Important factors when simulating the water and nitrogen balance in a tile-drained agricultural field under long-term monitoring.

Despite the effectiveness of tile drain systems as a water management practice in naturally poorly drained soils, they facilitate the transport of NO-N to surface water bodies. In order to improve the risk assessment of this significant transport under increased applications of N fertilisers in agriculture, it is imperative to delineate the controlling factors and processes. The aim of this study was to acquire such knowledge using the 1D Daisy model to simulate water and N balance based on comprehensive data from a ten-year monitoring study of a tile-drained loamy field in Denmark under the actual crop rotation of winter wheat, sugar beet, spring barley, winter rape and maize. The model simulated the cumulative drainage and NO-N leaching over the ten-year period satisfactorily with NSE of 1.00 and 0.87 respectively. While the annual N input to the model was 181 kg N ha, an average of 139 kg N ha was harvested in the crop, 22 kg N ha was leached through deep percolation, 17 kg N ha was leached to the tile drains, and 14 kg N ha was lost due to denitrification. Although the model satisfactorily captured the monitored data, the results of this study highlight: (i) the requirement for improved parameterisation of winter crops, (ii) the need to give further consideration in the model to soil surface and macropore processes that govern water infiltration and (iii) that measured and simulated NO-N concentrations in the drainage exceeded the limit defined by the European Drinking Water Directive and Nitrates Directive for drinking water and hence improved N management strategies are essential for tile-drained agricultural fields in temperate regions under conventional crop rotations.