dry zones. With regard to the geology, attention was paid to the watershed permeability during intense storms; in each basin the pervious area was estimated after identification of three permeability classes. The analyses demonstrated that the sign of the climatic index strongly discriminates the behavior of both the parameters f(A) and E[a], the former representing, for each basin, the characteristic total abstraction rate and the latter being the expected value of the basin area Contributing to the flood peak. In dry zones, f(A) was found to decrease with increasing A, and the ratio r = E[a]/A tended to decline linearly, as the percentage of pervious areas increased: On the contrary, in humid zones the basin area did not seem to have any effect on f(A), which was instead particularly sensitive to the climatic index. Moreover, in these zones the parameter r was steadily low without showing any significant dependence on climate, geology, and morphology of the basin. In this paper, an interpretation of these results is indeed provided.