Water saving efficiency and reliability of rainwater harvesting systems in the context of climate change

Rainwater harvesting (RWH) are receiving notable attention as a promising alternative to address water supply pressures in urban areas. In the context of climate change, however, knowledge about impacts of rainfall changes on RWH is limited. In this study, the impacts of rainfall changes on efficiency and reliability of RWH are investigated at three cities (i.e., Chengdu, Beijing and Urumqi in China). Rainfall change trends are detected using the Mann-Kendall test. A computational tool based on water balance equation is developed and three water demand scenarios (lawn irrigation, toilet flushing, and the combined demand) are considered. The results indicate that the commonly described "dry gets drier, wet gets wetter" rainfall change pattern does not hold in the 57 to 65 years rainfall records of the three cities. The declining trend of rainfall at both Beijing and Chengdu makes RWH less efficient in water saving and less reliable for meeting water demands, whereas the rising trend of rainfall at Urumqi is favorable for RWH. Rainfall increases in dry season at the three locations are beneficial for RWH while in wet season the impacts are bidirectional depending on the changing trends of rainfall. Impacts of rainfall changes on RWH are dependent on not only trends and extents of rainfall changes, but also tank sizes and water demand scenarios. These findings signify the importance of incorporating rainfall variations in the design and evaluation of RWH systems. Location specific and adaptive measures (e.g., adjusting tank or cistern sizes, contributing areas or water demand scenarios) should be adopted for RWH to accommodate climate change.