Deliverable 7.18

Synthesis of work at Vansjø-Høbol catchment
The Vansjø-Hobøl catchment in Norway is located southeast of Oslo in a lowland setting with forests and agriculture. The Lake Vansjø is a popular recreation site and serves as drinking water source for the city of Moss. The lake and its tributaries are highly polluted with chronic blooms of algae, in some years these produce toxins and the water is no longer suitable for bathing. The algal blooms are caused by too much of the nutrient phosphorus (P). The phosphorus comes mostly from runoff from agricultural land and household sewage wastes. The present-day loading of P to the lake is about two-fold that which is deemed “acceptable”, that is, the amount that the lake can tolerate without developing algal blooms.
Mitigation measures to reduce the P inputs have been implemented at the Vansjø-Hobøl catchment over the past 10-15 years, and these have reduced the P loading somewhat. But additional measures are necessary to meet the requirements of the Water Framework Directive. In addition future climate change can make the problem worse.
In the REFRESH project a set of linked mathematical models (climate, river hydrology, river water chemistry, lake chemistry and biology) was used to estimate the effect of future storylines of changed agricultural practices, land use and sewage treatment implementation alone and in combination with climate change. In general climate change will exacerbate the problem, but the largest effect will be that of changed land management practices.
The cost-effectiveness of the various measures was also assessed. It appears that there are sufficient measures available to “solve” the problem, even given future climate change. Several of the measures in agriculture have low cost per kg P removed, whereas more effective sewage treatment has a relatively high cost.
But cost is not the only factor affecting the mangers decisions as to which mitigation measure should be implemented. The measures must be deemed “fair” by the parties involved. Thus costly measures with high government subsidies may be chosen over low cost measures for which the costs are borne by individual farmers.


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