Is rainfall momentum the best index for estimating rainfall erosivity?

This paper shows the results of an experimental investigation carried out by the drop size distributions (DSD) measured in the period June 2006- April 2014 using an optical disdrometer installed at the Department of Agricultural, Food and Forest Sciences of University of Palermo and in the period July 2015-May 2016 at El Teularet experimental station in Spain. At first the DSDs are aggregated into intensity classes and then measured momentum values are determined by coupling at each raindrop of the aggregated DSD the corresponding terminal velocity, calculated by a relationship available in literature. For a fixed rainfall intensity the measurements show that both DSD and rainfall momentum measured at the two experimental sites are not coincident. However both datasets present a threshold value of rainfall intensity over which the rainfall momentum becomes quasi constant. Using the raindrop size distribution proposed by Marshall and Palmer a theoretical relationship is deduced, according to which the ratio between the rainfall momentum and rainfall intensity depends only on the median volume diameter of the distribution. The available measurements detected in different climatic contests by different measurement techniques allow to conclude that: (i) this theoretically deduced relationship allows reliable rainfall momentum estimates; (ii) the rainfall momentum is strictly correlated to the kinetic power of the precipitation.