The contribution of rainwater harvesting from roof catchments in increasing water resources for Nairobi City County residents
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Nairobi City County is faced with acute water scarcity. The daily deficit is currently 170,000 m3/day which accounts for about 23%. This deficit will escalate as population increases. However, streets are often flooded during heavy rains and water harvesting has been neglected. The County relies mainly on surface water sources from Athi River basin, Tana River basin and over 3000 boreholes. The objectives of this study were to assess volumes of rainwater that can be harvested from roof catchments, sizes of storage capacity required, water quality and adoption of rainwater harvesting from roof catchments. Five rainfall Thiessen polygons that covered the entire County were created using ArcGIS version 9.2. Samples of 10 maisonettes and multi-story buildings were obtained from each Polygon. More samples of 25 industries and 25 institutions were collected making a total of 150 samples. Data used included rainfall depths, roof surface area, size of tanks, physical, bacteriological and chemical properties of rainwater from roof catchments. Microsoft Excel, SPSS, descriptive analysis, regression and correlation analysis were appropriately used in the study. The result indicated that Kabete receives highest annual rainfall followed by Dagoretti, Eastleigh Moi Airbase, Wilson and J.K.I.A. Over 90% of buildings would have rainwater supplements of above 23% thus filling the current deficit. The size of storage tanks for maisonettes ranged from 5, 126 to 60, 840 litres while multi story buildings it ranged from 32, 400 to 344, 250 litres. A regression model for storage capacities and their cost were generated. The quality of rainwater from different roofing materials was not suitable for potable purposes but, it would be used for nonpotable purposes. Majority (93%) of residents depended on city council piped water, private boreholes (7%) and only 16% harvested rainwater as supplement to City council or private borehole sources. Institutions are leading in roof catchment rainwater harvesting (44%), followed by maisonettes( 20%), Multi-story buildings 6% and industries have least adopted rainwater harvesting(4%). The result indicates that if roof top rainwater catchment is done, water deficit that has been estimated in Nairobi County will be definitely offset. Industries and institutions will benefit more from roof catchment rainwater harvesting followed by maisonettes and multi-story residential buildings. Residents should be encouraged to adopt roof catchment rainwater harvesting by enforcing policies and by laws for roof catchment rainwater harvesting in Nairobi City County.