Rainwater harvesting (RWH) entails capture of rainwater falling on a roof and conveyance to a storage tank for later use as domestic water supply. During dry weather, dust and pollutants accumulate on the roof surface and are subsequently washed off with the ‘first flush’ at the beginning of the next rain. Diverting the first flush can improve the quality of stored water but at the cost of reducing the reliability with which the system can supply water. A storage-reliability-yield (SRY) analysis of RWH was completed for Bisate, Rwanda for a period of 20 years with a range of normalized storage volumes and yields. Reliability expressed as days per year on which demand was met was determined for alternative first-flush strategies and compared to the reliability of an otherwise equivalent system without first-flush diversion. Diversion of the first flush was found to reduce reliability by at most 8%. Analysis of three existing RWH systems in Bisate indicates that a recommended 1 mm first-flush diversion would reduce the number of days the system meets demand by no more than 7 days per year.

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