Raindrop size distribution and terminal velocity for rainfall erosivity studies. A review

The knowledge of the rainfall drop size distribution (DSD) at the land surface is essential for understanding precipitation mechanisms affecting soil erosion processes. Rainfall erosivity is defined as the potential of rain to cause erosion and it can be evaluated by rainfall kinetic power, which is determined by DSD and raindrop terminal velocity. This paper firstly deals with the raindrop terminal velocity estimate. Then the most widely used DSD are reviewed highlighting the difference between the raindrop size distribution per unit volume of air and that per unit area and time. The reliability of the available kinetic power-rainfall intensity relationships and their application in several part of the world is discussed, highlighting that the use of rainfall intensity is not sufficient to determine the rainfall kinetic power everywhere. Finally, the influence of seasonality on both raindrop size distribution and rainfall energy characteristics is investigated using DSD measurements carried out by an optical disdrometer placed at Palermo experimental area.