Stormwater treatment systems like filters and screens can reduce pollutant levels by removing contaminants without changing the flow of water they process. The performance of these systems is usually measured by how well they remove pollutants at a certain flow rate. However, their effectiveness can vary depending on the characteristics of each storm. A statistical method has been developed to estimate how well these treatment systems will work over time, taking into account variations in flow and pollutant concentration. This method can predict treatment performance more accurately than simulation models.