Hillslope runoff and erosion on duplex soils in grazing lands in semi-arid central Queensland. I. Influences of cover, slope, and soil

Abstract

Many soils in semi-arid grazing lands develop low pasture cover or bare areas (scalds) under heavy grazing and have a low tolerance to soil erosion, due to low total water-holding capacity and concentration of nutrients in the soil surface. Runoff and erosion was measured for 7 years on 12 hillslope plots with cover (pasture plus litter) ranging from 10 to 80%, slopes from 4 to 8%, with and without grazing, with and without tree canopy cover, on a variety of soils. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). One plot with low cover had a grass filter at the outlet. Runoff was strongly influenced by surface cover and was high with low cover (200–300 mm/year or 30–50% of rainfall). Runoff averaged 35 mm/year or 5.9% of rainfall with >50% cover. All soils fitted the same runoff–cover relationship. The grass filter had no effect on runoff and suspended load, but did reduce bedload. Grass pasture cover and tree litter cover were equally effective in controlling runoff and erosion. Total, bedload, and suspended load sediment concentrations increased linearly with slope in the range 4–8% for plots with low cover, and decreased exponentially with greater cover. Total and bedload sediment–cover relationships were similar for SS, MS, and MSe. However, plots on MSe had higher suspended sediment losses and thus slightly higher total soil losses. For all soils, erosion resulted in low sediment concentrations due to the hard-set surface soil, but total soil losses were high due to the large volumes of runoff generated. Concentration–cover relationships were different for bedload and suspended sediment. Consequently, suspended sediment was 20–40% of total soil loss for bare soil, and increased with cover to about 80% with cover >80%. The proportion of suspended sediment for bare soil was similar to the proportion of dispersed silt plus clay in the surface soil. About 90% of suspended sediment was fine-sized (<0.053 mm). Bedload was mainly coarse and fine sands, which were enriched compared with the surface soil. Grazing in semi-arid pastures should be managed to maintain >50% ground cover to avoid excessive runoff and soil erosion, and degradation of soil productivity, and to maintain good off-site water quality.