Soil erosion involves the detachment of soil particles from larger aggregates and the removal of the particles by flowing water and wind. In the climatic conditions which prevail throughout most of Zimbabwe fluvial erosion is dominant, that is the effects of rain splash and run-off, both surface and subsurface. Soil erosion is an extremely complex phenomenon varying both spatially and temporally.
Rates of soil formation in Zimbabwe are very slow (e.g. 400 kgs/ha/year), whereas rates of soil erosion are very much greater; estimates for average soil losses on crop lands and grazing areas on commercial farms are 5 tonnes/ha/year and 3 tonnes/ha/year respectively; the equivalent average for communal lands are 50 and 75 tonnes/ha/year. These rates have been generally accepted for a long time but more recent observations indicate that these rates may be largely exaggerated. These rates were the result of a survey carried out on a specific single soil type in one ecological region, thus the replicability of these findings across a country, with high variability, in terms of the erosive and stabilizing factors, is questionable.
The consequences of this erosion are seen in general declines in crop yields and very high rates of siltation of reservoirs, especially of the smaller dams used for rural water supplies. Small dams are likely to fill with sediments within 15 years of construction and even the larger irrigation schemes are being affected adversely by siltation problems.
Soil erosion is clearly an important problem especially within the communal lands. It is essential that conservation measures be considered a key part of development strategies in these areas rather than being treated as a token appendage.
No comments:
Post a Comment