Evaluation of Ruiru Reservoir Water Balance for Enhanced Operation and Management

Abstract

Ruiru reservoir is one of the main source of water supply to Nairobi City and its environs. During the wet season, the reservoir overspills. Meanwhile, the water demand keeps increasing every year in Nairobi city due to population increase. Lack of hydrological data for Ruiru reservoir makes it difficult to develop a sound management routine. In order to evaluate Ruiru reservoir water balance for enhanced operation and management, this study used the Hydrologic Engineering Centers Hydrologic Modelling System (HEC-HMS) 4.1 model to simulate runoff into the reservoir. Digital Elevation Model (DEM), and World Imagery were used to delineate the watershed in Watershed Modelling System (WMS) 10.1. The soil moisture accounting (SMA) parameters were computed in WMS using soil type, land use and land cover data. Daily rainfall and monthly evapo-transpiration for five years (2011-2015) were used as the meteorological inputs. The inflow and the outflow data obtained from the simulation were used to generate the mass curves of the reservoir. The analysis of the mass curves under different demand scenarios was used to determine the effects on Ruiru reservoir. The results showed a total runoff volume of about 203,000,000 m3 during the five years of simulation. The Efficiency of the model was 0.74 for calibration and 0.72 for validation. An evaluation of the water balance showed that the maximum storage of the reservoir was 2,516,499 m3 and the lowest storage was 1,116,190 m3. The month of May and December are the two peak storage months while September has the lowest storage within the five years. The analysis of mass curve under different demand scenarios showed that the amount of water being supplied by Ruiru reservoir to Nairobi City can be increased by 75%.