Factors affecting the runoff coefficient

Abstract. The runoff coefficient φ is a crucial parameter for flood peak discharge estimate in ungauged drainage basins. Tables and graphs generally allow the determination of φ in a somewhat empirical way that can lead to inconsistency in application; therefore, it is important to identify other parameters that can be utilized to assess φ more directly.

In the present paper, focusing on Southern Continental Italy, a simple analytical expression between runoff coefficient φ and soil potential maximum retention S is proposed; moreover, an improvement of this expression is provided by considering the pre-event moisture condition of the watershed through the use of a climatic factor. At this aim, the US Soil Conservation Service classification for soil permeability has been adopted, that allows the evaluation of S, according to its relationship with the runoff curve number CN, as a function of soil type, land use and antecedent soil moisture condition (AMC).