Samenvatting

This study evaluated the combined effect of vegetation and internal water storage (IWS) on evapotranspiration (ET) dynamics in bioretention mesocosms. Twelve mesocosms were arranged under outdoor summer conditions in four configurations: planted/free-drainage (PF), planted/IWS (PI), unplanted/free-drainage (UF), and unplanted/IWS (UI). ET was determined by mass balance from periodic weighing, while soil moisture was monitored at three depths using sensors. Results showed that vegetation was the dominant control of ET, nearly doubling cumulative water loss compared to the unvegetated systems (~500 mm in PI compared with ~200 mm in UI). IWS further amplified ET by maintaining root-zone moisture, particularly during dry periods. Crop coefficients reached 3.0–3.5 in late summer for PI, demonstrating strong plant-driven recovery of retention capacity. The findings highlight IWS as an effective design enhancement for climate-resilient stormwater control.