River flow regulation :a case study of the upper Ewaso Ngiro North River

Abstract

Water shortage during dry season and very high flows during wet seasons has been identified as the major challenges in water resources management in Ewaso Ngiro North River Basin, Kenya (ENNRB). The increasing water demand of the growing population, unequitable allocation of water resources and the uneven distribution of dry weather river discharges lead to tension and conflicts over the water resources in the basin. This study has investigated the feasibility of developing proposed Archers Post and Kihoto reservoirs to mitigate this problem of water shortage. The regulation of outflow from these reservoirs should be such that the downstream water users are assured of certain quantity of flow in the year round while the upstream water users are able to use the dry season river flow to meet their irrigation water requirements. The study objectives were to determine suitable reservoir sizes for the two sites and assess the performance of the reservoir in augmenting low flows. This was done through simulation of reservoir operations using Interactive River-Aquifer Simulation (IRAS) modelfor reservoirs with active storage capacity ranging from 50-million m3 to one with 350-million m3 at an interval of 50-million m3 for each site. The criterion of selecting the suitable reservoir size was based on the quantity of water that was made available by the reservoir during low flow periods over the simulation period which was 27 years (1960- 1986). If the resultant increase in volume of water made available by an increase in reservoir capacity was less than "15% of the volume of water made available by the preceding reservoir active storage capacity the increase was considered unsuitable. The next small reservoir size was considered the suitable reservoir capacity of the site. The assessment of reservoir performance of the selected reservoir sizes was done using low flow frequency analysis and the ability of the reservoir outflow to meet target releases. The reservoir size that was selected for the proposed Archers Post site has a gross capacity of 400-million rrr' while the one for Kihoto site has a gross capacity of 161-million rrr'. The dead storage for the Archers Post reservoir is 200-million m3 with useful life span of 80 years and for Kihoto reservoir is II-million rrr' with a life span of 100 years if the current sediment load conditions at the two sites prevail in future. The reservoir operating policy used to simulate reservoir releases allowed the releases such that the available water in the reservoir was adequate for 14 weeks. With this operating policy the two reservoirs did not run dry at any time of simulation. It was found that with this operating policy the Archers Post reservoir made available 3,001. 4-million m3 of water in the low flow periods during the 27 years of simulation while 897.25-million m3 of water was made available by Kihoto reservoir over the same period. This water, that was made available by these reservoirs, has improved flow reliability together with the return period of low flow events, which are shown to have a marked improvement at the two sites. This study is expected to be an important contribution to the management of water resources of ENNR, which Ewaso Ngiro North Development Authority (ENNDA) has identified as strategic resource for the development of the area under its jurisdiction. One way ENNDA can use results of this study will be to build on it to identify the water demand that is required to boost the economy of the region and can be satisfied by the outflows of these reservoir at a known reliability. It is also expected that based on the findings of this study ENNDA will now carry-out detailed economic analysis of these reservoirs to justify their development on the economic point of view.