THE PREDICTION OF NITRATE LEACHING WITH THE FIRST-ORDER ACIDITY BALANCE (FAB) MODEL FOR UPLAND CATCHMENT IN GREAT-BRITAIN

The relative contribution of N deposition to the acidification of fres hwaters in Great Britain has increased over the last few years as S de position has fallen in line with reduced emissions. In certain high de position areas of Great Britain, NO3--based acidity can equal or excee d the contribution of SO42--based acidity in some upland waters. Here we apply the first-order acidity balance model (FAB) to predict the ma ximum N leaching from 13 study catchments at future steady state. Usin g mean water chemistry and catchment soils data, along with long-term default values for N sink processes, we predict NO3-N leaching at much higher rates than currently are being measured in surface waters, wit h a mean increase of 10.5 kg ha(-1) yr(-1). As a result, mean acid neu tralizing capacity would decline to less than 0 meq L-1 at 4 sites. Wh ile there are uncertainties associated with model parameterization rel ating to the short-term storage of N within catchment soils and vegeta tion, model outputs do indicate much greater leaching of N at some tim e in the future as steady-state is achieved.