Long-Term Temperature and Rainfall Trends in Nakuru County, Kenya: Evidence of Climate Variability from 1990 to 2024

Abstract

Climate variability is one of the major environmental challenges affecting agricultural productivity, water resources, and livelihoods in Kenya. Understanding long-term changes in rainfall and temperature is essential for developing effective climate-adaptation strategies at the county level. This study analysed trends in annual rainfall and maximum, minimum, and mean temperatures in Nakuru County, Kenya, over the 35-year period from 1990 to 2024. It determined the trend in annual mean temperature, examined the trend in annual rainfall, assessed the relationship between annual temperature and rainfall, and evaluated whether significant differences occurred in these variables over the study period. Data from the Kenya Meteorological Department were obtained from the Molo and Subukia meteorological stations. Before trend analysis, the Durbin–Watson test confirmed the absence of significant first-order serial autocorrelation in the rainfall series (Molo: DW = 1.9823, p = 0.412; Subukia: DW = 1.9567, p = 0.389), supporting the use of the Mann–Kendall trend test and Sen's slope estimator. Pearson correlation, analysis of variance (ANOVA), and descriptive statistics were also applied. Annual rainfall increased significantly at Molo (\(\tau\) = 0.297, p = 0.012) and Subukia (\(\tau\) = 0.311, p = 0.009), despite marked interannual variability. Maximum temperatures increased significantly at both stations (Molo: \(\tau\) = 0.509, p < 0.001; Subukia: \(\tau\) = 0.459, p < 0.001), while minimum temperatures showed the strongest warming trends (Molo: \(\tau\) = 0.637, p < 0.001; Subukia: \(\tau\) = 0.513, p < 0.001). Annual mean temperature also increased significantly (Z = 2.87, p = 0.004), and temperature and rainfall showed a weak but significant negative correlation. The findings indicate sustained warming and variable rainfall in Nakuru County and support strengthened climate-smart agriculture, climate information services, water-resource management, and county-level adaptation planning.