Late Fall Site-Specific Soil Nitrate Upper Limits for Groundwater Protection Purposes

For temperate regions, such as Germany, a simple management model is developed, with which NO{sub 3} seepage losses during winter can be estimated, when the amount and distribution of soil NO{sub 3} in late fall are known. The semi-analytical, one-dimensional model is developed for homogeneous soils. The model is derived for steady-state flow conditions at a constant soil water content (field capacity). Essential part of the model is a solute transport equation based on mixing-cell theory. Sample calculations show that when the height {Delta}z of the mixing-cells is chosen such that {Delta}z = 2D/v, where D is the soil dispersion coefficient and v is the pore water flow velocity, mixing-cell model results compare well with results from convective-dispersive models. The advantage of mixing-cell model solutions frequently appear as simple mathematical expressions. With the model sample calculations concerning NO{sub 3} seepage losses during winter were carried out for a variety of site conditions. The calculations show that NO{sub 3} seepage losses vary considerably, even when the initial amount and the distribution of the soil NO{sub 3}-N are the same. The calculations also show that if a limit is set for the total amount of tolerable NO{sub 3} seepage losses during winter,more » site-specific late fall upper limits for soil NO{sub 3}-N can be derived. A table of such values, for a wide range of site-conditions, is presented. 25 refs., 4 figs., 6 tabs.« less