Management scenarios for reduced nitrate loads in a small catchment in Brittany (France) - the problem of data scarcity and the resulting predictive uncertainty.


The Soil and Water Assessment Tool (SWAT) has been applied to the Ic watershed, Brittany, France, to evaluate scenarios for reduction of nitrate in stream water. For the simulated period the model showed fair results with a mean index of agreement of 0.64 at the watershed outlet for discharge and nitrate loads. The management goal for the watershed is the meeting of drinking water threshold at the watershed outlet. An analysis of observed data revealed that nitrate loads would have to be reduced by at least 17% on average to reach that goal. Scenarios investigated cover fertilizer reduction and the introduction of wetland buffer zones. Decreased nitrogen inputs were realized on a) selected subbasins and b) all agricultural fields; wetlands were placed at three model subbasins. Most effective measures were a 50% fertilizer decrease on selected subbasins resulting in a range of 13 22 % reduction of nitrate loads with a high uncertainty. Consequently, none of the tested measures is likely to achieve a sufficient reduction. Combined measures such as enhanced fertilizer management and concurrent introduction of wetlands seem to be the most promising way to approach the drinking water threshold.

p 8 In: 5th International SWAT Conference. Boulder, Colorado, USA. 5. - 7.8.2009
Matzinger, A.
Matzinger, A.