An investigation into the effects of changes in land use on runoff and sediment transport rates from the Turasha catchment, Kenya.

Abstract

This study was conducted in the Turasha catchtnent in Nyandarua District, Kenya. The study traces the land use changes between the large scale farming practised by the white settlers prior to independence and subsequent small scale farming practiced by peasant African farmers after independence. The trends in land use are then related to trends in runoff and downstream sediment loads from the catchment taking into account the possibility of climatic change. The study furnishes information which shows the undesirable hydrological consequences caused by unplanned settlement. The study therefore furnishes information useful for the formulation of land use planning policies and conservation measures for the catchment. Land use types were delineated stereoscopically and visually on remote sensing pictures to determine their spatial trend between 1962and 1988. Similarly, annual suspended sediment loads were estimated to determine their trend between the same period. Long term discharge data recorded at the outlet of the catchment were subjected to Time Series Analysis to yield their trend over the study period. Annual peak flow events were applied to the Gumbel and Log Pearson Type III statistical models to reveal their trend. A linear multiple regression model was fitted to relate sediment load, as the independent variable, and the area covered by the various land use types as the dependent variables. It was observed during the study period that the area under grassland decreased by 34% while the forest area went down 4%. Both the area under arable and mixed farming increased by 31 %. The annual suspended sediment load showed an upward trend being

12;118 tonnes at the beginning and 307,960 tonnes at the end of the study period. Time series analysis of the discharge data showed random variation. The Gumbel and Log Pearson Type III analysis of flood frequency revealed floods depressed by a factor of 1.8 in more recent years. Multiple regression analysis between suspended load and area of land use types showed that both grassland and arable area play equally dominant but counteractive roles in generating sediments. While arable area tends to increase their production, the grassland area appears to perform a conservation role and hence a retardation in their production.