The effect of altitude on the prediction of momentum for rainfall erosivity studies in Mexico

Soil erosion is a mechanical process described by indices, such as kinetic energy or momentum of rainfall (M), which alters the protective cover of the ground surface. Optical disdrometer (PWS100) data, obtained between 2014 and 2018 at four sites across Mexico, were used to study the features of power and linear relationships between M and rainfall amounts (Acc). A comparison of the measurements showed that PWS100 usually estimated larger amounts of rainwater (PBIAS from 2 to 25%) with respect to gauges. The M-Acc relationships exhibited low dispersion, and the coefficients of the fitted expressions suggest an increase in M based on site altitude. Normalization of M based on the rainwater amount (M/Acc in N m(-2) mm(-1), hereafter M-mm) depended on the median volumetric diameter, D-0, and speed of raindrop fall. The D-0 was used as a proxy of raindrop spectrum broadness. Effect size results exhibited large values (d > 1.1) when Mmm means were compared between coastal and continental platforms. Furthermore, y-coefficients of the M-mm-D-0 relationships suggested an effect of the altitude (related to the air density in the sampling site) over M-mm estimates. Based on the v(D) empirical formulas, a set of expressions were proposed to fit the M-mm observations of the four sampling sites using D-0 and the corresponding atmospheric pressure or air density estimates. The results of performance measures showed that all equations predicted M-mm adequately thus suggesting that this erosivity parameter depended on both the amount of precipitation and the altitude (expressed as air density or atmospheric pressure) of the site at which the measurements were performed. These findings reinforce ideas regarding magnification of soil particle detachment via individual drops at higher altitude locations, as the force exerted is more significant with a decrease of air pressure and density.

Automated summary

The study explored the relationships between rain kinetic energy, rainfall amounts, and site altitude based on data collected by an optical disdrometer in Mexico. Results showed that the disdrometer generally estimated higher rainfall amounts compared to rain gauges, and the relationships between rain kinetic energy and rainfall amounts were influenced by both site altitude and raindrop diameter. Moreover, it was found that the force of individual raindrops plays a significant role in soil erosion at higher altitude locations due to the decrease in air pressure and density.