The Movement of Soil Materials During a Rainy Season in Western Nigeria

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Lewis L.A
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The relation between slope properties, precipitation, movement of soil materials and loss are examined in western Nigeria during a single growing season. During the first two years after the ground cover was cleared for agriculture, a prior study using the same twenty-two runoff plots, indicated that loss increases as slope gradient increases. Results from this investigation indicate that losses of soil materials, five years after the land was cleared, are greatest not on the steepest slopes but on the 5-percent slopes. This occurs even though a thick weathered layer of soil materials still exists on the steeper slopes. Additionally, significant loss occurs in only a few rain events per wet season. The majority of rain events cause soil-particle movement by rain splash, but only rains having intensities greater than about 2.5 cm/h result in actual loss. For individual storms, the maximum movement of soil particles is about 4 m. Thus, soil materials appear to move downslope in a conveyor-belt progression for high-frequency rains. No relation is found between plot length and loss. Finally, a method for determining textural changes resulting from erosion over short time periods is presented. This method which requires minimal equipment, appears useful for areas where soils are comprised of a wide range of particle sizes.
Bulletin, 25, p. 13-25