Hydrological Cycles over the Congo and Upper Blue Nile Basins: Evaluation of General Circulation Model Simulations and Reanalysis Products

Abstract

he simulations and predictions of the hydrological cycle by general circulation models (GCMs) are characterized by a significant degree of uncertainty. This uncertainty is reflected in the range of Intergovernmental Panel on Climate Change (IPCC) GCM predictions of future changes in the hydrological cycle, particularly over major African basins. The confidence in GCM predictions can be increased by evaluating different GCMs, identifying those models that succeed in simulating the hydrological cycle under current climate conditions, and using them for climate change studies. Reanalyses are often used to validate GCMs, but they also suffer from an inaccurate representation of the hydrological cycle. In this study, the aim is to identify GCMs and reanalyses' products that provide a realistic representation of the hydrological cycle over the Congo and upper Blue Nile (UBN) basins. Atmospheric and soil water balance constraints are employed to evaluate the models' ability to reproduce the observed streamflow, which is the most accurate measurement of the hydrological cycle. Among the ECMWF Interim Re-Analysis (ERA-Interim), NCEP?NCAR reanalysis, and 40-yr ECWMF Re-Analysis (ERA-40), ERA-Interim shows the best performance over these basins: it balances the water budgets and accurately represents the seasonal cycle of the hydrological variables. The authors find that most GCMs used by the IPCC overestimate the hydrological cycle compared to observations. They observe some improvement in the simulated hydrological cycle with increased horizontal resolution, which suggests that some of the high-resolution GCMs are better suited for climate change studies over Africa.