Estimation of the Share of Total Nutrient Load from the Territory of Estonia along the Narva River to the Baltic Sea ^†

The annual inputs of nitrogen and phosphorus by the Narva river into the Baltic Sea are distributed between Estonia and Russia according to the catchment area shares of 33% and 67%, respectively [1]. A comparison of the total nutrient load shows that the proportion between countries may vary depending on what input data and estimation methods are used. The aim of this study was to improve the comparability of estimates in order to obtain more accurate information on the distribution of nutrients from Estonia. The results were obtained from the testing of the Estonian and Russian calculated methods [2] and initial data for the period 2006–2018, as well as the distribution of sources calculated for the direct catchment area of the transboundary Narva River. Detailed calculations for each sub-basin were carried out using mainly national coefficients. A comparison of the calculated total input with the monitoring data showed that both countries’ estimates of nitrogen and phosphorus loads correlated quite well with the monitoring data. The main sources of Ntot in the immediate catchment were point sources and agriculture. The main difference was in the Ptot input from natural and urban areas and agriculture. Therefore, point sources, agriculture and runoff from urban areas can be considered the priority sources. According to the results obtained for the Estonian territory, the contribution of Ntot was 23% and that of Ptot was 29% [3].