Spring flood increases phosphorus discharges

The discharge of phosphorus is an important part of the puzzle for reducing eutrophication in lakes and seas. Research shows that phosphorus levels can increase significantly in connection with the spring flood. More measurements will improve knowledge.

The Swedish requirements for reducing the amount of phosphorus entering the Baltic Sea are being tightened up. However, increased knowledge of discharge sources and transport routes is required in order to know that the right measures are being taken and that they are cost-effective.

Previous calculations have shown that around 3,500 tons of phosphorus are transported out to the Baltic Sea via Swedish watercourses every year. New studies indicate that the discharges are probably greater, perhaps by as much as an additional 5-10%, due to the fact that the spring flood causes increases that were previously not known.

Increased levels of phosphorus

Measurements compiled a few years back at a number of estuaries in northern Sweden during the spring flood confirm that some rivers have dramatically increased levels of phosphorus.

“Despite there being relatively few watercourses showing increased phosphorus levels during the spring flood, the effect is significant, as we are talking about large watercourses and huge quantities of water,” says hydrologist Niclas Hjerdt.

“Intensive measurements reveal that the largest increase in phosphorus takes place just before or during the maximum level of the spring flood. This mainly involves the lower sections of unregulated rivers,” says hydrology researcher Johan Strömqvist.

Using computer simulations of the snowmelt process, it has been possible to increase knowledge of phosphorus transport. The conclusions are particularly important for improving current calculation models.

The maps show examples of calculations of phosphorus concentration in the river Kalix in connection with last year’s spring flood. The central map from 5 May corresponds to the highest level of the spring flood.
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More measurements to increase knowledge

As it is often a question of rapid processes in connection with the spring flood, more measurement programmes need to be started in order to increase knowledge of this development in detail. Frequent samples are being taken from watercourses in the Swedish county of Östergötland, in some cases as often as every other hour.

With the help of SMHI’s hydrological forecast model, researchers can follow 10-day forecasts for the water flows in detail and so can prepare to take measurements in the field at precisely the right time. Intensive measurements are taken using automatic equipment, among other things.

“We hope to be able to get a good picture of the process. We can see how much phosphorus is being added to the water and at what time,” comments Lotta Andersson, hydrology researcher.

An important part of the new sampling process is also trying to determine where the phosphorus comes from – if, for example, these are natural discharges or if they are caused by man, such as from outflows and leaching from cultivated land. Water samples are therefore taken from different places and will show whether the discharges originate from fields or outflows and whether they come from groundwater or surface water.

“We can gain basic understanding of the complicated processes involved in phosphorus transportation. Another benefit is that we can develop better measurement programs and learn to measure at the right time.

“Ultimately, it’s also a question of improving our calculation models to give us the basis for making the right decisions for reducing discharges,” concludes Lotta.

The study “Förbättrad modellering av fosforhalter i samband med vårflöden och höga flöden” (Improved modelling of phosphorus levels in connection with spring floods and high floods) has been carried out by SMHI and the Swedish University of Agricultural Sciences (SLU), within the framework of the SMED programme, on behalf of the Swedish Environmental Protection Agency. SLU is responsible for the measurement work.

The actual measurement of phosphorous in Östergötland is taking place within the framework of a research project financed by the Swedish Research Council Formas and the Swedish Farmers’ Foundation for Agricultural Research (SLF). It is part of two research projects in cooperation with Linköping University and the County Council in Östergötland.