Using the Southern Oscillation Index for improving rainfall prediction and agricultural water management in Ghana

The use of the Southern Oscillation Index (SOI) for rainfall prediction and subsequent management of water for rainfed crop production in Ghana was explored. Five sites were selected to represent the major vegetation zones in Ghana, three of which were located in the south and two in the north. For most sites, the occurrence of severe drought coincided with negative SOI (SOI below zero). It was also apparent that when the SOI was very low (below - 10) for long periods all five sites experienced severe drought. Generally, recovery from drought was very slow. In terms of agricultural productivity, some two seasons's crops would be adversely affected once a drought phase commenced before rainfall became normal again. Correlation between total seasonal rainfall and the average seasonal SOI was generally low at all sites and weakened from the south (r= 0.3) to the north (r= - 0.09),The use of these observations in water management was limited owing to the low correlations. However, lagged relationships between the SOI in March and the rainfall onset date in the south showed a fairly high inverse correlation (r= - 0.61). The onset was delayed with negative SOI in March and was brought forward with positive SOI in March, It was also shown that, for two sites in the south, rainfall at the peak crop water demand month (June) was significantly affected by the SOI in March/April SOI (P< 0.05). Positive SOI in March/April resulted in above average rainfall in June and vice versa. Hence appropriate water management strategies (e.g. supplementary irrigation or drainage) could be planed ahead of time depending on the SOI in March/April. The SOI-phase system showed high promise of rainfall prediction and water management in southern Ghana. In both simultaneous and lagged situations, positive or rising pre-seasonal SOI phases correlated well with above-average rainfall and while negative or falling SOI patterns resulted in below-average rainfall (P < 0.05). The use of SOI-phase system for water management is discussed in the text.