Impacts of compatibility between rainwater availability and water demand on water saving performance of rainwater harvesting systems

Rainwater Harvesting Systems (RHS) are widely implemented to mitigate urban water shortages, yet the impacts of compatibility between rainwater availability and water demand on their performance are often overlooked. This study introduces novel rainwater deficit and surplus indices to quantitatively assess the impacts of compatibility between rainwater availability and water demand on the performance of RHS across 16 cities in different climate zones and under 3 daily water demand scenarios. The rainwater deficit and surplus indices, and the water saving efficiency and reliability of RHS are calculated based on long-term (1991-2020) continuous simulations using a daily water balance model. Results show the rainwater deficit index effectively quantifies water supply pressure on RHS, while the rainwater surplus index quantitatively evaluates their water saving potential. Both of water saving efficiency and reliability of RHS significantly linearly decrease with the rainwater deficit index (P < 0.05), and the coefficients of determination (R²) decrease from 0.96 to 0.80 as tank size increases from 1 to 300 m³. Both of water saving efficiency and reliability increase along sigmoid curves with the rainwater surplus index (P < 0.05), and R² values increase from 0.91 to 0.99 with tank size. The results indicate that tank size optimization enhances water saving performance of RHS through reducing deficits and surpluses. However, the performance enhancement by tank size optimization could be limited by the compatibility between rainwater availability and water demand. Taken Kashgar (arid climate) as an example, the thresholds of water saving efficiency under all water demand scenarios are below 15% due to high rainwater deficit and low surplus. This study emphasizes that the compatibility between rainwater availability and water demand should be considered in planning and design of RHS.