Modelling water storage capacity in small, constructed wetlands – today and in the future

Sammanfattning

Wetlands, regardless of their size, contribute essentially to ecosystem services and are therefore vital to sustaining human life and the functioning of the environment. At the same time, wetlands are decreasing at an alarming speed and are exposed to considerable threats. Current policies and programs are not able to stop this trend which is amplified by a lack of research. Due to their potential to reduce runoff and prevent flooding, wetlands have recently gained more research attention. This study aims to model and assess the water storage capacity and thus the flood buffering capacity of ten constructed wetlands in Mälardalen, Sweden, under present and potential future climate scenarios. The hydrologic model PERSiST, a semi-distributed rainfall-runoff model, was employed to reproduce water level observations for constructed wetlands with hourly driving data for a 30-year period. Based on a conceptual model depicting water level patterns of all constructed wetlands with the underlying processes and interactions, observed water level data for 2023 was calibrated against hourly air temperature and precipitation ERA5-Land data. Driving data for the baseline scenario was produced with ERA5-Land data from 1971-2000 and was adapted with Regional Climate model data from SMHI to generate RCP4.5 and RCP8.5 scenarios. Extreme precipitation stretch scenarios were employed to model extreme precipitation events. Results for a subset of three ponds for the 12 different climate scenarios showed no indication of drying out nor a major difference between baseline and RCP4.5 and RCP8.5 scenarios. Stretched RCP and extreme precipitation stretch scenarios presented major variations in water level behaviour, especially in summer and fall. Important factors influencing the flood buffering capacity are the relative area of a constructed wetland, their purpose and design as well as interactions with their surrounding environment. The future climate scenarios suggest that current pond design makes constructed wetlands in Mälardalen resilience against an overall increase of temperature and precipitation but needs further research development regarding extreme precipitation events.