Urban rainfall runoff pollution is a major cause of water quality decline in cities. A new method was developed to quickly estimate pollution load from each rainfall event. By using a concept called "characteristic area" and a mathematical model, researchers found that pollution load is related to the size of the area and the amount of rainfall. The model can be calibrated with data from 3 or more rainfalls to make more accurate estimates. This method is simple, practical, and can be used in many different areas, reducing the amount of data needed. For smaller rain events, using similar rainfall data can improve the model's accuracy.