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

•Rainfall momentum increases with altitude.•A new parameter, momentum content Mmm, is proposed to study rain erosivity.•Mmm is related to the DSD shape and the pressure (air density) at the site.•Soil particle detachment by drops is magnified at higher altitude locations. 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 Mmm) depended on the median volumetric diameter, D0, and speed of raindrop fall. The D0 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 Mmm–D0 relationships suggested an effect of the altitude (related to the air density in the sampling site) over Mmm estimates. Based on the v(D) empirical formulas, a set of expressions were proposed to fit the Mmm observations of the four sampling sites using D0 and the corresponding atmospheric pressure or air density estimates. The results of performance measures showed that all equations predicted Mmm 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.