Two models for the leaching of a non-reactive solute to a mole drain

Two models of solute leaching to a mole-pipe drainage system are described. The first model is research-oriented. It simplifies two-dimensional water and solute flow to a mole drain by dividing the soil between the mid-mole plane and the mole into notional compartments. Solute movement between compartments is assumed to occur by convection and mechanical dispersion. Within each compartment a mobile and immobile solute phase is defined, with diffusion occurring between them. Rainfall intensity (over approximately hourly intervals) and basic soil hydraulic data are needed as inputs. An explicit finite-difference solution to the water and solute mass-balance and flux equations is used. The second simpler model is management-oriented. It uses daily time steps, and assumes the soil solution behaves as if it were well-mixed system. It requires only daily rainfall and evaporation data, the drainage coefficient of the mole-pipe system, and the soil macroporosity as inputs. In both models a source/sink term accounts for additions of solute in rainfall and fertilizer, and extraction by plant uptake. The models were used to simulate leaching losses of chloride to a mole-pipe drainage system in a silt loam under pasture, following the application of potassium chloride to the soil surface. The first model simulated leaching better immediately after ertilizer application, and during bypass flow induced by heavy rain. However, both models were able to simulate the measured losses over a 2-year period equally well.