Abstract

A new process in the application of airborne gamma radiation data (radiometrics) to identify variation in soil property patterns across the landscape is described. Soil properties such as texture, pH, and electrical conductivity (associated with salinity) and many others can be mapped using this technology. A case study of Cootamundra Shire is presented which highlights the value of radiometrics for land capability assessment. When combined with other spatial data layers such as vegetation cover, landform, and climate, the results of radiometric analysis can accurately determine land capability for a wide variety of applications. For example, in Cootamundra Shire, patterns of electrical conductivity were identified that clearly demonstrate a relationship between geology and salinity. Accurate identification can highlight areas at risk for agriculture and infrastructure. Such knowledge can lead to well informed planning and management decisions at all scales such as where to site roads, or how best to manage ‘salinity hot spots’ for agriculture. For example, accurate identification of recharge and discharge zones can provide a target for strategic revegetation for salinity abatement. When combined with terrain and climate data, accurate soil property data can also assist in site selection for any given enterprise. The radiometric mapping exercise in Cootamundra Shire has provided the Council with an invaluable knowledge baseline to underpin its infrastructure planning. Identification of the salt pathways has helped to define the risk of salinity affecting infrastructure such as roads, sub-divisions and landfills.

Introduction

This paper summarises the outcomes of a project undertaken by Environmental Research and Information Consortium Pty Ltd (ERIC) entitled Cootamundra Shire: Developing New Agribusinesses. The project was commissioned by the Cootamundra Development Corporation Ltd (CDC) and covers the Cootamundra Local Government Area (LGA), located on the southwestern slopes of NSW. The overall aim of this project was to match viable agribusinesses with the capability and suitability of the land resources and to attract agribusiness investment to the Cootamundra Shire. The focus of investment would generally be small holdings, but up to 200ha where the enterprise is feasible and suitable.

ERIC applied a variety of remote sensing and Geographic Information System (GIS) techniques to this project. These included; landcover classification using a recent Landsat 7 ETM satellite scene, soil properties mapping using SoilMap™, mapping of deep structural geology using airborne magnetics, as well as climate and terrain modeling. Processing of the data layers was undertaken using TNTmips integrated image processing and geographic information system. Initial processing was followed by field survey, lab and statistical analysis of soils, then reprocessing of data layers in TNTmips to produce final feature maps highlighting land use, climatic, geological and soil variability across Cootamundra Shire. Multi-layer analysis of the feature maps, based on defined criteria, produced maps highlighting the most suitable parts of the Shire for given land uses.

Following interviews and workshops with the local community, ERIC also undertook a market analysis of five short listed agribusiness opportunities that would be suited to Cootamundra’s environment. These included;

• Broadacre pulses (cool season),
• Processing – boutique oils, alternative oilseeds, pulses, lucerne
• Perennial tree crops - olives, walnuts, figs, viticulture
• Alternative livestock – deer, goats, alpacas.
• Aquaculture

Pulses, grains and oilseeds are already grown extensively in the district and were therefore not considered in detail.

This paper will focus primarily on the use of the SoilMap™ technique, developed by ERIC, to map soil properties across a landscape such as Cootamundra Shire. This paper will demonstrate the universal value of SoilMap™ in mapping soil properties that are useful for planning and management. Soil properties such as electrical conductivity (indicates salinity), pH (indicates soil acidity), texture and structure were mapped in Cootamundra Shire using SoilMap™.

SoilMap™ is based on the processing of airborne geophysical gamma ray data (radiometrics). An important outcome of this project was the accurate mapping of soil properties across the Shire using the SoilMap™ technique. When combined with other data layers such as landform (derived from a digital elevation model), landuse and climate, land capability and suitability for any given land use can be determined.

The study revealed a fact that is known anecdotally far and wide - Cootamundra boasts exceptional soil resources. The study concluded that the natural resources of the shire, as well as the social and economic infrastructure, could support any of the above agribusiness opportunities. Potential limitations to diversification into intensive and irrigated agriculture are the water resource. Surface water is limited, however, groundwater is extensive but of varying salinity. The study recommended that a Shire wide water study be undertaken to accurately quantify the extent of the water resource. This would give potential investors a reasonable guarantee of supply. However, ventures into tree and vine crops would require little irrigation, particularly on the richest basalt soils in the east and north east of the shire.

The project also highlighted potential threats to agriculture and capital infrastructure, such as salinity. The SoilMap™ analysis revealed a relationship between patterns of salinity, geological faulting and landform, which had caused costly damage to council infrastructure over a long period of time. The paper will provide a case study example of how the information yielded from the SoilMap™ analysis has helped answer questions regarding salinity in the Shire.

The natural resources baseline established for this project has yielded significant planning and management data for Cootamundra Shire. The outcomes of the soil mapping in particular have highlighted opportunities and threats to agriculture, infrastructure planning and investment. After a description of the SoilMap™, the remainder of the paper will provide a case study of how the SoilMap™ technique was able to map salt pathways and how this information has proven valuable to Council land use planning, with the potential to save Council alone hundreds of thousands of dollars. The paper will conclude with recommendations about how knowledge gained from this project can be transferred to the community of Cootamundra Shire to support planning, management and investment decisions.

Radiometric Data in Mapping Soil Properties

The usefulness of gamma ray data in soil and resource assessment has been recognised for some time (Beirworth, 1994; Foote, 1964; Coventry et al, 1994). ERIC has developed a soil mapping methodology that is based on the numerical processing of gamma ray data to yield patterns that relate to soil property variation in the landscape (Tunstall and Gourlay, 1994; Gourlay, 1995; Gourlay and Sparks, 1996).

Airborne radiometrics is a measurement of the radioactive emission from the surface 30cm of the soil, which results from the natural decay of isotopes in the soil. Airborne radiometric measurements have traditionally been obtained for four energy bands. These bands are indicative of total radiometric count, Uranium, Thorium, and Potassium. The data is usually obtained at an elevation of between 40 and 150m above ground level, with measurements about 50 – 60metres along flight lines. Flight line spacings vary with aircraft height above the ground from around 1.5km spacing at 150m elevation, to around 100m spacing at 50 – 80m elevation. Each measurement is split into 3 pixels across the flight line. The radiometric data for Cootamundra was at 150m spacing at an elevation of 80 – 100m. This resulted in a pixel size of 50m.

Emissions of gamma radiation from the land surface vary with many factors but essentially depend on the composition of radionuclides within 30cm of the soil surface. The composition depends on the parent material and the degree of breakdown, loss and/or accession, and therefore generally reflects parent material and weathering. As soils are essentially the product of parent material and weathering, the radiometric data provide an opportunity to remotely sense information relevant to soils. Variation in the radiometric signal across the landscape gives rise to patterns that relate to variations in soil properties (Figure 1). The ability to grid radiometric data to form an image therefore means the data can be used to provide a base map that can be interpreted to provide information on the distribution of soils (Gourlay & Sparks, 1996).

Figure 1 Unclassified radiometrics image of South East Cootamundra Shire.

2.1 Classification

The ERIC SoilMap™ technique involves firstly numerically classifying the four bands of raw radiometric data, based on spectral similarity and spatial adjacency. The initial classification defines a number of radiometric classes, which then provide a basis for field survey. In the case of the Cootamundra study, 23 radiometric classes were defined across the Shire (Figure 2). The initial classification is unsupervised and is derived as an independent data layer. Factors that would otherwise influence the production of a soils map, such as catenary influences or vegetation cover, are not considered during the initial classification. However, because parent material is a primary determinant of the radiometric signal as well as for soils, and because soils derived from different parent material can produce a similar radiometric signal, geology is used as a reference when undertaking the initial classification (Tunstall, 1995).

Figure 2 Classified radiometrics image.

2.2 Field Survey and Lab Analysis

Numerical classification is followed by field survey. To allow statistical analysis, at least five stratified soil samples are taken from each radiometric class across the study area. Ideally samples are taken from the A1, A2, B1 & B2 horizons, however, for the Cootamundra study, samples were only taken from the A2 and the B2 horizons. Measurements and observations were made in the field, such as depth of each horizon, field moisture content, land use and catenary position. Samples from the A2 and B2 for each site were taken back to the lab for measurement of variables such as pH, electrical conductivity, texture, Eh (oxidation/reduction potential), colour and structure.

2.3 Feature Mapping

Results of the lab analysis were then statistically analysed to determine mean soil property values and ranges for each radiometric class. These values were then applied to the original radiometric classification to produce feature maps displaying variation in the measured soil properties across the whole of the Cootamundra Shire. Figure 3 provides an example of a soil texture feature map of Cootamundra Shire.

Figure 3 Soil texture feature map of Cootamundra Shire.

The final part of the analysis was to highlight specific soil properties, and combine these with the other data layers such as terrain, climate and land use to provide an indication of land capability and suitability for defined land uses.

Salinity in the Cootamundra Shire

3.1 From the Perspective of Cootamundra Shire Council

Prior to the ERIC study, Council had been working on a new land use planning instrument (LEP). As part of this planning process Council had accessed all the published data available from government sources to assist in classifying and allocating land to a variety of uses. These included both environmental and general agricultural classifications as well as land types specifically set aside for more intensive horticultural use. During the process differing soil types, vegetation types, watercourse types and other characteristics were observed, which when combined with anecdotal and experiential evidence provided by land-holders gave Council a good understanding of the limitations and possibilities of the land.

However, in many ways Council was merely stating the bleeding obvious without being able to give more than a rudimentary explanation as to why the land showed the characteristics that it did. This was particularly so in the case of isolated areas of salinity. An example of this is an isolated half-hectare showing signs of salt tolerant vegetation growth and a couple of dead eucalypts, sited at nearly the highest elevation in the Shire(Photo 1). This location receives only a small amount of drainage water from the Council road network and was never known to be waterlogged; yet it was surrounded by highly productive cropping land. The fact that the soil structure changed slightly from east to west at this point (a fault line) gave some hint but not enough for the community to really claim an understanding of the process and develop a mitigation strategy.

Photo 1 Salinity effects on pasture and native vegetation atop Moonie Moonie fault.

The ERIC data showed that the salt in the Shire was moving laterally along fault lines or fractures (Map 4). Subsequently, Council and the landholders are now able to make informed judgements about the salinity process and the mitigation actions (Trethewey and Gourlay, in press).

3.2 Specific Electrical Conductivity in the Soils of Cootamundra Shire

The analysis of optical satellite data located the various types of land uses and land conditions, and demonstrated the highly developed nature of the Shire in terms of agricultural use. Radiometric data analysis of the soils revealed the ranges in soil properties such as pH, electrical conductivity (EC/salinity), texture, oxidation/reduction potential (pe) and soil depth. The main soil limitations evaluated for agribusiness capability were salinity (measured as ECe) and acidity (measured as pH). Modelling was undertaken to establish mean rainfall patterns and mean temperature ranges across the Cootamundra LGA for the major plant growing period between August and March.

Specific conductivity (ECe) of the soils in the Cootamundra LGA is highly variable. Mean ECe levels in the A2 horizon ranged from close to 0 to greater than 0.4 deci-seimens per metre. Mean ECe in the B2 horizon ranged from just above 0 to greater than 0.9 deci-seimens per metre. The ECe levels represent average conductivity levels for each soil class. In some localised cases ECe levels may be substantially higher due to accumulation of salt as a result of land use impacts and lateral movement of salt through the soil profile. Feature maps were produced to highlight the distribution of the radiometric class that displayed the highest mean ECe levels in the B2 horizon (Map 4).

This variability of salts in the soils of the Shire is well known to both the landholders and Council and has been a feature of the area since its settlement by Europeans. The early settlers are said to have named the Salt Clay Creek on the edge of Cootamundra Township after observing cattle and sheep licking the clay banks. Cootamundra’s original water supply dam, Hardy’s Folly, was never used, as the water became brackish immediately it came into contact with the clay bottom. Farmers have also known for many years that low lying areas may exhibit signs of salinity in the season following a dry season and may return to normal after another wet winter as salts are flushed away.

Map 4 Highest electrical conductivity class in both the A2 and B2 soil horizon occurring along geological faults.

There was a strong relationship between soils demonstrating high specific conductivity, in both the A2 and B2 horizons with geological fracturing or lineaments. Also, ECe levels, while well within acceptable agricultural limits were generally higher in the soil classes associated lower parts of the landscape (sinks). It is likely, given that all soils contain some salt, that this salinity is due to lateral movement of salt through the soil profile and then accumulation in fractures (pathway) or the lower parts of the landscape (sink).

It also appears in living memory that the most of the permanently saline areas have been essentially in the same locations. For example, the site referred to earlier in this paper sits atop the Moonee Moonee fault line near Wallendbeen. The location of the saline pathways and sinks has implications for Council in relation to new rural subdivisions, roadworks, selection of sites for gravel pits, and a myriad of other activities. This salinity baseline also has implications on the positive side of the ledger with the vast majority of the Shire not exhibiting any signs of increasing salinity outbreaks.

The salinity patterns within the catchment of Muttama Creek strongly correspond to the Cootamundra, Gundagai, Coolac and Moonee Moonee geological fault lines. Muttama Creek contributes about 1% of the flow of Murrumbidgee River above Gundagai, however it contributes about 15% of total salt to the Murrumbidgee above Gundagai (Webb,1999). The results of the ERIC survey point to these geological fractures as the preferred salt pathways that may be contributing salt to the Muttama Creek. On the other hand, in the areas of tertiary alluvium to the north of Stokinbingal, soils demonstrating the highest conductivity levels related somewhat to the patterns of alluviation. The data suggest that these areas are old salt sinks. The higher conductivity levels in these classes and the relationship to the geology and alluviation suggest that gamma data should be a fundamental data set for determining salt pathways and sinks and therefore the salinity risk to a shire.

Given the threat that salinity poses to rural enterprise development in the Murray-Darling Basin, identification of salinity risk in the landscape could lead to a target for new enterprise sites, which are also locations for salinity remediation measures, such as planting of deep rooted perennials. The identification of areas at risk of salinity, as well as identification of the underlying causes of the threat, can also be vital in infrastructure planning. For example, the salinity feature map demonstrated a strong relationship between a salt pathway and potentially damaging effects on a section of Olympic Way north east of Cootamundra. This section of the Olympic Way crosses over the Gundagai fault, an area that corresponds to the highest ECe class for the B2.

This 3-kilometre section of the Olympic Highway, referred to as the Jindalee Straight, has for many years been a drain on both Council and RTA resources. Continual waterlogging of the strip of land (Photo 2) bounded to the west by the Main Southern Railway Line and extending approximately 50 metres east of the Olympic Highway led to the transference of salts from this pathway up through the road pavements to the bitumen seal. This caused both the breakdown of the bitumen and the constant movement of the road pavements, leading to an annual repair bill of up to $50,000. The problem has been stabilised over the last 6 years by using geotextile blankets to intercept the rising water 1.2 metres below the pavement. However, this came at a cost of approximately $300,000/kilometre and has exacerbated the salt scalding in the paddocks adjacent to the road.

Photo 2 Easement between Olympic Way and the main southern railway line demonstrating signs of constant water-logging.

Council repaired this section of road for many years without ever being able to fully accept that runoff or recharge from the surrounding hills could have produced the volumes of water travelling along the straight. Many theories were considered to try and explain the problem in an attempt to justify and be reconciled to the constant cost of repairs. These included such things as blaming the Railways for damming the flows, blaming the farmers for clearing the land, blaming the settlers for placing the road in the valley, blaming anyone or anything. In real terms a pointless exercise other than to allow Council to deal with the frustration of an inherited and essentially natural process of salt movement (Trethwey and Gourlay, in press).

The ERIC data has helped Council to understand the mechanisms by which such a volume of water is reaching the Jindalee Straight, by showing the Gundagai Fault passing under the straight as a conduit for mobilising salt and water from much further afield than the surrounding hills (Map 4). Council must continue to live with the road in its present position but every piece of new information will assist in not repeating the same mistake again with new subdivisions or other land uses. This one example shows what a hit and miss approach was previously used to deal with salinity in the past but it is also a prime example of how modern mapping technologies can support land use planning and management.

Given that the Muttama Creek catchment contributes 15% of the salt with 1% of the flow to the Murrumbidgee River above Gundagai, the community of the Cootamundra LGA has a sound basis for developing a salinity management strategy. The salinity management strategy could include investment strategies or farmer incentives for water management in accordance with water quality targets set out in the Murray-Darling Basin Salinity Strategy.

3.3 Development of a salinity management strategy

In terms of investment potential, the Murray-Darling Basin Draft Salinity Strategy (2000), and the NSW Salinity Strategy (2000) point to measures that communities could adopt to combat the salinity threat to their own area as well as areas down stream. The combination of natural salinity processes and land use activities in one part of the Basin can have an impact on other parts of the Basin. The strategy recognises that initiatives undertaken in one area which have a positive benefit in terms of reducing salinity and improving water quality should be suitably rewarded. The concept is referred to as ecosystem services - environmental services that may not be of direct economic benefit to the landholder but may provide benefit to the community at large. This could include water quality and biodiversity protection, rural productivity, public infrastructure maintenance, etc. The Cootamundra Shire now has the baseline information to effect this action and contribute to the Basin strategy for salinity management.

Conclusion

The above case study discusses only one of the many outcomes that the Cootamundra agribusiness study. Identification of threats such as salinity have the benefit of also showing those areas that are not at risk. The fact that Cootamundra is largely free from the threat of salinity provides more confidence to investors that are interested in the area. In fact the study has also clearly highlighted those areas where premium growing conditions could occur for any of the commodities identified in the study.

The study recommended that the CDC, along with the Shire Council and other key stakeholders in the Shire, set up a land information business unit (LIBU) to assist in the transfer of natural resource information to customers such as farmers, planners, investors and community groups. Land Information Business Unit services would include;

• decision support for planning and investment,
• training or farmers and community groups in computerised property planning, landscape planning, and interpretation of remotely sensed imagery,
• Spatial data such as soils, property boundaries, and a recent landsat scene highlighting targeted features at appropriate scales (e.g. catchment, property).
• access to policy and promotional information from each of the natural resource and agricultural organisations operating in the area.

The study stated that a Land Information Business Unit would establish a natural resources knowledge base-line for the Shire, and provide a means by which members of the community, as well as planners and investors could access and interpret this knowledge. This base-line would naturally also be built upon over time. The study recommended that the biophysical base-line established through this project be incorporated into an environmental management information system (EMIS) cycle. An EMIS cycle establishes a process of base-line knowledge acquisition, monitoring, evaluation, reporting, up-dating the base-line and so-on. Once each EMIS cycle is completed, the knowledge base-line is reset and decision making is informed by this new base-line. Acquiring and applying knowledge within the context of an EMIS cycle informs decision making because it leads to a greater understanding of whether or not actions undertaken to affect an environmental system are leading to an improvement in the system. EMIS can be applied at any scale from the property, through the Shire to the region.

The Cootamundra Development Corporation has accepted the recommendations of the study and is now in the process of establishing a funding stream to enable a land information business unit to be set up in Cootamundra. If all goes according to plan, within two years the Cootamundra community should have access to the latest in remote sensing technology, training in the use of such technology and the appropriate tools to undertake spatial analysis for planning, management and investment.

References

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