Assessment of the Onset and Cessation of the Rainy Season in Kebbi State

The dates of rainy season onset and cessation are critical meteorological parameters that directly govern planting calendars and cropping decisions for farmers across Nigeria's North West zone. This study assesses temporal variability in rainy season onset and cessation dates in Kebbi State using daily rainfall records from NiMet stations in Birnin Kebbi and Yauri covering the period 2010 to 2023. Three operational definitions of onset, including the agronomic, meteorological, and modified Stern definitions, were applied and compared to assess sensitivity to definitional choice. Linear regression analysis and Mann-Kendall tests were used to identify trends in onset and cessation timing across the study period. Results indicate that the agronomic definition yields the latest mean onset date of 19 May, while the meteorological definition produces a mean onset of 28 April, with significant year-to-year variability across all definitions. A statistically significant trend toward later onset and earlier cessation, effectively shortening the growing season by an estimated 9 days over the 13-year period, is identified. Qualitative survey data from 120 farmers in Gwandu and Koko LGAs confirm widespread perception of shifting rainfall seasons and resulting adjustment of traditional planting dates. The study recommends adoption of a standardized national onset definition and delivery of LGA-level onset forecast advisories by NiMet to aid farmer planning. Keywords: rainy season, onset, cessation, Kebbi, agrometeorological.