The terrestrial water budget over a particular hydrological domain (e.g., a drainage basin) plays an important role in understanding the interactions among the energy and water cycles. In this work, we assess the closure of the water budget methods [i.e., terrestrial water budget (TWB) and the coupled atmospheric-terrestrial water budget (ATWB)] as well as the control of total water storage gain over Volta Basin of West Africa. To achieve this, we explore the available satellite and data products: GRACE-derived terrestrial water storage changes (TWSC), satellite altimetry, TRMM-measured rainfall, MODIS-estimated evaporation, atmospheric moisture storage and divergence (ERA-Interim reanalysis data), and in situ discharge. The closure of water budgets are assessed by comparison with GRACE-derived TWSC. The results indicate that the closure of water budgets over the Basin, considering the agreement with GRACE-derived TWSC, present an RMSE of 33.72 and 48.22 mm/month for TWB and ATWB, respectively. This implies that the net precipitation (precipitation minus evaporation) computed from ERA-Interim reanalysis data presents high uncertainties over the Basin. Further, the significant accumulated water gain of 162.84 mm for the period of January 2003-December 2012, is 48% controlled by Lake Volta.

Abstract:The potential of terrestrial water storage (TWS) inverted from Gravity Recovery and Climate Experiment (GRACE) measurements to investigate water variations and their response to droughts over the Volta, Niger, and Senegal Basins of West Africa was investigated. An altimetry-imagery approach was proposed to deduce the contribution of Lake Volta to TWS as "sensed" by GRACE. The results showed that from April 2002 to July 2016, Lake Volta contributed to approximately 8.8% of the water gain within the Volta Basin. As the signal spreads out far from the lake, it impacts both the Niger and Senegal Basins with 1.7% (at a significance level of 95%). This figure of 8.8% for the Volta Basin is approximately 20% of the values reported in previous works. Drought analysis based on GRACE-TWS (after removing the lake's contribution) depicted below-normal conditions prevailing from 2002 to 2008. Wavelet analysis revealed that TWS changes (fluxes) and rainfall as well as vegetation index depicted a highly coupled relationship at the semi-annual to biennial periods, with common power covariance prevailing in the annual frequencies. While acknowledging that validation of the drought occurrence and severity based on GRACE-TWS is needed, we believe that our findings shall contribute to the water management over West Africa.