Evaluation of the ISBA-TRIP continental hydrologic system over the Niger basin using in situ and satellite derived datasets

摘要

During the 1970s and 1980s, West Africa has faced extreme climatevariations with extended drought conditions. Of particularimportance is the Niger basin, since it traverses a large part of theSahel and is thus a critical source of water for an ever-increasinglocal population in this semi aridregion. However, the understanding of the hydrological processes overthis basin is currently limited by the lack of spatially distributedsurface water and discharge measurements. The purpose of this study is toevaluate the ability of the ISBA-TRIP continental hydrologic system torepresent key processes related to the hydrological cycle of the Nigerbasin. ISBA-TRIP is currently used within a coupled global climate model,so that the scheme must represent the first order processes which arecritical for representingthe water cycle while retaining a limited number of parameters and asimple representation of the physics. To this end, thescheme uses first-order approximations to accountexplicitly for the surface river routing, thefloodplain dynamics, and the water storage using a deep aquiferreservoir. In the current study, simulations are done at a 0.5 by0.5° spatial resolution over the 2002–2007 period (in order totake advantage of the recent satellite record and data from the AfricanMonsoon Multidisciplinary Analyses project, AMMA). Fourconfigurations of the model are compared to evaluate the separateimpacts of the flooding scheme and the aquifer on the water cycle. Moreover, the model is forced by two different rainfall datasets toconsider the sensitivity of the model to rainfall inputuncertainties. The model is evaluated usingin situ discharge measurements as well as satellite derived floodextent, total continental water storage changes and river heightchanges. The basic analysis of in situ discharges confirms theimpact of the inner delta area, known as a significant flooded area, on the discharge, characterizedby a strong reduction of the streamflow after the delta compared tothe streamflow before the delta. In the simulations, the floodingscheme leads to a non-negligible increase of evaporation over largeflooded areas, which decreases the Niger river flow by 15% to50% in the locations situated after the inner deltaas a function of the input rainfall dataset used as forcing.This improves the simulation of the river discharge downstream of thedelta, confirming the need for coupling the land surface scheme with the flood model. The deep aquifer reservoir improves Niger lowflows and the recession law during the dry season.The comparison with 3 satellite products from theGravity Recovery and Climated Experiment (GRACE)shows a non negligible contribution of the deeper soil layers tothe total storage (34% for groundwater and aquifer). Thesimulations also show a non negligible sensitivity of the simulations torain uncertainties especially concerning the discharge. Finally,sensitivity tests show that a good parameterization of routingis required to optimize simulation errors. Indeed, themodification of certain key parameters which can be observed fromspace (notably river height and flooded zones height changes andextent) has an impact on themodel dynamics, thus it is suggested that improving the model inputparameters using future developments in remote sensing technologiessuch as the joint CNES-NASA satellite project SWOT (Surface Water OceanTopography), which will provide water heights and extentat land surface with an unprecedented 50–100 m resolution and precision.