The Dynamics of Inter-basin Water Transfer under Increasing Water Demands and Climate variability: A Case of Water Transfer from Upper Tana Basin to Athi Basin

Abstract

Universal and equitable access to adequate, safe and affordable drinking water by all is one of the Sustainable Development Goals (SDGs). However, this goal is far from being achieved as issues of water scarcity remain a big challenge especially in sub Saharan Africa due to growing population, urbanisation, and climate variability. There have been several interventions to cope with the rising water demands, among them inter-basin water transfers (IBWTs). IBWTs link two or more basins resulting in complex economic, social, and environmental inter-dependencies among the donor and recipient basins. A proper understanding of these inter-dependencies requires a multi-disciplinary approach and in-depth scientific analysis, which may vary for different river systems. The aim of this research was thus to evaluate the hydrological and socio-economic implications of inter-basin water transfer systems under growing water demand and climate variability with a focus on the Northern Collector Tunnel (NCT I) in Upper Tana basin, Kenya. First, the study evaluated the reliability of Maragua, Irati and Gikigie streams to meet the proposed water transfers to Nairobi City under different climatic conditions using hydrological modelling system (HEC HMS) and stream flow variability indices. An innovative methodology that uses evapotranspiration (ET) from water productivity open access portal (WaPOR) as input to the model was used to improve the hydrological model performance and limit model uncertainty. The results showed that during the dry years, the reliability of Maragua, Irati, and Gikigie to meet the water transfer was 15%, 23%, and 33% respectively. In normal years, the reliability increased to 45%, 60%, and 62%; and to 83%, 98%, and 94% in the wet years. This implies that the project will not meet its objective of achieving over 70% supply coverage during the dry and normal years for which the dry years have a 5-year cycle of occurrence. In addition, Maragua stream showed a higher flow variability thus lower reliability whilst envisioned to contribute the highest amount among the three streams. Thus, it is recommended that the technical viability of large IBWTs should emphasis on the reliability of water sources taking into account climate variability. Using the water evaluation and planning model (WEAP), the study evaluated optimal urban water allocation strategies for Nairobi city with the current and planned IBWTs. While the government’s objective is to increase the supply coverage in Nairobi city to over 70%, this will only be achievable during the wet years. From the results of this study, even with demand management measures, NCT 1 will still not meet the desired supply coverage in the normal and dry years. However, from the results, with additional water sources (NCT phase II and Maragua dam), demand management measures provide opportunities of alleviating water shortages by achieving the desired supply coverage under all climatic conditions. For flood based IBWT systems, it would be vital to incorporate storage reservoirs to enhance supply reliability. There were concerns about the possible impacts of the Northern Collector Tunnel (NCT) I project on groundwater flow in the Upper Tana Basin. Therefore, this study investigated the barrier effect of the project on groundwater flow in the basin using vertical electrical sounding (VES) survey and modular finite difference flow (MODFLOW) modelling. Results showed the recharge zones are on the upstream side of the tunnel where many shallow springs of a maximum depth of 50m are located. The tunnel increased the local hydraulic head on the upstream side by up to 10% with a 2% decrease observed in boreholes located on the downstream side. Lastly, the study evaluated the distribution of risks and benefits in IBWTs among stakeholders in the xxii NCT 1 using the stakeholder analysis technique. Athi Water Works Development Agency (AWWDA) and the Ministry of Water and Irrigation (MoWI), which are national institutions, played a key role in the project. However, most stakeholders consider the devolved units; Water Services Providers (WSPs), Water Resources Authority (WRA) and Water Resources Users Associations (WRUAs) and Murang'a County as the most critical institutions. Major risks of the project emerged as reduced river flows, drying up of springs and local community’s inability to meet their water needs. Increased water supply to Nairobi City was perceived as the major benefit of the recipient basin. Thus, NCT I project presents both risks and benefits to the donor and recipient basins, however, without a specific framework to balance them. This calls for a water levy that covers the full costs of water abstraction, socio and environmental externalities (risks) and catchment conservation.