Assessment of Rainfall Energetic Characteristics Under Natural and Simulated Conditions for Soil Erosion Research

Rainfall is the primary driver of soil erosion by water, and its erosive potential depends on the size and velocity distribution of impacting raindrops. Rainfall energetic characteristics are typically estimated using optical disdrometers, specialized and costly instruments whose deployment is generally limited to dedicated research facilities and advanced monitoring stations, and therefore not widespread across standard measurement networks. As a result, these characteristics are often inferred from site-specific empirical relationships of limited accuracy, which constrain their reliable representation in erosion studies. The present doctoral research addresses these limitations through an integrated theoretical and experimental investigation of the energetic characteristics of rainfall under both natural and simulated conditions.For natural rainfall, the study relies on an extensive database of drop-size distributions measured with an ODM 470 optical disdrometer at three Mediterranean experimental sites characterized by distinct rainfall regimes. These data were used to examine the statistical structure of raindrop size distributions and their relationship with rainfall energetic properties. The analyses showed that the Weibull distribution provides a reliable representation of natural rainfall drop size distributions in these environments, and that the theoretical relationships developed in this study for estimating rainfall kinetic power and momentum are applicable under these conditions. Within this framework, the results confirmed that rainfall intensity alone is insufficient to describe rainfall energetic characteristics, as commonly observed in the literature, whereas the Weibull parameters capture site-specific differences in drop size distributions and therefore in rainfall energetic properties.A subset of the database collected at the Sparacia experimental site, where rainfall measurements were coupled with plot-scale soil-loss observations, was used to reassess the representation of rainfall erosivity at the event scale. The classic erosivity formulation showed limited ability to explain the variability of measured soil loss even when rainfall kinetic energy was derived from disdrometric data. A new event-scale descriptor, incorporating rainfall kinetic power and intensity at their measurement temporal resolution, yielded a substantially improved relationship with soil loss, highlighting the importance of rainfall energy dynamics during erosive events.Building on the Weibull-based framework, the research also advanced a patented measurement method to reconstruct rainfall energetic characteristics from three directly measurable quantities: rainfall intensity, drop concentration, and the mean diameter derived from the raindrop momentum distribution. This method enables the retrieval of Weibull parameters and the estimation of kinetic power and momentum without full disdrometric measurements. The concept relies on the analysis of electrical signals generated by piezoelectric sensing of individual raindrop impacts, offering the potential for low-cost measurement of rainfall energetic properties. Validation against disdrometric datasets confirmed the accuracy of the approach, particularly for rainfall momentum, supporting the feasibility of the patented measurement principle pending prototype development.Furthermore, an extensive experimental program involving different types of rainfall simulators (drip-type and pressurized) supported the development of a reliable methodology for their comprehensive characterization, enabling the assessment of rainfall-intensity spatial uniformity and associated energetic parameters. Specifically, for drip-type simulators, which generate droplets with constant diameter and fall velocity under fixed operating conditions, rainfall energetic properties are inherently deterministic. For the widely used Kamphorst simulator, this implies that kinetic power and momentum depend only on rainfall intensity and fall height, enabling the development of empirical relationships expressed solely in terms of these variables. This deterministic behavior also allowed the validation, across all investigated drip-type devices, of a literature-calibrated empirical relationship for estimating raindrop fall velocity as a function of drop diameter and fall height. By contrast, pressurized simulators produce more complex rainfall fields, consisting of composite droplet sprays with a wide range of sizes and velocities that more closely resemble the variability of natural rainfall. In this study, two pressurized configurations were considered: a newly proposed simulator developed in Palermo and a previously established facility in León capable of generating terminal fall-velocity conditions. Owing to this intrinsic heterogeneity, the deterministic approaches applicable to drip-type devices are not suitable for these simulators. Their comprehensive characterization, accounting for the spatial variability of drop size and fall velocity and their associated energetic properties, can therefore be achieved only through spatially distributed optical disdrometer measurements integrated with conventional volumetric calibration based on the Christiansen method.Controlled laboratory investigations enabled an assessment of the Parsivel2 optical disdrometer, designed for rainfall monitoring under natural conditions. The instrument provided reliable estimates of drop size distributions but showed limited accuracy in fall-velocity measurements. In particular, the experiments confirmed that the underestimation of raindrop velocity, well documented for natural rainfall, also occurs under simulated, non-terminal conditions. At the same time, the disdrometer-based velocity measurements allowed verification of the applicability of the above-mentioned empirical fall-velocity relationship across both drip-type and pressurized simulators, under sub-terminal as well as terminal fall conditions.The results obtained for both natural and simulated rainfall converge toward the future development of the patented rainfall energy measurement device. Once available, the instrument will first require validation under highly controlled conditions, ensured by the fully characterized drip-type simulators. Subsequent scaling to more complex rainfall fields will be supported by the pressurized simulator configuration proposed in this work. Final verification will then be conducted under natural field conditions.