RELATIONSHIP BETWEEN STREAM BANK CULTIVATION AND SOIL EROSION IN DEDZA, MALAWI

Abstract

The main objective of this research was to test whether there is a relationship between the extent of stream bank cultivation and the extent of soil erosion. Although stream banks have been cultivated over many years it is hypothesised that the practice is linked to increased levels of soil erosion. While considerable research has been conducted on the effects of riparian buffers on water quality and aquatic habitat, little is known about the influence of the removal of riparian vegetation on stream bank erosion. Therefore this study focused on the effects of stream bank cultivation on soil erosion. It is hypothesised that the removal of riparian vegetation by stream bank cultivation amplifies stream bank erosion. The study aimed at determining the relationship between the number of gardens as a surrogate for stream bank cultivation and distance away from stream, changes in the extent of stream bank cultivation over time, significant differences in the extent of soil erosion indicators (changes in soil surface levels and occurrence of gullies) and the relationship between the extent of cultivation and the extent of soil erosion indicators. Stream bank cultivation was determined using overlay analysis in GIS, aerial photographs of 1980, 1982 and 1995 and a SPOT satellite image of 2002. Soil deposition was determined in 21 sites of sizes between 2.5 and 5.0 m2, using the erosion pin method and measurements for length, depth and width were taken for 20 gullies. Bivariate relationships were determined using correlation and non linear regression analyses. The study revealed that a significant (α=0.05) negative relationship exists between number of gardens and distance away from the stream with most of the gardens located within 18 metres of the stream. Within this distance 74 percent of the gardens are under irrigation and 87 percent of the gardens are without any form of soil conservation measure. 52 percent have no buffer zones and for those that have buffers the mean width is 3.7 ± 6 metres. Though there is change in area under cultivation over 22 years there are no significant (α=0.05) differences along the two streams. Area increased between 1980 and 1982, remained constant until 1995 then decreased between 1995 and 2002. The study also revealed that though changes in soil surface levels occurred there was more soil deposition 82.86±104.738 (n = 70) than soil loss 60.96±69.857 (n = 20) along the two streams. However in terms of gulley occurrence no significant (α=0.05) differences were observed. Whereas there was a significant (α=0.05) positive relationship between number of gardens and soil deposition there was no relationship between number of gardens and gulley volumes. The study concludes that the extent of cultivation is contributing to the extent of soil deposition along the streams, and this is amplified by irrigation activities and the non use of soil conservation measures. The study therefore recommends that further studies be done to establish the origin of the deposited soils to ensure that appropriate mitigation measures are applied. The study also recommends similar studies over a number of years, under different ecological zones and different soil characteristics to test if the same relationships would emerge. In addition all irrigation planning may have to seriously incorporate appropriate soil conservation measures. The Malawi Government may also need to come up with practical regulations on stream bank protection and mechanisms for enforcing them considering the current food production- population imbalances that exist.