The effect of agricultural development on the physical and hydraulic properties of four Western Australian soils

Abstract

Changes to the physical and hydraulic properties of four wheatbelt soil types after agricultural development were determined so that the effect of clearing the soils on runoff and erosion could be estimated. Development resulted in an increase in the bulk density of three of the four soils and an increase in the dispersion in the surface of a sandy soil. It also decreased the water repellency of one of the soils. After agricultural development the average sorptivity of the soils, as determined using a disc permeameter, increased from 8.0 to 24.4 mm h-0.5 when the water was added at a 10 mm suction and from 6.5 to 10.4 mm h-0.5 when added at a 40 mm suction. The average steady state infiltration rate, also determined using a disc permeameter, increased from 12.6 to 475 mm h-1 after clearing for a 10 mm suction and from 6.1 to 11.3 mm h-1 for a 40 mm suction. A rainfall simulator added water at a rate of about 40 mm h-1 for 30 min to the four soils. Runoff was generated from all of the uncleared soils but from only one of the cleared soils. On three of the cleared soils, increasing the rate to 60 mm h-1 and the duration to 40 min still failed to produce runoff. The increased infiltration after clearing was unexpected and conflicts with previous results obtained using infiltrometer rings. We believe that methods which disturb the surface crust on uncleared soils are likely to result in higher infiltration rates than occur under natural rainfall events. Localized runoff is likely in uncleared areas with enhanced infiltration at nearby sites. Therefore the lower infiltration rates that we recorded do not necessarily result in higher runoff from uncleared areas. Infiltration in cleared areas is more uniform and the closure of high infiltration sites can result in more runoff from catchments. The point measurements of infiltration cannot be used to accurately predict runoff from large catchments.