Multi-Year Performance Evaluation of Rooftop Rainwater Harvesting Systems in Semi-Humid Mediterranean Climates: A 25-Year Design Matrix Simulation for Kadirli District, Türkiye

Abstract

Rainwater harvesting (RWH) has gained renewed attention as a decentralized water supply strategy in regions with strong seasonal imbalances between rainfall availability and water demand. This study develops a long-term simulation framework based on a complete 25-year dataset (1 January 2000 – 31 December 2024) for household-scale rooftop rainwater harvesting (RWH) systems in Kadirli, Türkiye, a representative semi-humid Mediterranean district. Daily meteorological inputs were sourced from the NASA POWER database, including precipitation, temperature, relative humidity, radiation, and wind speed. A design matrix of storage capacities (0–30 m³, 0.5 m³ increments) and first-flush depths (0–2 mm) were evaluated. Climate-responsive demand was estimated as proportional to daily reference evapotranspiration (ET₀) with a scaling factor α = 100 L/mm, yielding realistic daily household-equivalent demands. System performance was assessed using volumetric reliability (Rv), day-based reliability (Rd), and spillage ratio. Descriptive statistics of daily rainfall revealed high variability (CV > 2.6), strong positive skewness (≈4.5), and extreme kurtosis (≈29), demonstrating that RWH must cope with long dry spells punctuated by intense rainfall events. Results confirm that RWH in Kadirli is fundamentally supply-limited, with a maximum volumetric reliability of ~0.28 under a 100 m² roof, irrespective of tank oversizing. Shortages concentrate in late summer (August–October), while spillage occurs mainly in winter. Pareto analyses show diminishing returns beyond 15–20 m³ of storage, indicating that oversized tanks are inefficient in this climate regime. This study introduces a replicable, satellite-driven methodology for long-term, climate-aligned RWH evaluation, highlighting the role of RWH as a supplementary rather than standalone water source. The approach is transferable to other Mediterranean cities facing seasonal water scarcity.