Cities are increasingly implementing urban green spaces to cope with the challenges of climate change, including more intense droughts and heat stress. Urban green spaces currently rely largely on drinking water for irrigation, often at rates exceeding the actual water demand of the vegetation. Better understanding of this water demand could help improve irrigation efficiency and reduce pressure on drinking water availability.

This study investigates whether the minimal water demand of urban green spaces can be determined experimentally. A mass balance approach is applied to MeSUDa (Managed experimental Sustainable Urban Drainage Area), an isolated vegetated bioswale located at the Flood Proof Holland site on the TU Delft campus. The setup consists of three compartments with different drainage configurations. By measuring precipitation, outgoing drainage, and changes in soil water storage, evapotranspiration is determined as the residual term of the water balance. The observation period ran from 17 October 2025 to 19 January 2026.

Only one of the compartments could be used to apply the mass balance method. The cumulative precipitation over the observation period was 350.6 mm, though this carries uncertainty due to a 34% discrepancy with data from the official KNMI station at Rotterdam The Hague Airport. The cumulative outgoing drainage was 51.8 mm. The storage change component could not be fully quantified for the entire soil profile.

The soil moisture sensors, installed at 10 and 25 cm depth, only represent the upper 0.35 m of the approximately 1.15 m deep profile. Attempts to further constrain the storage change using a nighttime water balance analysis, an event-based rainfall threshold method, a graphical approach, and time lag analysis between soil moisture sensors did not produce results sufficient to close the water balance.

The cumulative water balance could not be closed due to an incomplete quantification of the storage change over the full soil profile, possible overestimation of precipitation, or a combination of both. The conclusion of this thesis is that, within the current setup and observation period, it was not possible to experimentally determine the minimal water demand of urban green spaces. The mass balance approach applied to MeSUDa is in principle well suited for this purpose, but reliable determination of all water balance components is a necessary condition.