Land Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Accelerated erosion and loss of surface soil (continued)
Condition
Surface soil loss: evidence from remote sensing, soil reference sites, and modelling [L Indicator 1.4]
The organic-rich surface layer of soil, containing the majority of soil biota, is of critical importance in the maintenance of ecosystem integrity (Pankhurst et al. 1997). Although surface soil is so important to ecosystem functioning, it is very difficult to obtain information on current changes in surface soil, other than through detailed research and monitoring sites, and then extrapolate these through modelling.
Remote sensing
Remote sensing is so useful for environmental monitoring that a study was undertaken for Environment Australia to assess its application to issues such as soil loss. Unlike the case of simply detecting bare soil (land without vegetation), the study concluded it was unlikely that current topsoil loss could be detected by remote sensing, except in the case of severe erosion events which resulted in wind scars or exposure of subsoil (Wallace and Campbell 1998).
The impact of rabbit calicivirus disease in the Intensive and Extensive Land-use zones
In October 1995, rabbit calicivirus disease escaped from quarantine facilities on Wardang Island, Spencer Gulf, and quickly spread through South Australia and into other rabbit populated regions at the rate of about 18 km per day. Rabbits inhabit almost all terrestrial environments south of about the Tropic of Capricorn, and further north in parts of eastern Queensland. In many areas rabbit numbers initially fell by about 90% as a result of rabbit calicivirus disease (RCD), and have since fluctuated between 15 and 20% of previous numbers.
A coordinated RCD monitoring program was set up, with monitoring sites in most regions. This has provided an unparalleled opportunity to assess the effect of rabbit populations, before and after populations changed so drastically, on vegetation, associated agricultural production and biodiversity. The reduction in rabbit abundance has been particularly marked in the arid and semi-arid regions. In the more humid Intensive Land-use Zone the reductions have fluctuated and been more patchy, for reasons that are as yet not always clear but include a number of predisposing environmental factors (Neave 1999). Rabbits are generally most abundant in arid and semi-arid regions that have soils suitable for warrens (burrows) and a grassy understorey.
In all but one of the intensively monitored sites, reductions in rabbit populations have led to increases in grass cover, shrub regrowth, woody regrowth and shrub seedling survival. There has been some increase in the abundance of other herbivores (e.g. kangaroos) as rabbit populations have decreased in extensively modified grazing regions such as the Tablelands of New South Wales, and Victoria.
In some semi-arid and arid woodlands in Southern Australia, sustained regeneration has not occurred because of a lack of seed sources, but there have been some very dramatic increases in vegetation; for example, in the northern pastoral regions of south Australia, and in recruitment of Acacia papyrocarpa (western myall) on the Nullarbor for the first time since 1900 when rabbits first colonised this region.
Soil reference sites
Paired soil sites, which can be used to compare the changes taking place in the soil profile under different systems of land use, provide the best evidence of change, but these do not cover all land uses or climates. A recent review of existing paired sites in Australia has identified only 30 sets out of some 765 soil research sites where undisturbed woodland or other native vegetation is compared with agricultural land uses (Webbnet Land Resources Services 1999). In this set the western half of the country is almost unrepresented, and there are significant gaps in Tasmania and the high montane regions of the Dividing Range (Figure 12).
Figure 12: Locations of soil profile sites in Australia.
Source: State agencies; CSIRO
Although a National Network of Reference Sites has been proposed and planned under the auspices of the Australian Collaborative Land Evaluation Program (ACLEP) for a number of years (McKenzie et al. 1997), this has not been implemented. From an environmental viewpoint, the most significant gaps at present are the lack of long-term reference sites to cover all principal biogeographic regions. Until such a set of monitored sites is available, the real- time changes that occur in association with climate, land use or land cover change must be extrapolated from indirect lines of evidence. Existing soil profile sites across Australia are not located in a statistically representational manner to cover biogeographic regions, soil types or land management systems (Figure 12).
In 1998, Australia becoming a signatory to the Kyoto Protocol to the United Nations Framework on Climate Change, creating a more urgent reason for identifying gains and losses of carbon in soil. Currently the Australian Greenhouse Office (AGO) is developing GIS-based spatial estimates of woody vegetation and associated soil carbon using 1970 baselines and a year 2000 status timeframe . Estimates for soil carbon for the Kyoto targets will rely upon the paired sites identified, and an additional network of paired sites to be installed and monitored during 2000-01. These will provide sufficient data on the changes that have taken place over the past decade (principally as the result of land clearing) to extrapolate using a model. Indirect evidence on loss of topsoil may be forthcoming, but that is not the object of the work.
Dry cultivation is often still practised, with consequent loss of organic-rich topsoil.
Source: Ann Hamblin
Decline in soil organic matter is a very common feature of soils that have been cropped, and follows a well-described asymptotic decline function that can be reversed with grass and nitrogen-fixing legume pastures (Greenland 1971). Most Australian cropping soils have probably lost about half their original topsoil organic matter through repeated cultivation and resultant oxidation of organic materials. Past soil surveys give us some feel for where the whole surface soil itself may have been lost as well through wind and sheet erosion, but unfortunately these are not sufficiently precisely located in time for any chronological documentation to be possible.
Modelled estimates of sheet and rill erosion
The 1996 State of the Environment Report described the spatial pattern of sheet and rill erosion across Australia, based on the work of Rosewell (1997). Areas of high erosion potential were associated with hill and mountain ranges, regions of cyclonic rain, and soils of natural erosivity (sandy and gravelly, with exposed surfaces). This assessment was not able to identify areas with a high risk of accelerated erosion as the result of current land uses that might expose the soil surface prior to the onset of the rainy season.
Recent work undertaken for the National Land and Water Resources Audit has been able to improve on Rosewell's estimates by coupling recent land-use mapping with seasonal rainfall distributions and improved knowledge of soil erodibility and slope.
The estimated soil loss from each major land-use class (NWLRA 2000) has been calculated (Table 6). While the largest total volume comes from those land uses that extend over the greatest area, such as grazed lands, including woodlands and rough pastures, the highest runoff and soil loss per hectare occur on the intensively used cropping lands. Sugar cane has the highest rate of sheet erosion, and is nearly three times the rate estimated for other intensive crops such as vegetables, potatoes, and specialist (non-cereal) crops such as tobacco, hops, tea, herbs, and nursery flowers.
| Land use description A | Approximate total area (km2) | Total soil loss (t/year) |
Average soil loss rate (t/ha/year) |
|---|---|---|---|
| Perennial watercourse and buildup area | 158 574 | 7 200 | 0.0005 |
| Non-perennial watercourse | 14 315 | 1 600 | 0.0011 |
| Closed forest | 25 116 | 1 986 700 | 0.7997 |
| Open forest | 285 796 | 10 271 600 | 0.3633 |
| Woodland (unmanaged lands) | 2 179 326 | 1 204 995 000 | 5.5899 |
| Commercial native forest fruits plantation | 157 460 | 2 940 400 | 0.1888 |
| Cereals excluding rice | 182 936 | 47 520 100 | 2.6262 |
| Legumes | 22 568 | 1 139 900 | 0.5106 |
| Other non-cereal crops | 687 | 957 600 | 14.0824 |
| Oilseeds | 6 242 | 2 810 000 | 4.5513 |
| Non-cereal forage crops | 270 | 122 000 | 4.5693 |
| Rice | 1 573 | 144 800 | 0.9306 |
| Cotton | 4 053 | 2 943 600 | 7.3425 |
| Potatoes | 441 | 227 700 | 5.2225 |
| Sugar cane | 4 736 | 17 776 300 | 37.9430 |
| Other vegetables | 540 | 688 100 | 12.8858 |
| Nuts | 200 | 328 900 | 16.6111 |
| Improved pastures | 200 295 | 37 571 800 | 1.8964 |
| Residual/native pastures | 4 257 824 | 3 792 659 600 | 9.0053 |
| National parks (mainly in north and centre) | 183 303 | 231 497 300 | 12.7679 |
| Continent total | 7 686 255 B | 5 356 590 200 | - |
| Average continental rate of soil loss | 6.97 |
A From national land use map BRS (2000).
B Actual total land area of Australia is 7 686 801 km2.
Source: Lu et al. (2001).
The cropping lands are most erodible (in terms of t/ha/year), but all these land uses occupy a very small area of the whole. Over half the total land area of the continent is grazed, and the influence of this form of land use on sheet and rill erosion is therefore overwhelming in terms of total tonnes lost per year. Woodlands also contribute substantially because of the large area they occupy.
The predicted continental soil loss from this analysis is about 5.4 billion tonnes per year. This figure is only half of the figure calculated by Rosewell in 1997. The difference is primarily because of the improved information available on seasonal cover, land use and soil erodibility that is now available.
The map shown in Figure 13 (from Lu et al. 2001) shows that there is a close relationship between some forms of agricultural land use, higher dissected topography, and the risk of erosion before the break of season rains. A comparison of this map with the earlier estimates shows that there is a greater differentiation between the northern and southern regions than was identified in the earlier analysis. The greater erosion of soils in native pastures and natural systems than from improved pastures also reflects the difference in location: most improved pastures occur in the southern regions, where rainfall intensities are lower and land cover is more continuous throughout the year.
Figure 13: Continental sheet erosion estimates, based on 1997 land use distributions, and 1990-1999 seasonal greenness (NDVI) and rainfall regimes.
Source: Lu et al. (2001)
The seasonal differences (shown in the seasonal soil loss maps in Figure 13) are very striking in the new analysis. Nearly all significant erosion occurs in the summer, both in the northern (summer-dominant rainfall) and southern (winter-dominant) regions. In the northern regions this is because the rainfall intensities are so great, whereas in the south it is because poor land cover coincides with occasional high intensity summer storms.
A further extension of this analysis has been undertaken that compares the estimated sheet erosion over the past 10 years with that which would have occurred before European impact on the continent (Figure 14).
Figure 14: Ratio of current to pre-1750 vegetation sheet erosion rates.
Source: Lu et al. (2001)
Erosion rates have increased in what is now a cropping region from the Darling Downs right round to the Western Australian wheatbelt. While erosion rates have increased over 100 times in some areas, the absolute difference between northern and southern Australia still existed in the pre-1750 era, because of the great intensity of tropical rainfall.
Implications
The very much higher erosion rates and losses in northern Australia and the higher elevated regions are clearly emphasised in the recent continental assessment of sheet and rill erosion. These northern regions are much more vulnerable to widespread erosion than are comparable central or southern regions, because of the higher intensity of rainfall and larger areas of dissected relief. Very low erosion and soil loss occur in areas which retain a full vegetation cover throughout the year (associated with low evaporative demand and/or high rainfall).
The distinction is also much clearer than in the earlier study by Rosewell (1997), because the analysis had the benefit of investigating the seasonal differences in vegetation response to rainfall, using the NOAA-AVHRR satellite imagery and detailed rainfall statistics. The very much smaller total value of 1.6 billion tonnes of soil loss from the continent that has been computed from the study by Lu et al. (2001) does not mean that there has been a real reduction in erosion since the earlier study, but that we now have a more reliable figure as a baseline.
The greater accuracy stems from the availability of more comprehensive data sets across a larger number of input variables. Extreme events and seasons, when break-of-season rains following long periods of drought produce the most erosion and runoff, are not captured by the present analysis for those parts of the continent that did not experience severe drought in the 1990s (see Figure 29).
In southern Australia there is sufficient green cover from vegetation to reduce erosion to low levels across the agricultural and southern pastoral belt. There are areas of higher ground in southern Australia where there is an ever-present risk of serious erosion, particularly in the Tablelands of the Great Dividing Range, the Adelaide hills and their northward extension into the Flinders Ranges, and small areas of steep land in Tasmania.
In the north, where the summer rainfall is much more intense, substantial accelerated erosion can occur before the country greens up after the opening of the 'big wet'. In the north, retaining land cover must always be of paramount importance to land managers. Vegetation clearing not only destroys native habitats; it can lead to the collapse of ecosystem function and the future productive use of the land.