Impacts of climate change on Blue Nile flows using bias-corrected GCM scenarios

M E Elshamy,I A Seierstad,A Sorteberg

doi:10.5194/hess-13-551-2009

M E Elshamy, I A Seierstad + Show 1 more

Open Access

https://doi.org/10.5194/hess-13-551-2009

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Abstract

Abstract. This study analyses the output of 17 general circulation models (GCMs) included in the 4th IPCC assessment report. Downscaled precipitation and potential (reference crop) evapotranspiration (PET) scenarios for the 2081–2098 period were constructed for the upper Blue Nile basin. These were used to drive a fine-scale hydrological model of the Nile Basin to assess their impacts on the flows of the upper Blue Nile at Diem, which accounts for about 60% of the mean annual discharge of the Nile at Dongola. There is no consensus among the GCMs on the direction of precipitation change. Changes in total annual precipitation range between −15% to +14% but more models report reductions (10) than those reporting increases (7). Several models (6) report small changes within 5%. The ensemble mean of all models shows almost no change in the annual total rainfall. All models predict the temperature to increase between 2°C and 5°C and consequently PET to increase by 2–14%. Changes to the water balance are assessed using the Budyko framework. The basin is shown to belong to a moisture constrained regime. However, during the wet season the basin is largely energy constrained. For no change in rainfall, increasing PET thus leads to a reduced wet season runoff coefficient. The ensemble mean runoff coefficient (about 20% for baseline simulations) is reduced by about 3.5%. Assuming no change or moderate changes in rainfall, the simulations presented here indicate that the water balance of the upper Blue Nile basin may become more moisture constrained in the future.

Highlights

It is widely accepted that Global Circulation Models (GCMs) are the best physically based means for devising climate scenarios
Results based on the output of 17 general circulation models (GCMs) for the SRES A1B emissions scenario are used to investigate the impacts of climate change on the upper Blue Nile flow at Diem
potential (reference crop) evapotranspiration (PET), and actual evapotranspiration (AET) are summarized as mean areal averages over the basin area

Summary

Introduction

It is widely accepted that Global Circulation Models (GCMs) are the best physically based means for devising climate scenarios. They reproduce the global and continental scale climate fairly well The main reason for this gap, between the spatial scale of GCM output and that needed for impact studies, is the coarse spatial resolution of GCMs which restricts their usefulness at the grid-size scale and smaller (Wilby and Wigley, 1997). GCM experiments show very different pictures of climate change over the Nile basin. While they all agree on a temperature rise, they disagree on the direction of precipitation change. Most of the analyzed experiments showed an increase in precipitation over the basin (up to 18%), some experiments showed a reduction (up to 22%), while one experiment showed almost no change (Elshamy, 2000)

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