Hydrology of the Sudanian Savannah in West Africa, Burkina Faso

As in all semi-arid regions, Tambarga, a small village surrounded by national parks in the landlocked country of Burkina Faso, is affected by the seasonality of the local hydrology. Seasonal and spatial variability of rainfall shapes the livelihoods of rural farmers who depend mainly on rain-fed agriculture. We instrumented the Tambarga catchment (area=~4 km2) to investigate the rainfall patterns, the resulting hydrologic processes (surface runoff, base flow) and the evaporation effects on the local water balance. Thus, we have measured hydrological, meteorological and soil parameters at high spatial and temporal resolution over the catchment since 2009. Data for this research were acquired from the 2010 to 2012 rainy seasons using a network of automatic wireless weather stations (SensorScope stations), two weirs, eight piezometric wells, and two surface energy balance stations. Rain at Tambarga is triggered by the convective mechanism, which is mainly controlled by the sensible heat flux. The daytime rain events at Tambarga are convective and are characterized by their high intensities and short durations. An overall increase in rainfall of 10-30% is observed in the savannah forest when compared to the agricultural field from 2010 to 2012. Biotic (leaf area index) and abiotic (well drained soil) factors, causing enhanced sensible heat flux at the savannah site, lead to an increased predisposition toward convective rainfall. The hydrologic processes concomitant to the rain events were identified over the basin and intermittent responses of streamflow were isolated. The rise of the perched water tables located in the upstream and downstream regions of the basin generated two separate flows in the riverbed after the first rain. The intermediate zone operates like a deep storage tank, and by filling, it creates a connection between the two perched water tables. This connection induces a continuous flow throughout the 2.8 km of the stream. The hydrologic response at the basin outlet is observed to alternate between two different states over the course of the season. At the start of the rainy season, when the soils are very dry and the groundwater level is deep, a typical single-peak hydrograph is observed. This evolves into a double-peak hydrograph when the rainy season is completely established. The single-peak hydrograph is correlated with rainfall intensity, while the double-peak hydrograph is also correlated with the antecedent soil moisture condition. The baseflow, meanwhile, occurs when the groundwater level is higher than the riverbed at certain locations in the basin. In contrast, during dry days (no rain) the baseflow exhibits a diurnal flow pattern. This diurnal pattern consists of decreased flow rate from sunrise (6:00 A.M.) to 1:00 P.M. and flow rate recovery to its previous level from 1:00 P.M. to sunset (6:00 P.M.). The diurnal pattern of the streamflow was found to be derived both from infiltration in the riverbed and evaporation over the river edges. The infiltration process dominates during the beginning of the season. While late in the season, when the groundwater network is interconnected and its overall level is higher than the riverbed elevation, evaporation controls the diurnal pattern. An evaporation contributing area is defined when the diurnal pattern is controlled by the evaporation. This area is about 0.6% of the basin area and could be rationally represented by the riparian area and the outlet wetland. Given the importance of rainfall-runoff processes over this catchment, a water balance was completed based on a simple lumped model. This model allowed for testing some water management strategies to improve agricultural production. Its implementation suggests that storing water underground for irrigation purpose with deeper wells is a way to achieve better agricultural productivity and therefore, reduce the vulnerability of the local people.