Rooftop Rainwater Retention Potential in Former Agricultural Areas Under a Changing Climate: A True Orthophoto-Based Assessment of a Suburbanizing Polish Village

Rapid conversion of agricultural land into low-density housing reshapes local water balances at a time when climate change is intensifying hydrological extremes in Central Europe. This study evaluates the rooftop rainwater harvesting (RWH) potential of a newly urbanized housing estate in Rokietnica (Greater Poland Voivodeship, Poland) using a publicly available true orthophoto (5 cm ground sampling distance) as a reliable geometric data source. Roof footprints of 41 single-family buildings (5305 m2 in total) were manually vectorized in QGIS and combined with monthly precipitation recorded at the nearest meteorological station (Złotniki) in a simplified volumetric model (V = A × R × C, with C = 0.95). To place the single reference year (2021) in its climatic context, the full 72-year precipitation record (1952–2023) was analyzed using the Mann–Kendall test and Sen’s slope estimator. The results indicate a harvestable volume of approximately 2642 m3 in 2021 (53–73 m3 per building; mean 64 m3), with May and August jointly accounting for almost one-third of the annual total. Annual precipitation at Złotniki exhibits a statistically significant increasing trend (+1.29 mm yr−1; p = 0.028) concentrated in winter and early spring, while summer totals remain trendless but highly variable. A monthly storage simulation driven by the recent 30-year record (1994–2023) shows that summer garden irrigation, the dominant practical application of harvested rainwater in Polish households, is considerably harder to meet from the roof alone than year-round indoor non-potable uses: a 5 m3 tank covers approximately 73% of the seasonal demand of a 100 m2 garden plot, while a 3–5 m3 tank would theoretically secure 91–98% of toilet-flushing demand for a four-person household. Under Poland’s national rainwater co-financing scheme (formerly “Moja Woda,” active 2020–2024, succeeded by “Mikroretencja” from June 2026), simple payback periods of roughly 3–7 years make household installations financially defensible. The findings suggest that true orthophotos enable rapid, low-cost RWH assessments in dynamically developing suburbs, and that storage sizing should anticipate the ongoing seasonal redistribution of precipitation under climate change. To our knowledge, this is the first study to combine free national true-orthophoto imagery, a 72-year Mann–Kendall trend analysis, and a subsidy-linked storage-reliability simulation within a single suburban rainwater-harvesting assessment.