10. Characterisation of Seasonal Rainfall for Cropping Schedules

Abstract

El Nino-South Oscillation (ENSO) phenomenon occurs in the Equatorial Eastern Pacific Ocean and has been noted to account significantly for rainfall variability in many parts of the world, particularly tropical regions.
This variability is very important in rainfed crop production and needs to be well understood. Thirty years of daily rainfall data (1976–2006 excluding 1991) from the Akatsi District in the Volta Region of Ghana were analysed to observe the variation of rainfall characteristics such as onset and cessation dates, seasonal rainfall amount and their temporal distribution with ENSO phase, namely El Nino, La Nina and Neutral. Using rainfallreference evapotranspiration relationships, the onset of rainfall during La Nina and Neutral seasons occurred within the same period, March 11–20, but about a month late (April 11–20) during El Nino. Without regards to ENSO phase, the long-term mean onset date of the rainy season occurred from March 11–20. Annual and major season (March–July) rainfall amounts decreased in the order of these ENSO phases; La Nina, Neutral and El Nino but showed an opposite decreasing order of El Nino, Neutral and La Nina during the minor seasons (September-November). The trend of variability of rainfall distribution during the major season was observed to be highest during El Nino years and least during Neutral years. The study also showed that the optimum planting periods on 10-day time scales during La Nina, Neutral and El Nino years were found to be March 13–22; March, 17–26 and April 20–29, with March 16–25 for the long-term situation. These observations seem to reveal that long-term or climatological observations alone are no longer sufficient for seasonal rainfall prediction to aid cropping schedules.