Assessment of the Impacts of Climate Change on Surface Water Resources in the Rift Valley Region: a Case Study of Narok County

Abstract

Over the past century, notable changes in precipitation and temperature have been observed, which has been greatly attributed to a change in climate. These changes have altered the climate system and considerably induce changes in the hydrological cycle, eventually affecting the hydrological system. Narok County is important to the country since it supports several farming activities of both livestock and crop farming, has a stretch of the Mau Forest Complex in the Northern region which is a principal water catchment not only in the County but the Country as well, and earns the country an extra income through tourism by the presence of Maasai Mara game reserve. All these are inter-linked such that, affecting one component eventually affects the other, hence the entire ecosystem. The main objective of this study was to assess the impacts of climate change on surface water resources with a focus on Narok County. Data employed include; monthly observed rainfall, temperature, and discharge. Rainfall and temperature datasets were obtained from Kenya Meteorological Department (KMD), which were supplemented with gridded datasets (CHIRPS and ERA5 datasets) from IGAD Climate Prediction and Application (ICPAC) and European Centre for Medium-Range Weather Forecasts (ECMWF) respectively. River discharges were from Water Resource Authority (WRA) and the climate projection datasets were obtained from the ESGF website domain. The historical datasets were for the period 1981-2018, while the projection dataset was for the period 2006-2055. Trend analysis was employed to analyze the past, present, and future patterns climate and hydrology, and tested by Man-Kendall (MK) non-parametric test. PCA was done regionalization and spatial distribution of rainfall evaluated. Spectral estimation was also performed over the study region to identify the cycles of extreme events in the observed rainfall and Probability distribution functions used to assess the change in the Mean, Variance, and skewness in both rainfall and temperature datasets with different time slices with the baseline. The validation was assessed by the use of RMSE, correlation analysis, model BIAS, and standard deviation. To assess the impacts of climate change on surface water availability, the hydrological modeling approach was employed aided by the Water Evaluation and Planning Model (WEAP), with an evaluation of two scenario approach; Synthetic scenarios and General Circulation Model Scenarios. From the results obtained in this study, six homogeneous rainfall zones were delineated, each with distinct climate characteristics. All zones exhibited high variability of rainfall in space and vi time with the dry months (Dec-Feb, Jun-Aug) recording the highest, while the lowest variability recorded during the wet seasons (March-May) and (Sept-Nov). The region also exhibits a bimodal rainfall pattern with much of the rains received in April during the long rain season (MAM) and short-rain season (SON), where rivers in this region follow the patterns of rainfall. Rainfall was also unevenly distributed, with the western region receiving much rain (1400mm) annually compared to the East and Central regions. Spectral estimation over the region identified three dominant spectral peaks; 2-3.2, 4-5.5and 6.5- 10 years cycles which are attributed to Quasi- Biennial Oscillations (QBO), El-Nino, and solar variability respectively. The future climate projections over the region were provided by two climate change scenarios; RCP4.5 and RCP8.5, with a multimodel Ensemble mean having the best skill in projecting the future climate. It was seen that there was a likelihood of the future climate being warmer and drier as seen in the significant increasing trends in temperatures in both scenarios. A shift in the mean values of both rainfall and temperature indicates a changing climate and this will inimically affect water availability (yields), and this calls for proper adaptation strategies in water management in the County. There will be a general decrease of water quantity in the region in both scenarios; -30% by 2030 and -23.45% by 2055. In comparison, RCP4.5 and Scenario3 (+2.5C, +10%P) had higher than RCP8.5 and Scenario2 respectively. There was also a clear indication that the region was highly sensitive to a perturbation in climate from the Synthetic scenarios. A change in either rainfall or temperature (or both) could lead to an impact on the amount of surface water yields. Findings from this study may be adopted in the water sector for management and both long and short-term development planning, evaluation and monitoring of surface water resources, and assessing the vulnerability of the county to a change in climate that will help the relevant authorities to come up with ideal adaptation and mitigation measures. WEAP hydrological model is recommended for Water Evaluation and Planning and assessment of the impacts of climate change in the county to improve on water-related issues. Policymakers can also adopt results from this study to solve issues related to water resources in the County, and further studies on incorporating other factors such as population, water demands, groundwater, and land use/land cover to assess the impacts of climate change in the region are recommended.