Evaluation of tolerable erosion rates and time to critical topsoil loss in Australia

[摘要] Soil is formed over such long time periods that it is considered to be a finite resource. In contrast, soil erosion in agricultural and grazing landscapes occurs at rates that may deplete the productive part of soil in a matter of decades. This report addresses the need to improve the monitoring and modelling of erosion processes around Australia as identified in a report (Leys et al. 2009) recently commissioned by the Australian Government Department of Agriculture, Fisheries and Forestry. This report’s goal is the identification of regions of Australia at risk of exceeding 'tolerable' rates of soil loss. The report provides the first quantitative evaluation of the amount of time land managers have to arrest soil erosion and establish a sustainable soil resource that balances erosion, deposition, and soil formation rates. In an agricultural context, ‘tolerable’ soil erosion rate is the maximum level of soil erosion that will permit a high level of agricultural productivity to be sustained economically and indefinitely. It is common to use the soil production rate to set an ‘indefinitely sustainable’ or ‘tolerable’ erosion rate, i.e., where sustainable net soil loss from a specified region is defined as equal to the rate of soil production. In this report we used measured and modelled pre-European long-term denudation rates to estimate soil production rates.Assuming an average soil production rate of 1.5 x 10-5 m yr-1 and a bulk density of 1.3 t m-3 uniformly over Australia gives a tolerable soil loss of 0.2 t ha-1 yr-1. In places where soil gain from dust deposition are important and equal to soil production, e.g., the western slopes of the Great Dividing Range, the tolerable soil loss becomes 0.4 t ha-1 yr-1.An extension of the idea of tolerable soil erosion is the idea of the life span of soil, or “time to soil exhaustion” which can be quantified as the critical time, Tc, it takes to erode through a soil profile. We define it by: .Evaluation of Tc requires data for soil thickness (S), net soil erosion rate (E, given by the balance of erosion and deposition), and the soil production rate (P). Herein soil thickness is defined in terms of the soil A-horizon, since this is the part of the soil profile that is directly related to agricultural productivity. We evaluate inputs required for calculating Tc, namely maps of soil thickness, erosion rates, and soil production rates, then we estimate Tc, and identify catchments at risk of exceeding tolerable soil loss. There are two broad-scale methodologies for mapping soil erosion. The first takes observed measures of soil erosion by water at the plot scale and generalises them using the Revised Universal Soil Loss Equation (RUSLE) approach so that the average rate of soil erosion by water on the hillslope can be predicted for all combinations of soil type, slope steepness, slope length, vegetation cover, and local rainfall conditions. A hillslope sediment delivery ratio delivery ratio of 10% is assumed for the transport of eroded hillslope soil into streams. The second uses a budget of the radionuclide caesium-137 (137Cs) that labelled surface soils following the atmospheric nuclear weapons testing in the 1950s and 1960s. This method requires the use of geostatistics to interpolate between sparse measurements across the continent made during the 137Cs National Reconnaissance Survey in 1990. The RUSLE modelling and the 137Cs-based technique give inconsistent results with around one order of magnitude difference observed in some regions. The two methods also identify different areas at risk of exceeding tolerable soil losses. RUSLE predictions suggest that northern Australia and coastal Queensland are most at risk. However, according to the 137Cs-based map WA and Victoria are most threatened. A map showing the composite of these results is shown below. Estimates of the time to critical soil loss (Tc), defined here as the time for complete erosion of the soil A-horizon,

[发布日期] 2010-08-01 [发布机构] CSIRO

[学科分类] 地球科学(综合)