ENSO Episodes and its Impacts on Seasonal Rainfall Characteristics with Reference to Crop Production in Kenya
Bosire, Emily Nyaboke
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EI Nino-Southern Oscillation (ENSO) plays an important role in the interannual variability of rainfall in Kenya. The effects of ENSO on the seasonal rainfall characteristics were investigated. The seasonal rainfall characteristics of concern are the onset, cessation, duration of seasonal rainfall and seasonal rainfall total. The determination of these seasonal rainfall characteristics is useful information for the planning of land preparation and planting activities. The seasons considered were March-April-May (MAM) and October-November-December (OND). Daily rainfall data were collected from eleven synoptic meteorological stations representing homogeneous climatic zones for both MAM and OND seasonal rainfall. The seasonal rainfall characteristics were determined for each station for all the years using the INSTAT software. The onset dates were determined using two conditions: considering a dry spell and not considering a dry spell within the growing season. The cessation dates were determined using the water balance approach; the duration of the seasonal rainfall was determined by subtracting the onset dates from the cessation dates. Lastly the total seasonal rainfall was obtained by cumulating daily rainfall from March-May and October-December for the MAM and OND season, respectively. The average onset, cessation, duration and seasonal rainfall totals were determined for each of the stations for the EI Nino years, La Nina years and all the years. The overall mean of onset date, cessation date,duration and seasonal rainfall totals were compared with mean values during the EI Nino and La Nina years. The differences between the average of the rainy season index obtained from all the years and index during EI Nino or La Nina years were obtained. Positive anomalies show earlier than normal onsets and cessation dates while negative anomalies show later than normal onsets and cessation dates. The results of the onset of rains show no clear pattern within the area of study with some areas experiencing late onset and others early onset in all the years and also during EI Nino and La Nina years.Cessation of rains occurs late during the EI Nino and early in La Nina years over most parts of the country.However, the cessation pattern is also not clear over most of the stations within the study area. Positive and negative anomalies show longer duration and shorter duration during both ENSO episodes.The duration of the seasonal rainfall is longer than normal during EI Nino years and shorter than normal during La Nina years. Seasonal rainfall total is also higher than normal in EI Nino years while in La Nina years seasonal rainfall total1s lower than normal.From these results we can conclude that 'crop production will vary from place to place within the study area.Crop production will be favoured duriflg normal years. During EI Nino and La Nina years there will be a reduction in cro-p pro-duction as a result of excess and lack of enough water, respectively within the soils.