The hydrology of small agricultural watersheds in the Guinea Savanna zone of west Africa

Water availability in wet lowland environments in the Guinea Savanna zone of Cote d'Ivoire is an incentive for expanding agricultural production into these areas. This dissertation presents research on recharge to shallow groundwater in the M'be experimental watershed, which is typical of small watersheds that are traditionally used for agriculture in central Cote d'Ivoire.;Groundwater fluctuations are key to understanding the variability of recharge in the M'be watershed. Recharge to the shallow aquifer results in subsurface flow from the watershed slopes towards the valley bottom. Localized zones of recharge that become sources of subsurface flow were identified on the slopes and in the hydromorphic zone of the watershed. A larger proportion of stream flow measured from the valley bottom was subsurface flow originating from these recharge zones. Using an analysis of flow in a sloping soil layer, the volume of subsurface flow was successfully predicted at storm scale. The analysis showed that the duration of subsurface flow was consistently about 10 days, irrespective of rainfall amount or the extent of the area drained. It was concluded that the duration is dependent only on aquifer properties.;A water balance procedure was used to estimate recharge to the shallow aquifer. Recharge in the watershed was highly variable, both spatially and temporally. Effects of variability were apparent from the differences in groundwater response patterns in the different landscape units that make up the watershed. The recharge process was discontinuous, as it was dictated by rainfall amount and distribution. Recharge was greatest in the valley bottom fringes from which subsurface flow was subsequently generated. Average recharge estimated by the.;water balance procedure was in close agreement with direct recharge computed from groundwater levels changes, and was consistent with the upper range of the Savanna regional estimate cited in literature, which ranges from 0--200 mm. Average recharge for the 1998 rainfall season ranged from 4% of rainfall on the slopes to 18% of rainfall in the valley bottom fringes.