EVALUATION OF NITRATE LEACHING RISK AT SITE AND FARM-LEVEL

At the 'De Marke' experimental farm a dairy farm was set up with the a im of meeting environmental and economic goals. The farm management wi th respect to nitrogen emphasized reduction of fertilization and a cat tle grazing system that should result in nitrate concentrations in the groundwater below the EC-directive level of 11.3 mg l(-1) nitrate-N. At six sites in six different fields of 'De Marke', these concentratio ns were monitored for 4 years. A direct comparison with the chosen lim it was possible for these sites, but an evaluation of the environmenta l achievements of the farming system at farm level was also required. This was achieved by using simulation models and additional informatio n about soils and field management. Based on multiple soil profile des criptions, frequency distributions of model output were generated, all owing a risk assessment for the total farm. The probability of exceedi ng the chosen threshold value of 11.3 mg l(-1) nitrate-N during the pe riod of summer 1991- spring 1995 was 63% for the whole farm, with mark ed differences between years, crops and hydrological conditions.