A method for evaluating rainfall kinetic power by a characteristic drop diameter

Empirical relationships linking kinetic energy to rainfall intensity are commonly used to estimate rainfall erosivity. In this paper, using the drop-size distribution (DSD) proposed by Marshall and Palmer, a relationship for calculating the rainfall kinetic power was firstly deduced. This equation requires the simultaneous measurement of both the median volume diameter and the rainfall intensity and its application also reveals the drawback of needing the measurement of the whole DSD by an expensive disdrometric technique. Then, using a fictitious precipitation, which is both constituted by droplets having the same diameter (mean volume diameter) and having the same volume of the actual rainfall, a new expression of the rainfall kinetic power was deduced. This last equation has the advantage of allowing the estimate of the rainfall kinetic power by using the measurements of both rainfall intensity and the total number of droplets, reaching a horizontal unit area in an unit time, which can be acquired by a cheap device using a piezoelectric force transducer. This new relationship for estimating the rainfall kinetic power was tested using disdrometric measurements carried out at two experimental sites in Italy (Palermo) and in Spain (El Teularet). These experimental tests demonstrated that the rainfall kinetic power estimated by the mean volume diameter has an accuracy similar to the literature relationships proposed by Wischmeier and Smith, Brown and Foster and McGregor et al. Finally this investigation also showed that this novel approach can be applied estimating the median volume diameter by the mean volume diameter and the standard deviation of the measured drop diameters, whose measurement can be obtained by a force transducer.