A usable mechanistic model of nitrate leaching I. The model

SUMMARY A mechanistic model to simulate the leaching of nitrate in soil is described. Three flow patterns are accommodated: conventional convective‐dispersion; preferential flow, in which water and solute moving in the larger water‐filled pores fails to equilibrate with that in the smaller pores; and by‐pass flow, in which water (and solute) moves rapidly down cracks and fissures without displacing the soil water. The pattern of flow depends on the rainfall intensity and the hydraulic conductivity of the water‐filled pore space. The performance of the model is examined in terms of its ability to reproduce solute profiles calculated from analytical solutions to transport equations, and in the sensitivity of the output to changes in the main parameters. A second paper (Barraclough, 1989) illustrates the use of the model to reproduce nitrate leaching patterns observed in the field.