Spatiotemporal land use land cover dynamics and rainfall-runoff responses in the urbanizing Gdynia, Poland

Urban expansion and precipitation variability synergistically intensify runoff generation and shorten the time of concentration. This study quantitatively assessed the impact of surface runoff and runoff coefficients in Gdynia, and particularly in the Kacza River basin. A synthetic precipitation event with a 1% probability of occurrence (100-year return period) for durations of 15 min, 60 min , and 24 h is analyzed together with daily precipitation projections under RCP4.5 and RCP8.5 scenarios. Results indicate a decline in agricultural land, green areas, and sparsely vegetated (bare) surfaces by 7.39%, 6.54%, and 2.16%, respectively, between 1985 and 2024. Over the same period, forest cover and built-up areas increased by 5.87% and 10.21%. Notably, 5.37% of the expansion in built-up area occurred within districts located in the Kacza River basin. Correspondingly, surface runoff increased from 0.69 to 1.63 mm (15-min event), 3.27 to 5.27 mm (60-min event), and 25.88 to 31.69 mm (24-h event), reflecting reduced infiltration capacity and a greater share of precipitation converting directly into runoff as urbanization progressed. Under future precipitation projections, surface runoff is expected to rise by 1.09% (2050) and 2.37% (2100) under RCP4.5, and by 2.70% (2050) and 7.33% (2100) under RCP8.5 relative to the baseline. Runoff coefficients will exhibit a comparable pattern, increasing by 0.77% and 1.55% under RCP4.5 and by 1.48% and 2.96% under RCP8.5 for 2050 and 2100 respectively. Despite uncertainties in event representativeness and spatial detail linked to synthetic precipitation event assumptions and historical land-use re-mapping, the methods and findings are expected to add insights into scientific knowledge as well as contributes to water conservation and adaptive urban planning. Currently, changes in runoff patterns are estimated under LULC and projected precipitation changes using daily cumulative synthetic precipitation event as a baseline; future research will extend this framework by simulating outflow dynamics and evaluating the role of technical solutions through hydrological modeling.