Modeling of nitrogen cycle and nitrate transfer in regional hydrogeologic systems

A model of the nitrogen cycle and nitrate leaching in soil, called MORELN, was developed to calculate the nitrate flux percolating from soils into aquifers, It uses a ''coupled model,'' which simulates both surface and subsurface flows, A third model, NEWSAM, is linked to the above models to simulate nitrate migration in an aquifer system, This methodology gives a quantitative global description of nitrate transfer, taking into account both leaching in soils and migration in aquifers, Three examples of application tested this methodology (especially the nitrate leaching code, MORELN) at different scales and with different emphases: (1) in a soil plot of several square meters; (2) in a small experimental basin of several square kilometers; and (3) in a regional hydrogeologic system of several hundred square kilometers. Satisfactory results were obtained for the simulation of nitrate leaching at the small scale, Major differences between the measured and simulated nitrate concentrations were observed in a regional aquifer, where the measured concentration does not spatially correlate and the larger spatial variability of the input data were averaged in the model.