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.

雨水收集系统（RWH）是世界范围内的实用供水，但是在伊朗，尚未完全评估城市RWH的直接和/或间接收益。此外，气候变化对RWH作为主要因素之一的影响在伊朗仍未得到探讨。在这项研究中，评估了伊朗马什哈德市在季节和年度尺度上气候变化对RWH参数（即节水，可靠性和水安全）的影响。使用Taylor图和Nash-Sutcliffe模型效率系数（NSE）在基线期（1981-2000年）中选择季节性尺度上性能最佳的通用循环模型（GCM）。所选模型用于预测RCP8.5排放情景下2030-2044年的日降雨量。每日水平衡模型用于模拟RWH系统在基线和未来期间的性能。在考虑25至300 m的各种屋顶区域的情况下评估RWH参数对于一个4人家庭，其需求水平为20和50 L / ca /​​ day，分别称为²和0.5–10 m ^3的储罐尺寸，分别称为基本需求和中间需求。结果表明，仅针对季节性规模的基本需求，RWH参数才能得到充分满足，主要是在最大屋顶面积300 m ²和储罐尺寸10 m ^{3的情况下发生的}。此外，结果表明，基于年度RWH参数选择最佳储罐尺寸高度取决于需求水平，对于基本需求中除25和50 m ²以外的所有屋顶区域，10 m ³储罐尺寸可获得最佳RWH参数。对于中等需求水平，最佳储罐尺寸为0.5–10 m ³ 根据给定的屋顶面积。