Evaluating Household Rainwater Harvesting Potential in Nepal

Water scarcity forces mid-hill households in Nepal to migrate, weakening climate resilience; thus, local adaptation is crucial to combat water stress in the hilly region. Rainwater harvesting (RWH) presents a promising approach to addressing this issue, owing to its cost-effectiveness, self-sufficiency, and strong government support. Among the components of an RWH system, the tank is widely considered the most critical component. This study investigates the reliability and water-saving efficiency (WSE) of RWH at 29 locations in Nepal, considering various climatic scenarios. The average roof area at each location was extracted using Google Earth imagery, and to account for future rooftop growth, the projected values for 2020-2050 were considered. The NEX-GDDP-CMIP6 dataset was used to generate daily rainfall data for the period 2024-2050. Missing values in the historical dataset (1993-2023) were imputed using statistical and machine learning techniques. The performance of RWH was modeled using the Yield After Spillage algorithm. Our investigation revealed that, when using historical data, reliability and WSE of RWH systems range from 43.73% to 87.51%. When projected rainfall was used, performance declined, with reliability and WSE ranging from 40.04% to 82.18% for the SSP585 and SSP245 scenarios, implying that future climate scenarios will negatively impact RWH performance when there is no change in rooftop area. However, when future rooftop area projections were also considered, performance differences gradually reduced over the decades, potentially indicating that increased rooftop areas may offset future climate change-induced performance reductions. Moreover, the difference between the highest and lowest reliability and WSE was generally smallest for 0.5 m 3 tanks, increased for intermediate sizes, and decreased again for 8 m 3 tanks and larger. This trend suggests that, on average, RWH performance is most consistent for the smallest and largest tank sizes. However, while smaller tanks show smaller differences, larger tanks not only offer similar consistency but also demonstrate higher overall performance. Thus, an RWH system with a larger storage size is expected to be more resilient to future climate scenarios.