Soil erosion, in particular rainfall erosivity
Soil erosion is closely related to land use and often upsets the natural balance in the landscape. More soil will be eroded than can be reproduced, and the increased amount of sediment delivered to the channel system may partly block the channels, and cause silting of water reservoirs. Therefore, proper land-use planning is essential to avoid devastating soil erosion.
Gullying of crusted soils in a semi-arid area in Tanzania.
The release of soil is caused by different erosion processes, such as rainsplash, sheetwash, rill and gully erosion or by mass movement processes, such as landslides, slumping, and soil creep.
In tropical and sub-tropical areas erosion, especially splash erosion, is mainly caused by rain falling at intensities above 25 mm per hour.
Rainfall intensity-duration curves, based on records at Morningside in Tanzania 1.8.1963 - 31.7 1971. From Rapp, Axelsson, Berry, and Murray-Rust 1972 (Geogr. Ann. 54 A).
The highest recorded half-hour intensity at Morningside during the above given 8-yr period was 97 mm per hour. Half-hour intensities of 50 mm per hour were equaled or exceeded on the average 3 times a year. The highest recorded one-hour intensity was 65 mm.
A suitable measure of the erosion potential or erosivity of a rainstorm is the product of the total kinetic energy of the storm and its maximum half-hour intensity. The erosivity of a single storm may be very high. One example of this is a storm at Morningside on March 4, 1967. This storm had a duration of only 75 minutes but a maximum half-hour intensity of 97 mm per hour. The erosivity of the storm amounted to about 16 % of the total annual value.
The studies of soil erosion should be based on land-use mapping and on the application of remote-sensing techniques. In order to quantify the effect of land use on soil erosion and reservoir sedimentation it is necessary to establish river stations for sediment sampling that are situated not too far from the source areas for sediment production.
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