The revised and improved functions are the result of ongoing work in collaboration with Luleå Technical University with a scientifically reviewed publication on quantification of uncertainties in the StormTac Web application (Wu et al., 2020). The features use a method based on The Law of Propagation of Uncertainties (Taylor, 1994), where absolute uncertainties are calculated from relative uncertainties. This is described in the guide, with reference to the publication (Wu et al., 2020) for more detailed description.

The improvement uses a new function when different values are summed, which reduces uncertainty, while a multiplication gives increased uncertainty, according to a scientifically reviewed paper by Hedbrant and Sörme (2001). This new breakdown using different equations gives changed resulting relative uncertainties for total flow, total load and total concentration before and after treatment, respectively, with consequent changes in calculated absolute uncertainty.

As before, various absolute uncertainties for different substances are obtained, but based so far on assumed relative general uncertainties for concentrations and runoff coefficients from literature values. However, there is a continued collaboration with Luleå Technical University to develop specific uncertainties that take into account the uncertainty in concentrations for each substance and land use from StormTac’s database, as well as for runoff coefficients per land use. That work will be described in a new paper that will also be scientifically reviewed. Implementation of this more advanced uncertainty model is expected to take place within a few months.