Water balance in Unsaturated Soil under a Sundano-Sahelian Climate: Estimation of the Groundwater Recharge
A reliable estimation of the recharge rate of an aquifer is a prerequisite for the efficient management of the groundwater resources. A variety of methods for recharge estimation in Sudano-Sahelian areas are available, but most of them do not take into account all mechanisms of water transfer in the unsaturated zone; as a result, the estimated recharge values for a given region are often dissimilar. Field equipment was used to measure water content and capillary tension at various depths in the unsaturated zone at an experimental site at Ouagadougou, Burkina Faso, in order to study infiltration and evaporation processes. Several water content profiles and water tension profiles were used to determine mass balances and water fluxes. The calculation of the head gradients allows the determination of flow direction. This study, carried out both in deep lysimeters (7 m) and on undisturbed soil over two years, has allowed: - A description of water dynamics at different soil depths. The soil can be divided into three parts. The water movement (infiltration and evaporation) takes place in the upper part of the soil. Its extension reaches 2.5 m depth. Drainage at this depth could not be measured and can be neglected. Below this zone, the water movements are very slow. The water content is constant over time and is equal to about 22%. The third zone, which lies between 5.5 m and groundwater table (7 m depth) is influenced by the groundwater table fluctuations. - An estimation of the depth at which water can be taken up by evapotranspiration and of the amount of this water at different depths. This depth is equal to 2.5 to 3 m under our experimental conditions (soil and climatic conditions). At the end of the dry period, about 180 mm water has been extracted from the soil by evaporation: 50 mm from the first meter, 100 mm between 1 m and 2 m depth and 20 mm for the last 50 cm. - A reevaluation of the effective minimum soil capacity by taking into account the soil-plant- atmosphere exchange depth. The results show that this effective minimum soil capacity, usually calculated on an agricultural basis, is under-estimated for hydrogeologic purposes. The hydrodynamic behavior of the lysimeters and the undisturbed soil are similar. Under the studied climate conditions and soil hydrodynamic properties, groundwater recharge by direct infiltration can be neglected.