Hydrology

As indicators of the impact of climate change on the hydrology of the basin, we use average annual runoff into Lake Volta and the variability in annual runoff as expressed by the coefficient of variation (= standard deviation/mean). A dry year is defined as a year with less than 75% of the average runoff during the period under consideration.

Table 9.2 and Fig. 9.3 show the inflow into Lake Volta for the historical and future periods using the WEAP hydrological model (WEAP, 2002). WEAP used both the A2 and B2 climate change scenario as input. The results show an important increase in runoff under each scenario. The underlying relative increases in rainfall were much less, but the non-linear response to slight absolute increases in rainfall causes the dramatic rise in runoff into the lake. The Hadley A2 simulation for 2070—2099 shows less increase and even a slight decrease in the coefficient of variation. Interestingly, the other simulations show an important decrease in the coefficient of variation. The B2 scenarios show in general a more important increase than the A2 scenarios, something that does not concur with the general idea that increase in temperature (which is higher under A2 than under B2) accelerates the hydrologi-cal cycle.

Inflow Lake Volta

Inflow Lake Volta

Fig. 9.3. Comparison of modelled flows for mid-term (2020-2039) and historical (1961-1990) periods using output from the Hadley A2 and B2 models.

Years of simulation

Fig. 9.3. Comparison of modelled flows for mid-term (2020-2039) and historical (1961-1990) periods using output from the Hadley A2 and B2 models.

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