Caractérisation et modélisation des nappes d’eau souterraine au voisinage de petites retenues d'eau d'irrigation en zone de socle : cas de Kierma et de Mogtédo (Burkina Faso)

Small surface water reservoirs have been massively built in sub-Saharan Africa to provide alternative source of water during drought. They contribute to the socio-economic development and food security of populations through the practice of irrigated agriculture in the dry season. Unfortunately, these surface water reservoirs tend to dry up prematurely before the last harvest. One solution to these irrigation water shortages is to ensure the conjunctive use of surface and groundwater. In this respect, a better knowledge of groundwater resources is essential for integrated water resources management.
The objective of this work is to contribute to a better understanding of the surface water reservoirs in the vicinity of irrigation water reservoirs for agricultural purposes. The present study is part of the "Programme d'Appui au Développement de l'Irrigation (PADI)" (Irrigation Development Support Programme). The watersheds of Kierma (area of ≈100 km2) and Mogtédo (≈500 km2), which all have surface water reservoirs for irrigation, were chosen for the study. They are located in the climatic context of semi-arid and on crystalline and crystallophyllous basement geological formations.
The methodological approach was based on several steps. The first consisted in characterising the geometry and structure of the aquifer systems of the two basins, mainly through the analysis of lithologs and electrical geophysical investigations (electrical resistivity tomography, ERT). Then, the hydrodynamic properties of the aquifers were determined on the basis of the interpretation of slug tests and the results of pumping tests from the drilling campaigns. This was followed by the characterisation of the interactions between the surface water reservoirs and the groundwater tables through the spatio-temporal analysis of the piezometric chronicles and water levels in the reservoirs coupled with the physico-chemical analyses of the groundwater. The natural and local recharge of groundwater tables was estimated using methods based on the water balance. Finally, the results of these hydrogeophysical and hydrogeological investigations were used to develop a hydrological model integrated with the HydroGeoSphere programme to simulate surface and groundwater flows in the Kierma watershed.
The results indicate that the aquifer system of the watersheds consists of a layer of weathering which integrate alluvium, and a layer of fissured/fractured rock. These aquifers contain hydraulically connected water bodies. The results also show that small surface water reservoirs are in interaction with groundwater bodies while favouring focused recharge. The rate of focused recharge is a function of the geological context and the texture of the sediments in the reservoirs. This approach, applied to the surface water reservoirs of Kierma and Mogtédo permitted to estimate an average focused recharge of 1,350 ± 540 mm/year at Kierma and 1,080 ± 540 mm/year at Mogtédo for average annual natural recharges of 82 mm/year at Kierma and 48 mm/year at Mogtédo. In addition, the hydraulic conductivity values are between 0.01 and 2 m/day at Kierma and between 1×10-4 and 0.2 m/day at Mogtédo. These values of conductivity could make it possible to pump into large diameter dug wells with a significant flow rate of between 0.5 and 120 m3/day in Kierma and between 0 and 10 m3/day in Mogtédo to compensate for the early drying up of the surface water reservoirs for irrigation. In this way, plots of crops with a surface area of between 100 and 22,300 m2 in Kierma and between 0 and 1,800 m2 in Mogtédo could be irrigated daily. All the geological and hydrogeological information made it possible to carry out a numerical simulation that reproduces the observed underground flows and the interactions between and the groundwater.