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Figure 8.4
: 45-year annual rainfall deviation from the normal for study area.
The water year method uses natural climatic variations for the reference
scenario. A synthetic time series of 20 years was generated based on an
analysis of the recorded rainfall pattern from 1980 to 2000. The natural
climatic variations were chosen to depict the recorded rainfall pattern from
1980 to 2000 for the study area, where both extreme hydrological events
(very dry and very wet years) were experienced (Figure 8.4).
Andreini et al., (2000) showed that the rainfall-runoff pattern of the Volta
Basin is non-linear. The hydrology in the Volta Basin can be described as a
bucket type model/threshold. The model shows that after a particular
rainfall threshold is reached, about half of the additional rainfall becomes
runoff. Therefore, small changes in total rainfall can result in large changes
in total runoff. This accounts for yearly variability in rainfall runoff
coefficients.
The factors used for the climate variability were: very dry year = 0.76, dry
year = 0.89, wet year = 1.09 and very wet year = 1.27. These factors were
estimated by ranking the rainfall figures of Bolgatanga from 1961 to 2005.
The top 20% was classified as being very wet; the next 20% as wet, the
middle 20% being normal, the fourth 20% being dry and the last 20% being
very dry. The average of the ratio between the rainfall figures and the long-
term mean was used as the modifying factor for the classification.
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