Runoff Dynamics and Its Regime Changes in the Major River Basins of Africa From GRACE and GRACE-FO Observations

The major African basins supply freshwater to around 0.5 billion people, while monitoring runoff fluctuations in these basins is still challenging due to limited in-situ data and high costs. Hydrologic models are widely used for this purpose, but they have certain drawbacks with larger uncertainty and low accuracy in poorly gauged basins. This study attempts to address this issue by using Gravity Recovery and Climate Experiment (GRACE)/GRACE Follow-On (GRACE-FO) satellite measurements and remote sensing data to estimate runoff changes in five major African basins from 2003 to 2019. Moreover, a comprehensive framework was developed to quantify interannual and intraannual runoff regimes and their changes from the perspectives of magnitude, variability, and duration. Results showed that runoff changes estimated from GRACE/GRACE-FO in the major African basins were in good alignment with those from the Global Land Data Assimilation System, European Centre for Medium-Range Weather Forecasts Reanalysis 5, and Soil Conservation Service-Curve Number. Seasonal runoff increased in the Nile (0.12 to 0.46 mm/a, p < 0.05) and Congo (0.52 to 0.76 mm/a, p < 0.05) basins during 2003–2019, while it was decreased in the Zambezi (–1 to –0.34 mm/a, p < 0.05) and Orange (–0.54 to –0.24 mm/a, p < 0.05) basins during the same period. Further in-depth analysis showed that the impacts of climate change in the study area were the primary contributors to changes in runoff. Monthly runoff in the Nile, Congo, and Niger basins showed an increase in magnitude, duration, and variability. In contrast, the Zambezi and Orange basins experienced a decrease in runoff magnitude, along with reduced variability and duration. In addition, large-scale atmospheric circulations, such as El-Nino Southern Oscillation Index and Indian Ocean Dipole, have been found to be associated with changes in runoff within the study area, as demonstrated by correlation and wavelet analysis. Our findings provided valuable insights into long-term runoff changes in major African basins and enhanced the understanding of hydrologic processes in poorly gauged regions.