Annual and seasonal reliability of urban rainwater harvesting system under climate change

•High variability was observed in seasonal performance of GCMs.•High variability of rainfall data justifies GCM selection in seasonal scale.•Promoting RWH parameters by increasing tank size is highly dependent on a roof area.•Water demand has the highest impact on selecting optimum tank size in annual scale.•Optimum tank size for all roof areas except 25 and 50 m2 in the basic demand is 10 m3. Rainwater harvesting system (RWH) is a practical water supply worldwide, yet direct and/or indirect benefits of urban RWH are not fully evaluated in Iran. Besides, the effect of climate change on RWH as one of the major factors is still unexplored in Iran. In this research, the effect of climate change on RWH parameters (i.e., water saving, reliability and water security) in seasonal and annual scales was evaluated for the city of Mashhad, Iran. The best performing general circulation models (GCMs) in seasonal scale were selected over a baseline period (1981–2000) using Taylor diagram and Nash–Sutcliffe model efficiency coefficient (NSE). The selected models were used to project daily rainfall for 2030–2044 under RCP8.5 emission scenario. A daily water balance model was used to simulate the performance of a RWH system for the baseline and future periods. RWH parameters were evaluated considering a wide range of roof areas, 25 to 300 m2 and tank sizes, 0.5–10 m3, for a 4-people household with the demand levels of 20 and 50 L/ca/day, referred to as basic and intermediate demands, respectively. The results revealed that only for the basic demand of seasonal scale, RWH parameters will be fully met happening mostly for the maximum roof area of 300 m2 and tank sizes of 10 m3. Moreover, results show that the selection of optimum tank size based on annual RWH parameters is highly dependent on demand level and for all roof areas except 25 and 50 m2 in the basic demand, a tank size of 10 m3 results in the best RWH parameter. For the intermediate demand level, optimum tank size varies from 0.5–10 m3 based on a given roof area.