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 predictio n and subsequent management of water for rainfed crop production in Gh ana was explored. Five sites were selected to represent the major vege tation zones in Ghana, three of which were located in the south and tw o in the north. For most sites, the occurrence of severe drought coinc ided with negative SOI (SOI below zero). It was also apparent that whe n the SOI was very low (below - 10) for long periods all five sites ex perienced severe drought. Generally, recovery from drought was very sl ow. In terms of agricultural productivity, some two seasons's crops wo uld be adversely affected once a drought phase commenced before rainfa ll became normal again. Correlation between total seasonal rainfall an d 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 obs ervations in water management was limited owing to the low correlation s. However, lagged relationships between the SOI in March and the rain fall 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, fo r 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 i n June and vice versa. Hence appropriate water management strategies ( e.g. supplementary irrigation or drainage) could be planed ahead of ti me depending on the SOI in March/April. The SOI-phase system showed hi gh promise of rainfall prediction and water management in southern Gha na. In both simultaneous and lagged situations, positive or rising pre -seasonal SOI phases correlated well with above-average rainfall and w hile negative or falling SOI patterns resulted in below-average rainfa ll (P < 0.05). The use of SOI-phase system for water management is dis cussed in the text.