The utility of Burns's equation to describe solute movement through soil under various boundary and initial conditions

Burns's equation for describing solute movement through soil is attractive because it is simple and predicts adequately in many instances. However, the assumptions implicit in it are not inconsistent with preferential solute flow. We have explored the consequences of this by leaching initially resident chloride and surface-applied tritium and nitrate through 250-mm-long intact cores of a silt loam soil. The applied flow rates of 3 and 5 mm h(-1) (realistic rainfall intensities) produced unsaturated soil conditions, except near the base where free water dripped out. Burns's equation described the movement of the three solutes fairly successfully with the water content parameter having values between 0.29 and 0.48, similar to the actual volumetric water content of 0.47. The leaching of resident chloride to 450-mm-deep mole drains in the field was also successfully simulated using Burns's equation. However, simulation of the leaching of bromide applied to the soil surface as a solid salt was problematic. This resulted from uncertainty as to whether to treat the application as a pulse input to the flux or resident concentration. The observed behaviour fell about midway between the simulations for these contrasting initial conditions.