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Impacts of climate on water-use and water-use efficiency of woodlands in the Sudanese Sahel region: a modelling study
Syed Ashraful Alam and Mike Starr

We modelled the impact of various climate change and emission scenarios on the water-use (WU) and water-use efficiency (WUE) of woodlands across the Sudanese Sahel region for the 2080s. Modelled mean scenarios of climate variables (temperature, rainfall and cloud cover) were derived using 5 GCMs (CGCM2, CSIRO2, ECHam4, HadCM3 and PCM) run with A1FI (greatest climate change) and B1 (least climate change) SRES emission scenarios. Baseline (1961-1990) and scenario (2070-2099) climate data for 8 grids (1.0º latitude x 1.5º longitude), representing climate conditions across the region, were generated (TYN, UEA) grid data. A water balance model, WATBAL, was parameterized for the Acacia dominated woodland vegetation and two dominating soil types, arenosols (AR) and vertisols (VR), to give monthly actual ET values for the baseline data and each climate change scenario dataset. WU is assumed to be equal to ET and WUE (g C m-2 mm-1) was calculated as grid mean above-ground biomass C density (g C m-2) divided by ET (mm). Baseline annual rainfall ranged from 2 to 55 mm and temperature from 23.3 to 29.1 °C. Mean annual air temperature increased under all scenarios and for all grids (+1.2 to +8.3 °C), while rainfall either increased (+9 to +18 mm) or decreased (-1 to -16 mm), depending on scenario and grid. Baseline WU varied from 140 to 595 mm and WUE from 0.106 to 0.462 g C m-2 mm-1, depending on grid and soil type. Compared to AR soils, VR soils had equal or greater WU and equal or less WUE. Depending on scenario and grid, the relative WU varied from 0.54 to 1.88 for VR soils and from 0.61 to 1.89 for AR soils while that of relative WUE varied from 0.50 to 2.44 for VR soils and from 0.46 to 2.22 for AR soils. Our results indicate that future WU and WUE in the Sahel region will strongly depend on the nature of climate change and on the degradation of woodlands related to increasing population pressure.