Trees in dense urban environments are often planted in bioretention cells with an underlying trench (BC-T) providing both stormwater pretreatment and storage. The BC-T design is based on a water balance; however, some input data (tree water uptake and water-holding capacities of soil filter and trench substrate) are difficult to obtain. The goals of this paper were (i) to study the sensitivity of such data in the BC-T design (i.e., their effect on the size of the drained area which may be connected to the tree pit), and (ii) to recommend a possible simplification of the water balance for engineering practice. Global sensitivity analysis was performed for the setup of a BC-T used in Prague, Czech Republic, assuming three different trench exfiltration rates. The most sensitive variable affecting the size of the drained area is the available water-holding capacity in the trench. The simplification of the water balance is highly dependent on exfiltration conditions. At high exfiltration rates (18 mm·h⁻¹ and more) or for a trench with an underdrain, the water-holding capacity in the soil filter and the tree water uptake can be omitted; whereas, at low trench exfiltration rates (1.8 mm·h⁻¹, without an underdrain), both the water-holding capacity of the trench substrate and the potential tree water uptake have a significant influence and cannot be omitted.

Key elements of Blue-Green Infrastructure are vegetation and stormwater storage. A combination of a bioretention cell with an underlying trench (BC-T) serving as a tree pit is often used in dense urban environments. An adequate ratio of drained area to bioretention cell area is a crucial design parameter. The ratio is derived from the hydrological balance; however, input data are often difficult to obtain or uncertain. The goal is to study the sensitivity of such data (tree water uptake and water holding capacities of soil filter and trench substrate) in the BC-T design. Sensitivity analysis is performed for the setup of a BC-T used in Prague, Czech Republic. A 10-year rainfall series (1 hour resolution) is used as an input. Data that are subject to the sensitivity analysis are changed for different trench exfiltration rates, and the effect on the size of the drained area is studied. At low trench exfiltration rates (1.8 mm.h-1), both the water holding capacity of the trench substrate and potential tree water uptake have a significant influence (more than a 20% change in the size of the drained area) and cannot be neglected. At good exfiltration rates (more than 18 mm.h-1) or when the trench is equipped with an underdrain, all studied parameters can be neglected. However, it is recommended to reduce the size of the drained area by 10-20%.