Evaluation of the Soil Water Infiltration and Movement model for Assessing the Effects of Grazing Intensity on the Soil Water Balance

Abstract

There is an increasing concern that grazing animals cause soil structural damage. The pressures exerted by grazing animals are comparable to agricultural vehicles and, when soil conditions are conducive, can result in soil compaction. Compaction causes changes to soil physical properties which lead to changes in soil hydraulic properties affecting water entry, storage and movement within the soil. The aim of this study was to examine the effects of grazing on the soil water balance of a gleyed podzolic soil. The soil hydraulic properties, namely, the moisture characteristic and hydraulic conductivity function were measured, under a grazed and ungrazed pasture. The Soil Water Infiltration and Movement (SWIM) model was used to examine the consequences of changed hydraulic properties under grazing on the soil water balance. An evaluation was made of SWIM's ability to predict soil drainage. The study was carried out on a long term grazing trial near Armidale, NSW. The trial was set up in 1958. The two plots used in the experiments were an ungrazed plot and an adjacent plot that was stocked at 10 DSE per ha, situated on a gleyed podzolic soil type. The surface hydraulic properties were determined by both field and laboratory methods at three depths: above, within and below a compacted zone as indicated from soil strength measurements by Lemin (1992). The moisture characteristic was measured by the pressure plate method and unsaturated hydraulic conductivity was measured in the field using a negative head disc permeameter. Although there were no significant differences in the moisture characteristic between the two grazing treatments at 5-9 cm or 20-24 cm, surface infiltration was significantly higher in the ungrazed treatment compared to the grazed treatment. Differences in macroporosity and pore continuity at the soil surface could account for the higher infiltration rate. The treatment difference at the soil surface was for infiltration rates measured at tensions of 20 mm and 10 mm tension. The results suggest that the ungrazed plot has a greater number of pores that are greater than 1.5 mm in diameter and/or the pores are better connected.