Fresh water, a renewable but limited resource, is scarce in many areas of the developing world. This paper proposes a dual water supply consisting of both rainwater and groundwater to provide sustainable water for an arsenic contaminated area. In this paper, a novel model has been developed based on rainfall variance. A rainwater harvesting (RWH) system can obtain optimal performance by changing the demand for water use according to rainfall. This model was then used to design a RWH system for Cukhe village. Using rainwater for potable purposes on dry days and for all purposes on rainy days is recommended. The cost of RWH systems is also analyzed. In order to achieve 100% reliability for a four-member household, a RWH system should have a roof area of 100 m2. Additionally, a tank size of 2 m3 is recommended for overall cost efficiency. From the yearly water supply chart, it is recommended that rainwater be supplied for potable purposes only in January–April and October–December, while it should be supplied for all purposes in May–September.