Forschungszentrum Karlsruhe - Wissenschaftliche Berichte - FZKA 7240
Regional Climate Change
and the Impact on Hydrology in the Volta Basin of West Africa
G. Jung
Abstract
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 soilatmosphere
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
1.2-1.3ºC 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.
Regionaler Klimawandel
und der Einfluss auf die Hydrologie des Volta Beckens in West Afrika
Zusammenfassung
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