To meet the environmental quality standards (EQS), i.e., the goals of good status in surface waters, urban stormwater management must be designed with considerations given to the required pollutant treatment removals needed to protect the receiving waters according to the Water Framework Directive. The recipient model in StormTac Web calculates the annual pollutant load (kg/year) that can be transported to the recipient without exceeding the set criteria concentrations, i.e., the acceptable load (kg/year). The acceptable load (L_acc) of the watershed area (kg/year) represents which pollutant load (kg/year) can be allowed to come from the catchment area without exceeding EQS.

We have now updated the method for calculating L_acc with a higher accuracy even when there is no measured concentration in the surface water.

When there is measurement data, the same result is obtained as before when the model calculated L_acc from criteria pollutant concentrations and measured concentrations in the recipient’s water mass, together with calibrated pollutant load on the recipient. By now instead directly calculating L_acc from the criteria concentration (C_cr) and calculated outflow (Q_out) from the recipient, the same result is obtained as before when using measurement data. However, a different result is obtained when there is no measurement data from the recipient. The same equation is now used for both cases.

It can be said that L_acc does not vary directly (but indirectly) with changed pollutant load on the recipient (L_in), but it varies with changed outflow (Q_out) and thus inflow. Increased exploitation, which normally results in increased impermeable areas in the catchment area and increased annual flows, thus results in increased L_acc. In other words, it is not directly measured recipient concentration and calculated pollutant load per se that affect L_acc. The fact that increased flows result in increased L_acc is explained by the fact that the recipient’s retention time is decreased, which provides a greater opportunity to receive an increased pollutant load.