Spatially distributed lateral nitrate transport at the catchment scale.

In river catchments, N transformation and storage processes during lateral transport are important in controlling N loads of surface waters. There is a lack of approaches which capture lateral flows and associated N transformation in a spatially distributed way. The aim of this paper is to develop a new conceptual N transport and transformation model which simulates the lateral nitrate transport in subsurface flow from the source area to the receiving water body. The developed tool is based on the object modeling system (OMS) framework and consists of the analytical spatially distributed hydrological model J2000, the nitrate recharge model Meta Candy and a new groundwater N routing component. Nitrate degradation in groundwater is calculated stoichiometrically according to a predefined amount on oxidizable substrate. The new modeling approach was tested in a small agricultural lower mountain range catchment of Thuringia, Germany. The calibration of the N model using a 4-yr period showed reasonable results for nitrate load calculations with a Nash and Sutcliff coefficient of 0.78. The 3-yr validation period produced Nash-Sutcliff (NS) values of 0.75. There was a clear relationship of the goodness-of-fit between the hydrological simulations and the nitrate concentration calculations. Due to short residence times of the interflow nitrate degradation was restricted to slow base flow components. The new approach can be used to target N source areas within a catchment and assess the impact of these source areas on the N load of surface waters in a spatially distributed manner.

Impact of selected agricultural management options on the reduction of nitrogen loads in three representative meso scale catchments in Central Germany.

Nitrogen inputs into surface waters from diffuse sources are still unduly high and the assessment of mitigation measures is associated with large uncertainties. The objective of this paper is to investigate selected agricultural management scenarios on nitrogen loads and to assess the impact of differing catchment characteristics in central Germany. A new modelling approach, which simulates spatially distributed N-transport and transformation processes in soil and groundwater, was applied to three meso scale catchments with strongly deviating climate, soil and topography conditions. The approach uses the integrated modelling framework JAMS to link an agro-ecosystem, a rainfall-runoff and a groundwater nitrogen transport model. Different agricultural management measures with deviating levels of acceptance were analysed in the three study catchments. N-leaching rates in all three catchments varied with soil type, the lowest leaching rates being obtained for loess soil catchment (18.5 kg nitrate N ha(-1) yr(-1)) and the highest for the sandy soils catchment (41.2 kg nitrate N ha(-1) yr(-1)). The simulated baseflow nitrogen concentrations varied between the catchments from 1 to 6 mg N l(-1), reflecting the nitrogen reduction capacity of the subsurfaces. The management scenarios showed that the highest N leaching reduction could be achieved by good site-adapted agricultural management options. Nitrogen retention in the subsurface did not alter the ranking of the management scenarios calculated as losses from the soil zone. The reduction effect depended strongly on site specific conditions, especially climate, soil variety and the regional formation of the crop rotations.