Reliability and Efficiency Dynamics of Rooftop Rainwater Harvesting Under Climate Change across Altitudinal Zones in Usa-themi Catchment, Tanzania

Abstract

Rooftop rainwater harvesting can provide supplementary domestic water, but its performance may vary with climate change and altitudinal rainfall gradients. This study evaluated the reliability and storage efficiency of rooftop rainwater harvesting systems under projected climate change across altitudinal climate zones, using a simulation-based hydrological approach that integrates climate projections with water-balance modelling. The Usa-Themi catchment, located in the Arusha Region of Tanzania, was selected as a case study, where ground-station rainfall records (1994-2025) from three stations (Arusha Maji, Tengeru, and Themi) were collected. Climate projections (2040-2071) were derived using the Multi-Model Mean Ensemble of six climate models under SSP2-4.5 and SSP5-8.5 scenarios.  Reliability and storage efficiency were simulated using a yield-after-spillage water balance model driven by ground-station rainfall records and validated downscaled climate projections across varying tank capacities, roof sizes, and water demand scenarios. The rainfall projections suggested an increase in annual rainfall in the lowland and midland zones by up to 34.3 ± 5.3% under SSP5-8.5, while the highland zone showed a decline of up to 16.2 ± 3.4% under SSP2-4.5. However, the frequency of extreme rainfall events was projected to more than double across all zones. Wetter future conditions generally enhanced reliability, though spillage increased, thereby reducing storage efficiency in smaller tanks. Storage tanks were optimised within reliability and storage-efficiency ranges of 70–95% and 50–90%, respectively, at a demand of 30 L/person/day. The simulations indicated optimal capacities of 10–12 m3 for the lowland and midland zones, compared to 12–15 m3 for the highland zone. These findings highlight the critical role of zone-specific system optimisation in improving rainwater harvesting performance, enhancing water security, and strengthening long-term climate resilience. Further research should incorporate empirical field validation against observed system performance and economic feasibility at pilot scale.