AN ANALYSIS OF THE WITHIN-SEASON RAINFALL CHARACTERISTICS AND SIMULATION OF THE DAILY RAINFALL IN 2 SAVANNA ZONES IN GHANA

This paper has investigated the within-season rainfall variability at selected sites (Accra and Tamale) in two savanna zones of Ghana. The a verage duration of the dry and wet spells was estimated for each site from the long-term rainfall probability parameters. At Accra, the aver age duration of dry spells decreases from March (7 days) to June (2 da ys) and rises again in July (5 days). At Tamale, the average length of a dry spell during the rainy season is about 3 days. The observed pat terns of rainfall at the two sites seem to suggest that Tamale would o ffer more favourable conditions for rainfed crop production. However, a complete answer to such an important question needs to consider the water balance at each site including the water storage ability of the soils. A rainfall simulation model was also developed for the two site s, and simulated the rainfall patterns and amounts reasonably well at both sites. Simulated number of rainy days in each month of the rainy season for both; the short and long term agreed well with observations at Accra (R-2 = 0.98) and Tamale (R-2 = 0.95), provided the appropria te short- or long-term model parameters were used. This was also the s ituation for the simulated monthly rainfall totals (Accra R-2 = 0.97; Tamale R-2 = 0.97). It was, however, obvious that the model parameters derived from the long-term rainfall records could not be used to simu late the short-term rainfall fluctuations which were commonly observed , especially at Accra. In a quest to overcome this deficiency, we soug ht possible relations between the rainfall parameters and other global factors that affect or modify the local rainfall patterns at the site s. In particular, the study focused on the relationship between the ra infall parameters and the Southern Oscillation Index (SOI). A correlat ion coefficient as high as 0.78 was established between the SOI and so me of the model parameters. Thus, it appears possible to use informati on from a series of the SOI values to modify the numerical values of t he model parameters before their use in the simulation runs. Prospects of using such an approach to improve the simulation of rainfall are d iscussed. (C) 1997 Published by Elsevier Science B.V.