urren : C t R gy e o s l e o Njogu et al., Hydrol Current Res 2018, 9:4 r a r d c y h DOI: 10.4172/2157-7587.1000305 H Hydrology: Current Research ISSN: 2157-7587 Research Article Open Access Stream Flow Variability and Sediment Yield in North-West Upper Tana Basin, Kenya Imelda N Njogu*, Johnson U Kitheka and Hesbon Otieno Department of Hydrology and Aquatic Sciences, School of Environment, Water and Natural Resources, South Eastern Kenya University, Kitui, Kenya *Corresponding author: Imelda N Njogu, Department of Hydrology and Aquatic Sciences, School of Environment, Water and Natural Resources, South Eastern Kenya University, Kitui, Kenya, Tel: +254702781796; E-mail: [email protected] Received date: Sep 27, 2018; Accepted date: Nov 14, 2018; Published date: Nov 22, 2018 Copyright: © 2018 Njogu IN, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abstract The North-West Upper Tana River (NWUT) Basin is one of Kenya’s most important basins as it provides water for urban-rural water supplies, hydro-electric power (HEP) generation and irrigation. This study therefore investigated the influence of rainfall variability on the river discharge and sediment yield in the basin. The study relied on data archived by Water Resources Authority (WRA) and Kenya Meteorological Department (KMD) for the period 2010-2012. The methods applied in the study included the use of time series, double mass curve and the use of other statistical methods. The study established that there is a significant relationship between river discharge and rainfall in the basin. The variations in stream flow can largely be explained by variations in rainfall in the basin. There is however evidence of shift in rainfall patterns so that rainfall during the short rainy season seems to be more dominant than that experienced during the long rainy season. There is also a significant relationship between sediment yield and stream flow. The basin generally experiences high rates of sediment production due to inappropriate land use practices and lack of application of soil and water conservation measures on cultivated lands. Mathioya, Saba Saba, Thika and Maragua sub-basins exhibited high rates of sediment production rates due to high rates of soil erosion in these sub-basins. The high sediment yield in the basin has potential of reducing the benefits associated with Masinga Dam in terms of water supply, irrigation, flood control and HEP generation. The study recommends implementation of enhanced programmed for land and water conservation in the basin, including implementation of payment for Ecosystem Services (PES). Keywords: North West Upper Tana; Sediment yield; Sediment In addition to the impacts of hydro-electric dams, climate change in production; River discharge the Upper Tana Basin is also significantly exerting changes in the flow of the river. In the recent past, there has been increased variability of Introduction rainfall in the Central Kenya Highlands-the main water catchment area for the Tana river. Changes in rainfall patterns have led to changes Tana river in Kenya provides an important link between the Central in streamflow (reduction) and sediment production (increase) at Kenya Highlands and the Indian Ocean. This is in terms of freshwater, catchment level [4,11-14]. The impacts of climate change have been nutrients and sediment discharge. Through the discharge of various exercabated by landuse and landcover change in the basin. Since the materials, the river plays as key role in supporting diverse marine and beginning of the nineteeth century when the Central Kenya Highlands aquatic ecosytems in the Ungwana Bay part of the West Indian Ocean. were opened for settlements and agriculture by the European settlers, However, various developments in the upper parts of the Tana Basin, there have been significant landuse and landcover changes that have particularly the Upper Tana Basin has led to major changes in material led to high rates of soil erosion [2,4,13,15]. The extent to which these transport to the coast [1]. Past studies conducted in the basin have changes have impacted on the streamflow and sediment load of the shown that Masinga reservoir-the largest reservoir in the basin and in river has however not been fully established due to lack of data. Several Kenya, has significantly modified the flow of the Tana [2,3]. There has studies that have been carried out on river discharge and sediment also been reduction of the sediment load of the river due to trapping of yield in theUpper Tana catchment have not determined how key terrigenous sediments in the reservoir [1,3]. The siltation of the parameters such as rainfall variability and land use change overtime Masinga reservoir is progressively leading to the reduction of the affects sediment yield and river discharge in the catchment [9,16,17]. storage capacity and the life expectacy of the dam [4-6]. Some studies Although the main variables impacting on streamflow and sediment have estimated that Masinga reservoir has already lost approximately yield are known to be land use, vegetation cover and rainfall variability, 13.6% of its design storage capacity [7]. Without implementation of there is paucity of data to establish the interrelationship between these comprehensive soil and water conservation, including reduction in variables in tropical river basins of Africa. The influences of rainfall on population in the Upper Tana Basin, storage capacity of Kenya’s most streamflow and sediment yield are usually complicated by seasonal and important HEP reservoir would be reduced considerably before 2050 inter-annual changes in land use and vegetation cover [13,15,18,19]. [7-10]. This will mean that the benefits associated with the dam such as Therefore, without data on the seasonal and inter-annual changes in the provision of water, hydropower generation, flood control, streamflow as well as land use/land cover changes, it becomes difficult recreation including also ecological and environmental benefits will be to establish how the later is contributing to the current streamflow signficantly affected [7]. patterns in the Upper Tana Basin. Hydrol Current Res, an open access journal Volume 9 • Issue 4 • 1000305 ISSN: 2157-7587 Citation: Njogu IN, Kitheka JU, Otieno H (2018) Stream Flow Variability and Sediment Yield in North-West Upper Tana Basin, Kenya. Hydrol Current Res 9: 305. doi:10.4172/2157-7587.1000305 Page 2 of 9 The main objective of the study was to determine how streamflow and sediment yield are influenced by land use/landcover change and rainfall variability in the North-West Upper Tana catchment in Central Kenya. This study contributes to the current debate on the extent to which climate change and landcover/land use change influences river discharge and sediment yield in tropical river basins of Africa. The results obtained from this study will be invaluable in the formulation of appropriate and sustainable land and waters management strategies in the basin and elsewhere in Africa. Description of the study area North-West Upper Tana catchment is located in Central region of Kenya (Figure 1). The basin extends into Nyeri, Murang’a and Kiambu Counties. The study area is located between longitudes 36°30ˈ0ˈˈE and 37°40ˈ0ˈˈE and latitudes 1°10ˈ0ˈˈS and 0°10ˈ0ˈˈS (Figure 1). The basin covers a surface area of approximately 9,918 km2 with elevation ranging from 881 m to 3,844 m above sea level [20]. The main river tributaries in the basin are Sagana, Mathioya, Maragua and Saba Saba, all of which feeds into the river Tana [21]. The basin is therefore the Figure 2: The main sub-basins in the North West Upper Tana basin. source of Kenya’s largest and most important river system - Tana, which is used by Kenya Energy Generation Company (KenGen) to generate hydroelectricity power (HEP). Methodology River gauging stations and stream flow data This study relied on rainfall, river discharge and total suspended sediment concentrations (TSSC) data archived by Water Resources Authority (WRA) which is key government institution in Kenya responsible for collection of hydrological data. Stream flow data was obtained from WRA for nine (9) River Gauging Stations (RGS), namely Sagana (RGS 4AC03), Maragua (RGS 4BE01), Saba Saba (RGS 4BF01), Mathioya (RGS 4BD01), North Mathioya (RGS 4BD07), Irati (RGS 4BE03), Thiba (RGS 4DD01), Thika (RGS 4CC05) and Gikigie (RGS 4BE08). The archived river discharge data were mainly daily and monthly river discharges expressed in cumecs (m3/s). These were subjected to time series analysis to determine seasonal and inter- annual variability in river discharge. Rainfall data Rainfall data used in this study was obtained from Kenya Figure 1: The location of North-West Upper Tana basin in Kenya Meteorological Department (KMD) Head Office in Nairobi. Archived (Njogu et al.). rainfall data was collected for key stations in the basin, namely Kiritiri Chief‘s camp, Nyeri Ministry of Works, Nyeri Meteorological Station and Sagana Fish Farm (see also Kerandi et. al., 2017). Sagana Fish Five major HEP dams that have been constructed in the lower Farm Station and Nyeri Ministry of Works rainfall stations had more reaches of the Upper Tana basin are Masinga, Kindaruma, Kamburu, consistent and long-term data as compared to other stations [23]. The Gitaru and Kiambere. Together, these dams provide approximately study therefore relied on these two stations to infer on the long-term three quarters of electricity in Kenya and regulate the flow of the river trends in rainfall. Tana [7]. The basin experiences bimodal rainfall pattern due to the inter-tropical convergence zone (ITCZ). The long rain season is from March to May while the short rain season is from September to Total suspended sediment concentrations data December [22].