If measured pollutant concentration in the recipient surface water body is specified, the calculated total load on the recipient (Lin) is automatically calibrated against the outgoing load (Lout). Total load on the recipient (Lin) is then assumed equal to the outgoing load from the recipient (Lout). The difference between Lin and Lout entails retention within the catchment area (ditch, watercourse, etc.) and in the recipient, inaccuracies in the mapping of the catchment area and over or underestimation of land use specific typical values and volumetric runoff coefficients. Errors in the mapping of the catchment area can, e.g., be that areas for land use are incorrectly estimated.
By calibrating to the measured concentration in the water body of the recipient, the errors are reduced in estimated total load, acceptable load and required load reduction.