The Formation Process, Mechanism, and Attribution of Urban Impervious Surface Thermal Runoff

Abstract

Studies have shown that thermal runoff will form on the impervious surface after rainfall occurs in summer, which will cause thermal pollution to urban water bodies. However, the existing thermal runoff calculation models lack a simplified hydrothermal model suitable for typical impervious surfaces and do not continuously express the process of runoff formation by rainfall. In this study, a simplified urban rainwater runoff heat transfer model was established, and a temperature heat transfer model of urban impervious surface runoff based on numerical simulation was proposed in a continuous state. By comparing with the field-measured data, the model has a higher calculation accuracy (R² = 0.98, MAE = 0.16°C, RMSE = 0.17°C). The combination of the random forest model and Shapley Additive exPlanations (SHAP) method was used to evaluate and explain the main factors affecting surface runoff temperature, and it was found that the initial surface temperature was positively correlated with runoff temperature, which was the most critical factor affecting surface runoff temperature. All other things being equal, we find that for every 5°C increase in initial surface temperature, the event mean temperature (EMT) will increase by 0.2°C, no matter what kind of climatic conditions and underlying surface conditions in summer, the runoff EMT will gradually increase within 2 minutes of the rainfall and reach the maximum value, after which the runoff EMT will show a downward trend, but it will still be higher than the initial rainwater temperature.