Modelling water and solute transport in macroporous soil. II. Chloride breakthrough under non-steady flow

A model of water and solute transport in macroporous soils (Jarvis et al., 1991) has been evaluated in column breakthrough experiments under field conditions. Hydraulic properties were first measured in replicate soil monolith lysimeters sampled from grass ley and continuous barley treatments in a clay soil. A pulse input of 0.05 M KCl was then supplied by drip irrigation and measurements made of the water discharge and chloride leaching resulting from the natural rainfall over a 1-month period. The results showed that the macropores constituted the dominant flow pathway (accounting for 80% of the total water outflow) and that diffusive exchange of chloride between the two flow domains was the main factor causing variability in leaching. Larger hydraulic conductivities and macroporosities in the lower topsoil and at plough depth in the grass ley monoliths were taken as evidence of structural amelioration. Less of the applied chloride was leached in the grass monoliths than in the barley (means of 20% and 31% respectively). This was mainly due to a smaller effective aggregate size and thus a more efficient diffusion-controlled retention.