Assessment of Long-term Spatio-temporal Rainfall Variability over Ghana usingWavelet Analysis

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May 2016
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Rainfall variability has serious implications on food security and livelihood in West Africa since it modulates the socio-economic activities in the sub region. The interannual, inter-seasonal and inter-decadal rainfall variability in rainfall over Ghana has been studied and their periodicities analysed using wavelet analysis. A rainfall time series from 1901 - 2010 from GPCC was used in this analysis. It was observed that, high cumulative rainfall amounts ranging from 900 - 1900 mm are recorded over the entire nation per year with very high rainfall amounts between 1500 - 1900 mm recorded at the south-western part of the country and low rainfall amounts (900 - 1200 mm) recorded in the Savannah and east coast of the country. In general a decreasing trend is observed for the inter-annual rainfall over all the agro-ecological zones except for the coastal zone where a slight rise in trend of 0.1600 mm per year is seen. The seasonal trend analysis reveals a significant decreasing trend at 99% confidence level in all the agro-ecological zones except for the Savannah during the DJF season indicating an intensification of the Harmattan. The Coastal zone records lowest mean rainfall values for all the seasons with the highest of about 150 mm recorded in MAM. The Forest zone on the other hand records very high rainfall values for all the seasons with the highest of about 200 mm recorded in JJA followed by about 170 mm in MAM. The Transition zone however records almost similar rainfall values (varying between 120 and 170 mm) for all the seasons except for DJF where a lower mean value of about 50 mm is recorded. On inter-decadal time scale, below normal rainfall values is observed between the 1901 - 1920 and 1980 - 2010 periods for all the agro-ecological zones except for the Savannah which shows above normal rainfall values within the 1901 - 1940 period. These variabilities confirm the effects of strong El Niño and La Niña episodes on the region. The wavelet analysis also revealed a strong annual periodicity over all the agro-ecological zones except for the Coastal and Forest zones where the annual periodicity is accompanied by 4 - 8 months signal. The results of both the 5 year moving average and the inter-decadal anomaly confirm that rainfall amount is significantly decreasing even in the transition zone which is the food hub of the country. This will likely have negative consequences on agricultural practices, water resource management and food security.
A Thesis Submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology, in partial fulfillment of the requirements for the degree of Master of Philosophy (Meteorology and Climate Science) College of Science,