Characterization of the hydrological functioning of the Niger basin using the ISBA-TRIP model

Abstract

During the 70s and 80s, West Africa has faced extreme climate variations with extended extreme drought conditions. Of particular importance is the Niger basin, since it traverses a large part of the Sahel and is thus a critical source of water in this semi arid region. However, the understanding of the hydrological processes over this basin 5 is currently limited by the lack of spatially distributed surface water and discharge measurements. The purpose of this study is to use the ISBA-TRIP continental hydrologic system to explore key processes related to the hydrological cycle of the Niger Basin. The scheme accounts explicitly for the surface river routing, for the floodplains dynamic, and for the water storage using a deep aquifer reservoir. In the current study, 10 simulations are done at a 0.5 by 0.5◦ spatial resolution over the 2002–2007 period using the atmospheric forcing provided by the AMMA Land surface Model Intercomparison Project (ALMIP). The model is intensively compared to in situ discharge measurements as well as satellite derived flood extent, total continental water storage changes and river height changes. The flooding scheme leads to a non-negligible increase of 15 evaporation over large flooded areas, which decrease the Niger river flow by 15% to 50%, according to the observed station and the rainfall dataset used as forcing. This contributes to improve the simulation of the river discharges confirming for the need to incorporate flood representations into Land Surface Model. The model provides a good estimation of the surface water dynamics and accurately simulates the endorheic prop-20 erty of the Northern part of the basin. Moreover, the deep aquifer reservoir improves Niger low flows and the recession law during the dry season. This study also gives a basic estimation of aquifer recharge and of the total terrestrial water budget. The comparison with 3 satellite products from the Gravity Recovery and Climated Experiment (GRACE) is really optimistic and show a non negligible contribution of the deeper soil 25 layers to the total storage (26% for groundwater and aquifer). Finally, sensitivity tests have shown that a good parameterization of routing models is required to optimize simulation errors. Indeed, the modification of some key parameters has non-negligible