Land water storage variability over West Africa estimated by GRACE and land surface models

Land water storage plays a fundamental role on the West African water cycle and has an important impact on climate and on the natural resources of this region. However, measurements of land water storage are scarcely available at regional and global scale and, especially, in poorly instrumented endhoreic regions, such as most part of the Sahel, where little useful information can be derived from river flow measurements and basins water budgets. The GRACE satellite mission provides an accurate measurement of the terrestrial gravity field variations from which land water storage variations can be derived. However, its retrieval is not straightforward and different methods are employed to do this, resulting in different water storage GRACE products. On the other hand, water storage can be estimated by land surface modelling but, again, significantly different results can be reached by using different models. In this study, land water storage by six GRACE products and soil moisture estimations by nine land surface models (run in the framework of the AMMA Land Surface Intercomparison Project, ALMIP) are evaluated over West Africa, with a particular focus on the Sahelian area. The water storage spatial distribution, including the zonal transects, its seasonal cycle and its interannual variability are analysed between 2003 and 2007. Despite the not negligible differences within the different GRACE products and within the different models' results, a general good agreement between satellite and model estimates is found over the West Africa study region. In particular, GRACE data are shown to well reproduce the water storage interannual variability over the Sahel for the 5-years study period. The comparison between satellite estimates and ALMIP results allowed the identification of processes needing improvement in the land surface models. In particular, our results point out the importance of well simulating slow water reservoirs as well as evapotraspiration during the dry season for accurate soil moisture modelling over West Africa.