Estimation of the soil moisture distribution in the Tamne River Basin, Upper East Region, Ghana

Abstract: Northern Ghana experiences one dry season per year, but severe droughts may occur more infrequently. This is of concern to the local farmers of the Tamne River Basin (northeast Ghana) who depend on adequate rainfall amounts during the rainy season. Unfavourable precipitation amounts may lead to catastrophic consequences. It is therefore crucial to develop methods for predicting the distribution of soil moisture in the region. In this way, locations more sensitive to drought may be delineated. A high quality DEM was constructed over the region as topography is considered to be an important control on soil moisture. This was accomplished by combining elevation and linear features representing streams in the interpolation routine. The importance of topography relative to soil moisture was examined by using an index that reflects both the specific drainage area and local slope. As soil type and vegetation are also characteristics controlling the hydrologic response of a catchment, they were also included as two simple indices. The aforementioned variables were evaluated against 94 soil samples, distributed along seven transects, from which soil moisture was derived. The analysis showed that a statistical model that provides consistent results in terms of predicting soil moisture distribution was difficult to achieve for the entire basin. Only one statistical model was applicable to the whole basin; the one including soil texture. It is of limited utility however, since detailed data concerning the spatial distribution of soil properties was not available. The assumptions underlying the construction and use of the wetness index limit its application to homogeneous subbasins. This is shown by the more reliable results from the transects. Topography is shown to be less influential than expected. Conversely, soil texture provided the key for mapping the distribution of soil moisture under wet conditions.