Application of simple leaching models in heterogeneous soils

Abstract

The simple leaching model described divides the soil into layers, each containing an amount of mobile water ω m, defined simply by ω m > 0 and an amount of immobile water ω r, that is determined by the soil moisture characteristic. Solute moves downward through the mobile water only, but may also move laterally between the mobile and immobile water. Water and solute entering each layer are added to ω m, and each day a proportion, α, of ω m and the solute it contains move to the layer below. Of the input variates ω r and α have most control on leaching, so to test the model for the effects of input variability these variates were supplied as single mean values or as distributions in simulations of a field experiment. In this experiment nitrate was applied to the soil surface of twelve plots and allowed to leach under natural winter rainfall for two months before the plots were sampled in 15 cm layers to 75 cm. Supplying ω r and α as single mean values gave quite good simulations of the mean percentage recoveries of applied nitrate in the layers, and these could be improved slightly by replacing the single value of ω r by its distribution. No further improvement in the simulation of the mean recoveries was achieved by supplying α as a distribution. The variances of the percentage recoveries in the layers were simulated reasonably successfully, but only when both ω r and α were supplied as distributions.