A survey of soil erosion was conducted in Australia using the fallout radioisotope caesium-137 as an indicator of topsoil redistribution. Two hundred and six sites were sampled, 100 within rotational cropping and horticultural use, 52 within uncultivated permanent pasture and forest, and 54 in rangelands. Average net soil losses were approximately equal for cultivated cropping lands and rangelands ( ca . 5.5 t ha − 1 yr − 1 ), and just over 1 t ha − 1 yr − 1 for pasture and forest. The Mann Whitney U Test revealed that losses under cropping and rangeland conditions were significantly higher (p < 0.05) than under uncultivated pasture and forest. Soil loss was negatively correlated with mean annual rainfall and slope gradient, and positively correlated with slope length (Spearman's rank correlation). There was no correlation between rates of soil loss and a rainfall erosivity index. An assessment of erosional events was provided by landholders for 104 sites, with their ranking being weakly but significantly correlated with soil loss estimates (r = + 0.35). Sixty percent of sites had net soil losses greater than 1 t ha − 1 yr − 1 , and 74% of sites had losses of more than 0.5 t ha − 1 yr − 1 . This latter rate may be regarded as a limit for a tolerable level of soil loss. These high rates of soil loss have occurred since the mid-1950s despite there being significant landholder awareness of the soil erosion hazard.
A sediment budget was developed for the 1.7 km2 Maluna Creek drainage basin located in the Hunter Valley, New South Wales, Australia, for the period 1971-86. The impact of viticulture, which commenced at Maluna in 1971, was studied using erosion plots, with caesium-137 as an indicator of both soil erosion and sedimentation. Two methods were used to estimate vineyard soil losses from caesium-137 measurements. Sediment output from the catchment was measured for three years, and extrapolated from readings taken a t a nearby long-term stream flow gauging station for the remaining 13 years. Relative amounts of soil loss from forest (60 per cent basin area), grazing land (30 per cent) and vineyards (10 per cent) were calculated. Soil losses by rain splash detachment were ten times greater from bare/cultivated sufaces than from the forest. Erosion plots of area 2 m2 showed no significant differences in soil loss between forest and grassland but, under bare soil, losses were 100 times greater. The 13'Cs method was employed to calculate net soil loss from all vineyard blocks using both a previously established calibration curve and a proportional model. The latter method gave estimates of soil loss which were 3.9 times greater than by the calibration curve, and indicated that average soil losses from the vineyard were equivalent to 62 t ha-' y- ' (1971-86). It was estimated that the forest contributed 1.8 per cent, the grazing land 1.6 per cent, and the vineyard 96.6 per cent of the total soil loss during that period. Sediment storages within the fluvial system adjacent to the vineyard ws 9460 t for the period, whereas sediment output was equivalent to 215 t km-' y-l. Independent measurements of soil erosion, storage, and output showed that 56 per cent of the eroded sediment remained in the catchment, and 34 per cent was transported out by Maluna Creek. The budget was able to be balanced to within 10 per cent.
Caesium-137 from fallout from nuclear weapons testsis adsorbed on fine sediments and becomes an efecrive tracer. It is hypothesised that within a drainage basin, sites undergoing little or no erosion accumulate 137Cs in their upper layers; cultivated soils will have 131Cs uniformly distributed within the cultivated layer; eroded soils, cultivated or not. will have relatively less I3'Cs, depending on the severity of erosion. Accumulated sediments will have characteristic 137 Cs profiles rejecting temporal fallour variations and sedimentation history. This hypothetical model is largely confirmed by results from Maluna Creek basin, where erosion and accumulation of sediments has taken place. Soils under viticulture have about one third the "'Cs content of soils with grass cover, indicating more severe erosion under cultivation.Caesium-137 projles in alluvial f a n and flood plain deposits correlate with sediment layers and known cultivation history.
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