Rainfall erosivity and variability in the Northern Ethiopian Highlands

Abstract

The Ethiopian Highlands are subjected to important land degradation. Though spatial variability of rain depth is important, even at the catchment scale, this variability has never been studied. In addition, little is known on rain erosivity for this part of the world. The objectives of this study are (a) to assess the spatial variation of rain in a 80 km 2 mountain area (2100–2800 m a.s.l.) in the Northern Tigray region, and how this variation is influenced by topography, geographical position and lithology, (b) to analyse the temporal variations and (c) to quantify rain erosivity and the different factors determining it, such as rain intensity, drop size and kinetic energy. Spatial variation of rain was measured over a 6-y period by installing 16 rain gauges in the study area. Topographical factors, especially general orientation of the valley and slope gradient over longer distances, determine the spatial distribution of annual rain, which is in the order of 700 mm y −1. Precipitation is highest nearby cliffs and other eminent slopes, perpendicular to the main valleys which are preferred flow paths for the air masses. Rain intensity is smaller than expected: 88% falls with an intensity <30 mm h −1. High intensities have a short duration; maximum recorded rain depth over 1 h (32 mm) is only 2 mm less than that over 24 h. Using the blotting paper method 65,100 rain drops were sampled. For all observed rain intensities, the median volume drop diameters ( D 50) are significantly larger than those reported for other regions of the world. A relation between rain intensity ( I) and volume specific kinetic energy (Ek vol) was developed for the Ethiopian Highlands: Ek vol = 36.65 ( 1 − ( 0.6 / I ) ) ( R 2 = 0.99 , n = 18 ) , ( Ek vol in J m − 2 mm − 1 , I in mm h − 1 ) . Due to the occurrence of large drop sizes, probably linked to the prevailing semi-arid to subhumid mountain climate, this relation yields, within the intensity range [0.6–84 mm h −1], larger values for Ek vol than elsewhere in the world. It is recommended to use this new relationship for calculating Ek vol of rain in the Ethiopian Highlands, as well as for the computation of Universal Soil Loss Equation's rain erosivity factor on yearly basis.