Aarhus University Seal / Aarhus Universitets segl

No. 306: Streams 2017

Thodsen, H., Tornbjerg, H., Rasmussen, J.J., Bøgestrand, J., Larsen, S.E., Ovesen, N.B., Windolf, J. & Kjeldgaard, A. 2019. Vandløb 2017. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 74 s. - Videnskabelig rapport nr. 306
http://dce2.au.dk/pub/SR306.pdf

Summary

This year’s report presents topics on the nitrogen and phosphorus run off to Danish coastal waters. All data presented were collected in accordance to the National Monitoring Program for the Aquatic Environment and Nature (NOVANA).

Nitrogen and phosphorus load to the sea

The concentrations of nitrogen and phosphorus in streams have been markedly reduced since 1989. The main reason is reduced leaching from cropped areas resulting in a mean reduction of approximately 43 % for nitrogen due to several general regulations of farming practices (e.g. fertilization norms, seasonal regulation of manure application, catch-crops), whereas an approximate 40 % reduction in phosphorus is due to improved treatment of residential- and industrial waste water. However, concentrations of nitrogen and phosphorus in the streams are still approximately 4 and 2 times higher, respectively, than in un-impacted streams.

Similar reductions in the total load of nitrogen and phosphorus from land to Danish coastal waters have been estimated for the period 1990-2017. The reductions are 49% and 67% for nitrogen and phosphorus, respectively, if calculated as discharge weighted mean concentrations to take climatic differences between years into consideration. Considering the diffuse run-off of nitrogen alone, including leaching from cropped as well as uncropped areas and contribution of wastewater from scattered dwellings, the reduction is approximately 37%.

In 2017, the total load from land was estimated to about 64 000 tons of nitrogen and 2500 tons of phosphorus. For most months, these amounts being significantly lower than the average for the period 1990-2017. This should be seen in the context that total runoff of water was 9% larger in 2017 than the yearly average for the period 1990-2017.

As it is impossible to measure all water transported to the Sea, there is some statistical uncertainty on the estimated transports, as these needs to be based on both measurements of discharge and nutrient concentrations combined with model estimates for ungauged catchments.