Calculation of net inputs
The net input from a country to a basin comprises of the waterborne inputs discharging from that country to the Baltic Sea, plus the waterborne transboundary nutrient inputs via downstream countries, and subtracted by waterborne transboundary nutrient inputs caused by emissions from upstream countries. In case of nitrogen, the proportion of the airborne inputs to Baltic Sea caused by emissions in the country are also added to the net inputs.
EMEP models the atmospheric deposition attributed to emissions from Eurasian countries and some other sources. Thus, the contribution from each HELCOM country, international shipping on the Baltic Sea and other sources to the atmospheric deposition to each basin is readily obtained from the EMEP data. In addition, EMEP performs a normalization procedure so that natural variability that occurs due to interannual differences in wind and precipitation patterns is removed and thereby results of changes in nitrogen emissions can be identified with shorter time-series.
Annual inputs from rivers and coastal direct point sources are provided to the PLC-water database by the HELCOM countries based on monitoring and modelling. In addition to the reported data, corrections and filling of gaps due to missing data were performed within the PLC 5.5 project for data 1994-2010. Gaps in data for 2011 and 2012 were filled using the same methodology within the PLC-6 project. For later years, only very few values have been filled.
Some rivers are crossing country borders and therefore the nutrient inputs to the Baltic Sea from these rivers are originating also from these upstream countries. These contributions are denoted transboundary inputs. In the preparation of the country allocation calculations behind the 2013 Ministerial Declaration, data on inputs at the borders were collected and combined with estimates of the retention in the rivers downstream the borders to estimate the contributions of transboundary inputs. For the present follow-up, full time-series of the loads at the borders have been used in most cases and the loads have been flow normalized. However, for example, for German contribution to Oder and Belarus to Vistula a given percentage of the river loads are attributed as transboundary.
The riverine inputs are flow normalized according to generally accepted where a linear correlation between the logarithms of flow and load is assumed. Each river is flow-normalized individually before aggregated into country by basin division. The aim of the flow normalization is to remove interannual variations in nutrient inputs that are caused by natural variations in water flow.