Evaluating the performance of Blue and Green Infrastructures in an urban area through a fine-scale water balance model

Due to climate change and urbanization, increased extreme weather events and impervious urban surfaces have increased flood and drought risks. Blue and Green Infrastructures (BGIs) that can enhance stormwater management can help mitigate these negative effects. Nevertheless, the long-term performance of BGIs in an urban environment still requires further investigation. For this purpose, a fine-scale hydrological model and long-term water balance analysis are necessary. Accordingly, the first objective of this study is to develop a fine-scale water balance model that simulates runoff formation and propagation and incorporates BGIs. The second objective is to perform a long-term water balance analysis to determine the effectiveness of BGIs in mitigating urban floods and droughts. The proposed water balance model is developed in Python. Model inputs include meteorological data such as rainfall and evapotranspiration, and catchment characteristics such as land use and pipe networks. This model divides the catchment into underground pipe reservoirs and surface reservoirs based on land use. All reservoirs are then connected by upstream and downstream relationships according to topological information. BGIs can be implemented by altering the properties of the reservoirs where they are proposed. To calculate the generated runoff, the continuous Soil Conservation Service Curve Number (SCS-CN) method is used in permeable reservoirs with infiltration capability, whereas the single-bucket approach is employed in impermeable reservoirs. The continuous CN method utilizes a dynamic CN and accounts for the recovery of initial abstraction between storms. In the single-bucket approach, all impermeable reservoirs are assumed to have inputs, outputs, and a certain amount of storage capacity. Rainfall and inflow from upstream reservoirs can be considered inputs, while discharge to downstream reservoirs and reuse for rain tanks can be considered outputs. These two rainfall-runoff calculation methods are validated by using monitoring data provided by the Czech Technical University in Prague and the University of Ljubljana, respectively, on green roofs and an urban park. After calculating runoff, the linear reservoir function is used as the runoff routing approach to simulate stormwater propagation according to reservoir connection relationships. Following the above processes, the water balance for the catchment can be analyzed, accounting for evapotranspiration, infiltration, reuse, overflow, and discharge at the catchment outlet. The case study is done for the campus “Arenberg III” at the University of Leuven (KU Leuven) in Belgium. Three BGIs are proposed: permeable pavements, rain tanks and green roofs. Then five BGI scenarios are developed to evaluate the effectiveness of both single BGIs and combined BGIs: (1) campus without new BGIs, (2) every building has a green roof, (3) each building has a rain tank, (4) replacing impermeable parking lots with permeable pavements, and (5) all the BGIs listed above are implemented. The long-term water balance analysis is performed for the period 2010-2019. Initial results show that the combined BGIs scenario (5) yields the best results, as it can significantly reduce runoff and overflow, as well as provide substantial rainwater reuse and infiltration. Therefore, combining BGIs with different functions can be effective in mitigating both urban floods and droughts.