Assessment of land cover changes and hydrologic response of Tamne River Basin

Abstract

Northeastern Ghana, West Africa, is characterised by a high population growth rate and adverse climatic conditions, e.g., a long dry season followed by a short-duration but intense rainfall pattern. Land degradation in this part of the country has caused farmers to extend their agricultural activities into marginal lands including flood plains. The consequence is an increase in flood damage to cropland, livestock, infrastructure and human lives. In this study, the nature of vegetation change in the Tamne River Basin was assessed by integrating remote sensing and geographic information systems (GIS). Inter-annual variation in rainfall and streamflow were assessed using statistical hypothesis testing. Gumbel Extreme Value distribution was used to estimate peak flow magnitudes for selected return periods. Field observations and interviews with farmers provided first hand information about the farming systems in the Tamne River Basin. There was no significant change inter-annual variation in rainfall and hydrologic response and the estimated 100 year peak flow was 59.9 m3/s. Less than one percent of the total area in Tamne River Basin changed between 1975 and 1991. The distribution of the potential land cover changes confirmed the assertion that, farming systems constitute a primary cause of land degradation in the Upper East Region. The potential land cover changes have little or no effect on the hydrologic response in the Tamne River Basin. The study also provided a broad framework for detailed watershed analysis at the basin and sub-basin level, including the monitoring of land use change and distributed hydrologic modelling. In addition, the study highlighted some o f the problems related to incomplete and outdated environmental information, and their implications on the effective integration of remote sensing and GIS for resource management and policy formulation in Ghana. Suggestions have been made for: integration of ‘expert systems’ into GIS and image processing in Ghana; quantitative accuracy assessment for the potential land cover changes; and, event-based hydrologic modelling for sub-catchments in the Tamne River Basin.