Application of Mann Kendal Sen's Slope Estimator in Trend Analysis of Historical And Future Precipitation and Temperature in the Kilombero River Basin

Abstract

This study examines  historical (1981-2020) and future (2020-2070) trends in rainfall and temperature in the Kilombero Basin using the Mann-Kendall method with Sen's slope estimator. Data were obtained from the Tanzania Meteorological Agency and from simulated historical and future climate data sourced from the Coupled Model Intercomparison Project 6 (CMIP6). The CMIP6 datasets were downscaled and bias-corrected using the CMhyd tool. The basin exhibited a bimodal rainfall pattern with an average of 1400 mm, peaking around April. The CMIP6 models successfully simulated monthly rainfall, Tmax, and Tmin at most stations. No definitive trends in rainfall were observed, but Tmax and Tmin showed significant increases under both SSP2-4.5 and SSP5-8.5 scenarios. More warming is predicted under SSP5-8.5 by the mid-21st century, raising Tmax and Tmin at all stations. This rise in temperature could potentially increase evapotranspiration demand, negatively impacting freshwater availability. The average annual rainfall showed a slightly increasing trend post-2000, from 1403.96mm/year (1981-1999) to 1433.38mm/year (2000-2020), an increase of 2.05%. Sen's slope analysis, however, revealed varying trends across stations, with most showing a decreasing trend. Notably, only Ulanga Met Station showed a significant increasing trend with a slope value of 14.70 and a p-value below 0.05. The study concluded that both temperature and precipitation in the Kilombero Basin are on the rise