Drainwaterkwaliteit in de kleigebieden in de periode 1996-2001

Management of agricultural farms (nitrogen surplus) can influence drain-water nitrate the most clearly. This is because nitrate appears to be more dominant in younger drain water, while other chemicals have lower concentrations in younger drain water. About 40% of the farm mean nitrate concentrations exceed the European threshold value of 50 mg/l. Besides management, precipitation excess also influences drain-water nitrate. This is the result of testing a new design for a national monitoring network, where drain water from 66 farms was sampled during the 1996-2001 period. The network was found to be capable of detecting, with 80% probability, a future decrease in drain-water nitrate caused by changing farm management. Detection is conditional to the absence of other influences, such as the precipitation excess, or to knowledge on other influences