Integrating Climate Change Scenarios In The Hydrological Study Of Thika River, Kenya

Abstract

One of the most important factors influencing hydrological processes in a river basin is its weather/climate. The main objective of this study was to create a hydrological response model under changing rainfall and temperature conditions and specifically to determine climate and discharge trends and create a rainfall-runoff model for the Thika River. Data used were mainly from secondary sources. Time series analysis was used to determine climate and stream-flow trends. Thika Riverwatershed was delineated using Soil Water Assessment Tool (SWAT) with Digital Elevation Model (DEM) data, and simulated historical and projected streamflow using climate scenarios for RCP 4.5 and RCP 8.5. Double Mass Curves were used to test homogeneity and all the data used in the study depicted a straight line plot showing that they were good for the study. Results showed an increase in both minimum and maximum temperatures and decrease in annual and monthly rainfall in all stations except at Thika Agro Met Station. This was found out that Thika Agro Met station was located in a different agro climatological zone as the other stations. Mean annual streamflow analysis depicted a negative trend. However, rainfall projection under RCP 4.5 and RCP 8.5which represented medium and high emission scenarios respectively showed decrease in rainfall in the climate period 2021-2050. However, results from standardized rainfall anomalies for climate period of 2061-2080 showed an increase in rainfall in all the stations in the catchment area under all the emission scenarios except in Thika Agro Met where rainfall is expected to have a negative trend under RCP 8.5. Results from temperature projection showed that, the period 2020-2057 is expected to be cooler than average while climate period 2060-2100 temperature anomalies were found to be positive with the highest anomalies expected to be towards the end of 2100. Using SWAT model, water yields decreases in early 2020 and 2030s and increases in 2061-2080. There is an increase in water yields in early 2020 and 2030s in the catchment, decrease trend in early 2060s and an increase towards the end of 2070s during (2061-2080). The highest water yield in the catchment is expected to be in the year 2077 while the lowest water yield is expected to be in the year 2066. From this study it was concluded that changing climate could have an impact on water and the obtained results could be useful for water resources management and policy making so that climate change could be incorporated in the plans and management for sustainable water supply to local residents and other cities such as Nairobi and other satellite towns