Developing a sustainable water management plan for Ruiru, Thiririka and Ndarugu subbasins in Kenya using WEAP Developing a sustainable water management plan for Ruiru, Thiririka and Ndarugu sub basins in Kenya using EAP

Abstract

Ruiru, Thiririka and Ndarugu Sub-Basins have been subjected to intensive agricultural, industrial activity and population pressure. The growing demand over a limited endowment of water has generated competition between upstream and downstream users and is causing conflicts over its use. The increased competition for water is putting great pressure on the local hydrology and ecosystem. The increase in population in Juja, Jomo Kenyatta University of Agriculture and Technology, Gatundu, Ruiru and Nairobi and the increasing economic activities have increased the demand for water for domestic use, food security and industrial development. Two dams have been proposed by the Government for construction along Ndarugu and Ruiru rivers to supply water to Nairobi. With all these competing water demands, there is need to develop strategies for sustainable water resources management in the basin. The study utilizes the water allocating model, WEAP, in simulating and predicting the effect of water allocation scenarios and possible management options. The drainage area for Ndarugu, Thiririka and Ruiru sub-basins are 395km2, 328km2 and 476km2 respectively.

The data that was used in the study were collected from the Ministry of Water and Irrigation, Water Resources Management Authority Regional offices in Machakos and Kiambu, Central Bureau of Statistics, Meteorological Department in Nairobi and Agricultural Offices in Thika and Ruiru. This data include population, discharge, agriculture, rainfall and other climatic data. The Global Positioning System (GPS) was used to determine the co-ordinates and altitudes of towns, institutions, farms and point pollution. Water samples were collected from the upstream of the three rivers in Gatundu and Githunguri area and downstream of the three rivers at Kalimoni primary school and Juja farm. The water samples were collected during the dry season in August, 2009 and in January, 2010. The water quality parameters that were analyzed were Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Electrical Conductivity (EC), Chloride (Cl), Turbidity and Copper (Cu). The WEAP model was used to simulate water use in urban, agriculture, institution and industries sectors over a period set to 1980 to 2008 of varying rainfall and river flows. Calibration and validation was done by comparing the simulated and measured flows for stream flow gauges located at 3CB05, 3BD05 and 3BC12. The crop coefficient and reference evapotranspiration values used on the study were varied until an acceptable value of correlation coefficient (R2) was achieved. The correlation coefficient (R2) between the simulated and observed monthly stream flow was assessed for the three gauging stations. For station 3BC12, an R2 of 0.803 was obtained during calibration and 0.811 during validation. The coefficient of correlation for station 3CB05 was 0.801 during calibration and 0.804 during validation. For station 3BD05, the R2 of 0.849 was achieved during calibration and 0.88 during validation.

Four water plan strategies were identified and tested for their effectiveness in meeting future water demand through the WEAP model. The strategies are: Urban water use efficiency, surface storage, conjunctive management and groundwater storage and land use planning and management. The results indicate that by combining the four strategies that were identified, it is possible that all the water demand in the subbasin could be met. From the study, the high value of correlation coefficient (R2 of 0.849 and 0.88) between simulated and observed flow for model calibration and validation respectively at 3BD05 gauging station implies an acceptable performance of the WEAP model in stream flow generation in Ruiru and Ndarugu basin. The model could therefore be used to predict water demand and supply for future scenarios for Ruiru, Thirirka and Ndarugu sub-basins. The projections of the water demand by the WEAP model in the study area indicates that it will grow from 75.1 MCM in 2010 to 96.3 MCM and 129.2 MCM in 2020 and 2030 respectively. The proposed reservoirs under the surface water strategy should be constructed to meet the current and future water demand.