Nitrate leaching in a tile-drained silt loam soil

Nitrate (NO3) leaching was studied for a winter leaching period in a layered calcareous silt loam with tile‐drains at about 1‐m depth and 12‐m spacing. Groundwater levels, drain discharge rates, and NO3 concentrations in the drainage water were monitored, and the soil hydraulic characteristics were measured for the different soil layers. The data were interpreted using the two‐dimensional water flow and solute transport model SWMS_2D. This model uses Darcy's law for water flow and the convection–dispersion equation for solute transport for both the saturated and unsaturated zones. A nitrogen‐production term of 39 kg N ha−1 was used to account for the net N mineralization in the topsoil during the leaching period. The model was calibrated by varying the hydraulic conductivity at saturation (Ks) for the different soil layers, using the measured groundwater level–drain discharge rate relationship as calibration target. Peaks in NO3 concentrations in the drainage water are well explained by the temporal two‐dimensional behavior of convective transport. Measured NO3 leaching was 11 kg N ha−1 yr−1 and simulated NO3 leaching was 15 kg N ha−1 yr−1 in the relatively dry winter leaching period 1991–1992. The two‐dimensional transport model SWMS_2D is a useful tool to evaluate the relative effects of management practices to reduce N leaching.