Enhancing SWMM-UrbanEVA for continuous long-term water balance analysis of green infrastructure

The EPA Stormwater Management Model (SWMM) is widely used to simulate runoff quantity and quality in drainage systems. With the SWMM-UrbanEVA extension, evapotranspiration of green infrastructure is better represented by incorporating vegetation-specific components, improving the analysis of the water balance for sustainable cities. However, accurate modeling of the long-term water balance is challenging due to the sensitivity of results to the chosen reporting time step. In SWMM-UrbanEVA, the results at each reporting time step represent instantaneous values. This leads to reduced accuracy in capturing temporal trends when larger reporting intervals are used to reduce the output file size. To address this limitation, we propose several changes to the code of SWMM-UrbanEVA. This adaptation, termed hereafter SWMM-UrbanEVA-AvgTimestep, computes and outputs the average of all computed time steps for subcatchment results between two reporting time steps. This change maintains the accuracy of water balance simulations by ensuring that total evaporation, infiltration, and runoff are correctly represented, regardless of the reporting time step. Our results indicate that while small reporting time steps are preferred for detailed peak analysis, the averaged results provide a reliable alternative for comprehensive, long-term planning and climate adaptation studies. In addition, we added a new option to export the results as a text file rather than just a binary file. It allows users easier access to the output results. SWMM-UrbanEVA-AvgTimestep empowers engineers and urban planners to analyze the long-term water balance, which is crucial for analyzing the performance of urban green infrastructure and climate adaptation for more sustainable cities.