Reservoir sedimentation in an arid and semi-arid river basin; a case study of Kalundu dam in Kitui County, Kenya

Abstract

In Kenya, several water reservoirs located in arid and semi-arid lands (ASALs) are undergo the trouble of accumulation of sediments. Kalundu Dam in Kitui County is a typical reservoir located in ASAL that has been experiencing periodic siltation since 1950s when it was commissioned. However, the patterns of siltation within the dam including the hydrological processes influencing sedimentation processes have not been investigated in this reservoir as with other reservoirs in Kenya’s ASALs. Lack of data on the hydrological processes and land use practices has narrowed execution of strategies for controlling sedimentation in ASAL reservoirs. The objective of this study was therefore to determine the hydrological influences and land use practices that have led to silt accumulation in Kalundu reservoir in the period 2000-2021. Datasets used in this study included: (i) hydrological data that was obtained from three (3) sampling stations located along Kalundu River and three (3) stations in the reservoir; (ii) sediment samples from various locations in the reservoir; (iii) reservoir bathymetric survey; (iv) satellite data for land use and land cover (LULC) change and (v) household water uses data that was obtained through questionnaire survey. Remote sensing tools and the Modified Universal Soil Loss Equation (MUSLE) model were used in spatial analysis and estimation of soil loss, respectively. The methods of data analysis that were used in this study included descriptive statistics, regression and correlation analyses. The results of the study showed that the total amount of sediment discharge into the reservoir during both short and long rainy seasons were 43,121.53 m3/yr (64,360.49 ton/year). The trap efficiency of the reservoir was higher during the long rainy season (55.91%) as compared to the short rainy season (47.73%). This difference was attributed to relatively low river discharge experienced during the long rainy season that leads to limited flushing of sediments out of the reservoir. The analysis of bathymetric data showed that the storage capacity of Kalundu Dam had decreased from 500,000m3 in 2013 to 149,902m3 in 2021 with an estimated sedimentation rate of 65,317 tons/yr; and trapping efficiency of 55%. The analysis of particle size distribution of the sediments deposited in the reservoir showed that fine sediments were mostly deposited at the middle section (clay 36%) and coarser sediments before entry into the reservoir (sand 48%). The investigation of soil erosion rates using MUSLE showed that highest rates of soil erosion occurs in the lower parts at 51,450 tons/ha/yr. The analysis of land use and land cover showed that croplands and built-up areas have increased significantly within a period 20 years (2000-2020). Poor farming practices and clearing of natural vegetation have strongly contributed to the increased sedimentation of the dam in the period from 2010 to 2020. This resulted to high water turbidity that subsequently reduces socio-economic benefits and the livelihood of the local community. The study emphasizes the need for implementation of comprehensive soil and water conservation strategies in sub-basins that forms important catchment areas for reservoirs constructed in ASALs and especially for sustainability of Kalundu Dam reservoir.