Surface Hydrology: II—Distribution of Raindrops

Nonuniformity of rainwater distribution over a given soil area is due to two mechanisms. Raindrop splashes produce a net horizontal flow that is proportional to the slope and to the rate of the rainfall. A change in slope causes excess concentration of rain splashes in concave spots and reduces the rain over convex points. The concavity can be measured geodetically. The size of the horizontal discharge was deduced theoretically and measured on a sloping plane in a rain simulator. Both indicated the same order of magnitude. The horizontal discharge over short and steep slopes can reach a tenth of the total rain. The local excess rain can be up to half of the average rain. The other mechanism responsible for localized excess rain is the slanting of rain over slope facing it, as distinct from a soil slope in the rain shadow. This mechanism can lead to local doubling of the rain intensity.