Rainfall Erosivity over Rhodesia

Abstract

The paper discusses the basis of the determination of the erosive potential of rainfall, termed 'erosivity'. Using bare soil as a standard, a measure linking kinetic energy and a maximum sustained intensity (EI3o) is found to be the most generally applicable erosivity parameter over large areas of Rhodesia. This is consistent with American findings. Data for 8ooo storms show that erosivity varies much more widely between years than mean annual rainfall and that on a yearly basis there is no relationship between the two. Of more significance to the erosion process is the finding that many stations, especially in the lower rainfall areas, have a higher proportion of erosive storms in the early rainy season when vegetation cover is poor. A relationship is established between long-term mean annual rainfall and erosivity and this relationship is used to construct a composite erosivity map of Rhodesia for the use of conservation planners and as an extension aid. EROSIVITY is the ability of rainfall to cause erosion.1 The amount of erosion caused by a certain storm is a function of the many factors at play at the air/soil interface. It is the purpose of this article to consider only those aspects of the rainfall that contribute to the erosion process and to describe the application of experimental plot results to obtain a composite erosivity map for Rhodesia. The mechanics of the rainfall erosion process is a subject little studied by geographers, yet it is fundamental to every study of soil erosion, be it erosion under natural conditions or erosion induced by man. Also, since the erosivity of rainfall varies over space and time, the subject is of especial interest to geographers. There is very little information on these variations outside of the United States and even there knowledge on the spatial and temporal distributions is meagre. Therefore, the basic principles behind erosivity parameters and their choice is discussed and a parameter is applied to an area of subtropical Africa, namely Rhodesia, where rainfall erosion is severe partly because of the intense nature of the rainfall.2 From theoretical considerations energy and momentum are parameters of rainfall that might be significant to erosion, for erosion is a work process involving the expenditure of energy in the breaking down and splashing of soil particles. However, it is not necessarily true that a simple kinetic energy parameter is the best predictor of erosion. The erosion process also involves overland flow which is more likely to be a function of intensity or total precipitation. Similarly,