RAINFALL VARIABILITY AND TREND ANALYSIS IN OYO STATE, SOUTHWESTERN NIGERIA (1984–2024)

Abstract

Rainfall variability remains a major source of climate uncertainty in tropical regions, particularly in 
southwestern Nigeria where livelihoods, agriculture, and water resources are highly dependent on seasonal 
rainfall. This study investigates rainfall variability, temporal trends, and climate uncertainty in Oyo State, 
southwestern Nigeria, over a 41-year period (1984–2024). The research utilizes high-resolution ERA5-Land 
reanalysis rainfall data obtained from the Copernicus Climate Change Service (C3S) and processed within 
Google Earth Engine. Analytical techniques include descriptive statistics, coefficient of variation (CV), 
skewness and kurtosis analysis, and non-parametric trend detection using the Mann–Kendall test and Sen’s 
slope estimator. Results reveal pronounced inter-annual variability in rainfall, with values ranging from 486 
mm to 1,498 mm. Descriptive statistics indicate moderate variability (CV = 0.2) and a negatively skewed 
distribution (skewness = −1.2), suggesting a higher frequency of wetter-than-average conditions interspersed 
with occasional extreme dry events. Trend analysis shows a statistically significant downward trend in annual 
rainfall (Z = −3.29, p < 0.01), with Sen’s slope indicating a decline of approximately 8.29 mm per year (≈82.9 
mm per decade). This reflects a persistent long-term reduction in rainfall over the study period. Spatial
temporal analysis further highlights a shift from relatively stable and wetter conditions in the 1980s–1990s to 
increasingly variable and drier conditions in recent years, particularly after 2015. The findings highlight the 
unpredictable nature of rainfall and climate uncertainty in Oyo State, with important implications for 
agriculture, water resource management, and climate adaptation planning.