Catchment yield studies for water supply schemes, using- Mampong water supply catchment as a case study

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In Ghana catchments that have contributed raw water for water supply projects are undergoing great landuse changes such as deforestation, urbanization and cultivation. Manifested consequences on supply schemes have - raised expressed concerns .The situation calls for a review of the traditional approaches to the prediction of future yields for water supply projects. However, the enormous influence of catchment landuse on the characteristics of raw water supply sources has still not dawned on many. It has, therefore become necessary to sensitise engineers and planners on the need to incorporate future landuse as a factor in hydrological studies for water supply schemes. This study sought to achieve the above by evaluating the effects of catchment landuse on the availability of surface water from a selected catchment. Catchment yields were estimated on the basis of identified and predicted landuse scenarios, and a rainfall runoff model developed by the United States Soil Conservation Service. The following are some of the findings + The direct total runoff from the catchment is currently about eight times the value when the catchment was much like a virgin forest (1960), + The value when the catchment is fully urbanized further develops, and gets more impervious will be thirty - two times the initial value. These results show that current dry season flows are lower than dry season flows when the catchment was mainly forest. They also show that the dry season flows will further reduce .when urban landuse tends towards more imperviousness. Thus the streams will gradually lose their perennial character and become intermittent. Then, larger reservoirs, and more robust dams will be required if dry season water demand are to be met. In addition provision will have to be made to curb the consequences of higher flow rates and flooding. Obviously therefore, out water supply projects could fail, if we continue to base plans on yields estimated by methods that do not give due regard to possible changes in catchment landuse.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment f the requirements for the award of Degree of Master Science in Water Supply and Environmental Sanitation, 2001