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

Abstract

For temperate regions, such as Germany, a simple management model is developed, with which NO3 seepage losses during winter can be estimated, when the amount and distribution of soil NO3 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 Δz of the mixing‐cells is chosen such that Δz = 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 models compared with convective‐dispersive models is that mixing‐cell model solutions frequently appear as simple mathematical expressions. With the model sample calculations concerning NO3 seepage losses during winter were carried out for a variety of site conditions. The calculations show that NO3 seepage losses vary considerably, even when the initial amount and the distribution of the soil NO3‐N are the same. The calculations also show that if a limit is set for the total amount of tolerable NO3 seepage losses during winter, site‐specific late fall upper limits for soil NO3‐N can be derived. A table of such values, for a wide range of site‐conditions, is presented.

Calculation of nitrogen mineralization and leaching in fallow soil using a simple dynamic model

“…In LIXIM, the layer thickness has been taken as z = 1 cm, which would correspond to the dispersivity λ = 0.5 cm. We checked that the transport submodel of LIXIM gave similar results to those published by Van der Ploeg et al . (1995) with this dispersivity.…”

“…Using the terminology defined by Addiscott & Wagenet (1985), the water and solute leaching submodel in LIXIM is a ‘deterministic functional model with a layer approach’. It is derived from the approach proposed by Burns (1974) and the ‘mixing‐cells’ concept developed by Van Ommen (1985) and Van der Ploeg et al . (1995) for nitrate movement.…”

“…Therefore the equivalence between the functional (Burns or mixing‐cells) and the mechanistic (CDE) models is true if the layer thickness and the soil dispersivity are small enough and in the ratio 2:1. Comparisons between the two models relative to NO 3 – leaching have been made by Van der Ploeg et al . (1995) who found good agreement between the two approaches when the dispersivity was less than 5 cm.…”

A semi‐analytical model for solute transport in layered dual‐porosity media

“…Note that the approach differs from that used by van der Ploeg et al (1995), who approximated the spatial derivatives by finite differences and integrated over time whereas in this present approach time is differenced and space is continuous.…”

“…In temperate regions, solute leaching occurs predominately in the winter period when soils are approximately at field capacity. Consequently, water-flow conditions may be characterized as nearly steady-state (van der Ploeg et al, 1995). It has been shown that under such conditions, the convection-dispersion equation (CDE) yields comparable results to more comprehensive transient models if solute leaching is considered as a function of cumulative infiltration (Wierenga, 1977).…”