Simulation of N leaching from a small agricultural catchment with the field scale model SOILNDB

The SOILNDB modelling system was used to calculate the nitrogen (N) leached from fields in an intensively monitored, mainly tile-drained, agricultural catchment for a 4-year period. A new version of the model was tested in which leaching can be divided between flow to tile drains and groundwater. Input data for the simulations were crop and field management data and soil characteristics for each field in the catchment, as well as daily climate data for the area. After adjustment of the parameter governing potential groundwater flow, we obtained a good agreement between dynamics in water discharge simulated from the fields and that measured in the stream outlet, reflected by a bi-weekly model efficiency value of 0.68. We also obtained a rather small (4%) difference between the simulated and measured accumulated water discharge. The simulations resulted in an N leaching from the fields of 44 kg ha−1 as an area-weighted mean annual value, whereas measured N transport in the stream outlet was 40 kg ha−1. This difference could be attributed to dilution of stream water from other non-agricultural land uses that typically have lower N concentration. The retention was probably small, which was indicated by the almost equal load in simulated N leaching from the fields and estimated net loss of N from arable land (i.e. transport in stream after subtraction of estimated loads from sources other than agriculture). The results indicate that applying a modelling system like SOILNDB at small catchment scale is feasible and that it can be a useful tool when detailed analysis, for example of the relative effects of alternative crop management practices on N leaching, is needed.