Flood is the greatest environmental challenge for Hamadan city. In the absence of urban sewage systems, in most parts of the study area, rainwater harvesting (RWH) from roofs, parking areas and roads could play a crucial role in reducing flood proneness and providing household consumption. In this study, to assess effect of RWH on magnitude of runoff volume and various household consumption, firstly, flood volume with various return periods was calculated by using SCS method in the HEC-HMS model as a popular and effective method in flood hazard studies. In the next step, to assess effect of RWH system on flood volume magnitude with the assumption that the entire residential buildings in the city of Hamadan use RWH system, a divert element with constant flow whose amount was as much as RWH from rooftops was subtracted from each sub basin in the HEC-HMS model. Result indicated that reduction effect of the RWH system on runoff volume magnitude varied from 145.95 to 333.06 with 24 h precipitation for 2–100 years. On the other hand, this amount of reduction effect could meet household consumption need from 202.50 to 462.11%, and its economic aspect ranged from 10,008.98 to 22,840.65 (USD).
