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Universität Augsburg (2006)

Regional Climate Change and the Impact on Hydrology in the Volta Basin of West Africa

Jung Gerlinde

Titre : Regional Climate Change and the Impact on Hydrology in the Volta Basin of West Africa

Regionaler Klimawandel und die Auswirkung auf die Hydrologie im Volta Becken Westafrikas

Auteur : Jung Gerlinde

Université de soutenance : Universität Augsburg

Grade : Doctoral Thesis 2006

Résumé
The Volta Basin is a climate sensitive, semi-arid to sub-humid region in West Africa. Livelihood of the population is mainly dependent on agriculture and therefore highly vulnerable to rainfall variability and climate change. For an investigation of the impact of a possible global climate change to regional climate and surface, as well as sub-surface hydrology in the region of the Volta Basin, coupled regional climate-hydrology simulations were performed. Therefore, the mesoscale meteorological model MM5 was set up, fully coupled to a 1-dimensional SVAT (Soil Vegetation Atmosphere Transfer) model, to account for soil-atmosphere feedback mechanisms. After a validation was performed, MM5 was used as a regional climate model to simulated two 10-years time slices : 1991-2000 and 2030-2039. The emission scenario IS92a output of the global climate model ECHAM4 was downscaled dynamically, to a final resolution of 9km, for the Volta Basin. These regional climate simulations were then coupled to the physically based, distributed hydrological model WaSiM, after the calibration and adaptation of the hydrological model to the study region. A comparison the GCM output, as well as the RCM output for present-day climate simulation to observations showed a wet bias over the Sahel and a sufficient accuracy in temperature representation for the ECHAM4, present-day simulation (1961-1990). In the regional climate simulations, the displacement of the Inter Tropical Discontinuity (ITD) to the North at the beginning of the rainy season, as well as the displacement South, at the end occur too early. Rainfall also showed a negative deviation along the coast but a sufficient accuracy in the Volta Basin. The results of the MM5 and WaSiM simulations show an annual mean temperature increase by 1.2-1.3°C in West Africa and the Volta Basin. This temperature change significantly exceeds inter-annual variability. Mean annual precipitation increases for both, the sahelian and the coastal region of West Africa. Averaged over the region of the Volta Basin, this increase is about 5%. Only in the Sahel, the mean annual change signal exceeds simulated inter-annual variability. Spatially the increase is highly heterogeneous, reaching from -20% to +50%. A dipole pattern of rainfall variability in the Sahel and the Guinea Coast region was detected for June and July. An overall increase in precipitation was found for September, and a strong decrease for April. Causes for the rainfall variability were found in the dynamics of the Tropical Easterly Jet (TEJ), the African Easterly Jet (AEJ) and in the position of the ITD. For the Volta basin it is demonstrated, that the decrease in April at the beginning of the rainy season is not only connected to smaller rainfall amounts, but also to a delay in the onset of the rainy season. In addition, inter-annual variability in the Volta Basin increases in the early stage of the rainy season, while annual mean aridity in the Volta Basin does not change significantly. No significant changes in discharge follow the precipitation decrease at the onset of the rainy season. During the rainy season, most of the surplus rainfall evaporates, due to the increase in potential evaporation, as a consequence of higher near surface air temperatures. The study demonstrates the ability of the coupled modelling system to reasonably simulate West African climate and hydrology conditions. For the selected scenario and time slices, the change signal in precipitation, as well as surface and subsurface hydrology variables lies with few exceptions, within the range of inter-annual variability, whereas temperature shows a clear increase

Mots clés : Hydrologie ; Klimamodellierung ; MM5 ; Volta ; WaSiM ; Wasserverfügbarkeit ; Westafrika Volta ; West Africa ; climate ; climate modelling ; hydrology ; meteorology ; modelling ; water availability

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Page publiée le 5 novembre 2014, mise à jour le 6 janvier 2019