Long-Term Assessment of Rainwater Harvesting and Storage Reliability at a Mediterranean University Campus

Rainwater harvesting (RWH) may contribute to sustainable water management in Mediterranean regions, where water availability and demand are highly seasonal. This study examined the potential for RWH and the reliability of storage at the Kestel Campus of Alanya Alaaddin Keykubat University in Alanya, Türkiye, using monthly climate data from 2000 to 2024. The analysis considered variations in precipitation and air temperature, FAO-56 Penman–Monteith reference evapotranspiration (ET0), the climatic water balance, campus land cover, potential runoff, non-potable water demand, and six storage-capacity scenarios. The campus area is 231,700 m2, of which 35,700 m2 represents the potential roof catchment area. The average annual precipitation was 1104.1 mm, and the mean annual ET0 was 1241.6 mm, resulting in a mean climatic water deficit of 137.5 mm per year. The average theoretical roof runoff was 35,475 m3 per year, and the modelled annual non-potable water demand, including toilet flushing and irrigation of 1 hectare of green space, was about 19,804 m3 per year. With the assumed demand, runoff, and monthly operating conditions, the volumetric reliability rose from 72.3% for a storage capacity of 2500 m3 to 99.0% for 10,000 m3 and reached 100% at 12,500 m3; further increasing the storage to 15,000 m3 did not provide any additional benefit in terms of reliability. These storage capacities should be understood as planning scenarios only, not as design recommendations, since economic feasibility, event-based operation, and water quality were not assessed. The results show that a long-term seasonal water balance analysis is important for planning rainwater harvesting at the campus level in Mediterranean climates.