Hydrological Modelling of Impact of Small Reservoirs in Upper Ewaso Ng’iro River on Downstream Flow: Case Study of Nanyuki River

Abstract

The Nanyuki River originates from west of Mount Kenya and flows through the county of Laikipia, where perennial water scarcity problems lead to socioeconomic and ecological issues. Upstream of the basin, river waters are abstracted, reducing downstream flows drastically leading to disputes between rival water users. Excess river water storage (Flood storage) and management has the potential to reduce abstraction of the natural flows and prevent eminent conflicts. Nevertheless, there is very limited information on the extent and impact of storage specifically for the Ewaso Ng’iro River. The overall focus of this study was thus to identify suitable sites for dams along the Nanyuki River and evaluate the potential of flood water storage for use during dry spells to maintain/regulate downstream water flows. The specific objectives were three-fold: (i) To determine suitable dam sites and identify reservoir characteristics, (ii) To simulate downstream flows under different operational regimes using HEC-HMS model; and (iii) To evaluate the impact of the reservoirs to downstream flows using WEAP. Documentation of possible dam sites as a strategic initiative for water resources management was evaluated using an integrated approach involving MCDA and GIS in consideration of thematic attributes. Six thematic features were considered: Stream Order, Slope, Elevation, Curve Number, Land use/Landcover and Lithology. The Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) was adopted for continuous simulation of river discharge in the basin at the identified dam locations which are not gauged. Model output was harmonized with observed discharge data to establish the suitability of model in predicting streamflow. To assess the impacts of small reservoirs on maintaining the downstream flows and reduce the impact of over-abstraction especially during the dry season, the Water Evaluation and Planning (WEAP) model was adopted. Two dam locations deemed suitable were sited downstream of the catchment at location 0o 12’ 35.5’’N, 37o 0’ 29’’E and 0o 15’ 45’’N, 36o 57’0.95’’E for dam 1 and 2 respectively. Dam 1 axis elevation is 1750 meters (dam width is 460 meters and height is 12 metres), while Dam 2 axis elevation is 1694 meters (width of 600 metres and height is 14 metres). For Dams 1 and 2, the estimated water storage volumes are 5.8 and 6.1 Mm3 respectively. Model performance was based on statistically computed parameters in addition to the visual inspection and comparison of the resultant hydrographs. It has been noted that increasing domestic and agricultural demands were the main causes of over-abstraction - in particular - in the upstream catchments. This results to water scarcities leading to conflicts among different users in the downstream of the catchment. The study recommends flood storage in a series of small dams in the upstream to regulate and maintain dry-season flows in the downstream. Although, the benefits of water storage for the case of Ewaso Ng’iro may outweigh the negative impacts, there must be a limit beyond which such impoundment may have adverse impacts to the ecosystem functioning resulting to a shift in the balance.