Abstract:[Background] Rainwater harvesting, as one of the important soil and water conservation measures to alleviate shortage of water resources and to relieve pressures on water supply and drainage systems in urban areas, has been increasingly used as an effective practice for the Sponge City and Water Saving City construction. Design and construction of rainwater harvesting systems should meet future water demand scenarios and rainfall conditions, however, most of current studies of rainwater harvesting systems use only historical climate and water demand data. Rainwater harvesting systems designed with historical data may not be able to sustainably meet water saving or stormwater management requirements under future climate conditions.[Methods] In this study, the plausible impacts of rainfall change on water saving and stormwater management performance of rainwater harvesting systems were investigated by comparing the water supply reliability (g), tap water saving efficiency (w) and runoff reduction efficiency (η) of rainwater harvesting systems in Fuzhou and Urumqi calculated using the future (2020-2050) and historical (1985-2015) daily rainfall time series. A downscaling technique based on linear and nonlinear regression analysis and the CLIGEN model was evaluated and employed to generate the future daily rainfall with the projections of 21 Global Circulation Models. A daily water balance model was developed to simulate the hydrologic operation of rainwater harvesting systems. Toilet flushing water demand was considered in this investigation.[Results] The simulated rainfall data of Fuzhou and Urumqi from 1960 to 2005 using the regression analysis for spatial downscaling and the CLIGEN model for temporal downscaling is close to the measured rainfall data. The downscaling technique performs well in simulating the occurrence of daily rainfall at both of the cities. Therefore, the CLIGEN model could alternatively be used at Fuzhou and Urumqi for rainwater harvesting system analysis. The downscaling rainfall data shows that mean annual rainfall in Fuzhou and Urumqi in 2020-2050 is going to increase by 2.67% and 28.23% compared with that in 1985-2015, respectively. The water saving performance of rainwater harvesting systems is positively affected by the increases in future rainfall, as rainwater harvesting systems with smaller tank sizes and/or smaller catchment areas could supply adequate water for demand at a desired tap water saving efficiency or water supply reliability. The stormwater management performance, however, is negatively affected under the conditions of future rainfall change, as a larger tank size or larger catchment area is required to achieve desired runoff reduction efficiency.[Conculsions] The responses of water saving and stormwater management performance of rainwater harvesting systems to rainfall change are varying with not only the system dimensions (i.e., storage capacity and catchment area), but also locations with different rainfall conditions. Rainwater harvesting systems in a drier city with less rainfall are more sensitive to rainfall change. Rainwater harvesting systems with larger storage capacity and/or larger catchment area are expected to be more resilient to rainfall change. Therefore, the ability of rainwater harvesting systems to adapt to rainfall change could be increased by designing larger reservoirs and catchment areas.
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