Reservoir sedimentation in an arid and semi-arid river basin; a case study of Kalundu dam in Kitui County, Kenya
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In Kenya, several water reservoirs located in arid and semi-arid lands (ASALs) areundergo the trouble of accumulation of sediments. Kalundu Dam in Kitui County is atypical reservoir located in ASAL that has been experiencing periodic siltation since1950s when it was commissioned. However, the patterns of siltation within the damincluding the hydrological processes influencing sedimentation processes have not beeninvestigated in this reservoir as with other reservoirs in Kenya’s ASALs. Lack of data onthe hydrological processes and land use practices has narrowed execution of strategies forcontrolling sedimentation in ASAL reservoirs. The objective of this study was thereforeto determine the hydrological influences and land use practices that have led to siltaccumulation in Kalundu reservoir in the period 2000-2021. Datasets used in this studyincluded: (i) hydrological data that was obtained from three (3) sampling stations locatedalong Kalundu River and three (3) stations in the reservoir; (ii) sediment samples fromvarious locations in the reservoir; (iii) reservoir bathymetric survey; (iv) satellite data forland use and land cover (LULC) change and (v) household water uses data that wasobtained through questionnaire survey. Remote sensing tools and the Modified UniversalSoil Loss Equation (MUSLE) model were used in spatial analysis and estimation of soilloss, respectively. The methods of data analysis that were used in this study includeddescriptive statistics, regression and correlation analyses. The results of the study showedthat the total amount of sediment discharge into the reservoir during both short and longrainy seasons were 43,121.53 m3/yr (64,360.49 ton/year). The trap efficiency of thereservoir was higher during the long rainy season (55.91%) as compared to the shortrainy season (47.73%). This difference was attributed to relatively low river dischargeexperienced during the long rainy season that leads to limited flushing of sediments outof the reservoir. The analysis of bathymetric data showed that the storage capacity ofKalundu Dam had decreased from 500,000m3 in 2013 to 149,902m3 in 2021 with anestimated sedimentation rate of 65,317 tons/yr; and trapping efficiency of 55%. Theanalysis of particle size distribution of the sediments deposited in the reservoir showedthat fine sediments were mostly deposited at the middle section (clay 36%) and coarsersediments before entry into the reservoir (sand 48%). The investigation of soil erosionrates using MUSLE showed that highest rates of soil erosion occurs in the lower parts at51,450 tons/ha/yr. The analysis of land use and land cover showed that croplands andbuilt-up areas have increased significantly within a period 20 years (2000-2020). Poorfarming practices and clearing of natural vegetation have strongly contributed to theincreased sedimentation of the dam in the period from 2010 to 2020. This resulted to highwater turbidity that subsequently reduces socio-economic benefits and the livelihood ofthe local community. The study emphasizes the need for implementation ofcomprehensive soil and water conservation strategies in sub-basins that forms importantcatchment areas for reservoirs constructed in ASALs and especially for sustainability ofKalundu Dam reservoir.