THE MONITOR 3 - Site Evaluation (1.1) project news

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Project news by Jody Bruce

The focus over the next six months is to continue improving the weathering model and to complete the landscape modelling work at Green Hills.

Improving methods to measure soil thickness and manage for better water holding capacity

by John Gallant, Research Scientist (terrain analysis, CSIRO Land and Water)

Soil depth has substantial influence upon the productivity of a plantation and understanding soil thickness is important for plantation managers, particularly at the planning stage. Soil thickness varies from site to site, so if soil isn’t thick enough at any individual site there may not be enough water storage for trees to survive during drought-affected summers. For example, in Western Australia at least four metres of soil may be required to hold adequate water for trees in plantations but in Tasmania thinner soil layers can be tolerated as the climate is cooler, with less evaporation and more summer rain.

Soil-landscape surveys have been the traditional approach for delivering soil thickness information - these contain plenty of interpretive information and typical hillslope profiles but often lack explicit mapping of soil properties at a stand scale; spatial variation of soil thickness within polygons boundaries is described but not mapped, requiring that managers interpret descriptive information in the reports and make a best guess about soil thickness variation within a coupe.

Digital soil mapping techniques based upon geophysical remote sensing and digital terrain analysis provide an alternative approach to soil landscape surveys that require less interpretation but provide more explicit mapping of specific soil properties. This allows the user to define boundaries appropriate to the end use and more easily understand the variability within a system. Relationships between soil properties, their position in the landscape and geology can be modelled in a quantifiable manner, allowing continuous improvement as new data and methodologies become available.

Soil thickness is controlled by the balance between addition and removal of soil materials. Transported sediment accumulates in lowland depositional areas resulting in a thick layer of soil, loose rocks, decomposing rocks and other materials (known as the regolith) above the solid bedrock. The remaining areas (classed as erosional) can have thin or thick soils depending upon the balance between the supply of soil-building material (by rock weathering or wind-blown dust) and the removal of soil via erosion. Thick soils develop where the rate of soil removal from the hillslope is limited by the ability of water to transport soil (transport limited slopes). Transport limitation may be due to low slope, thick vegetation cover, highly cohesive soils or some combination of these factors. Thinner soils are found where the rate of removal of soil is limited by the rate of supply of material (supply limited slopes). Soil thickness on these slopes tends to be strongly dependent upon the position of the slope, with thin soils high on the slopes and thicker soils found on the lower slopes where soil is also washed down from higher slopes.

Mapping soil depth is therefore dependent upon distinguishing between depositional and erosional areas, and between transport-limited and supply-limited slopes in the erosional areas. A terrain-based technique, MrVBF (see also The Monitor 2), can identify areas of sediment accumulation with reasonable reliability by identifying flat low-lying areas. Distinguishing between transport-limited and supply-limited hillslopes is more difficult and appears to depend upon topography, geology and climate.

Topography is expressed mostly through slope, with steep slopes having a much higher ability to transport soil. Different geologies have markedly different weatherability and hence rates of supply of raw material for soil production. Climate has a substantial influence through the availability of water to drive the weathering process and through temperature, which alters reaction rates. None of these factors is dominant: a particular rock type might produce deep soils in a wet climate and shallow soils on the same slope in a dry climate; similar slopes in the same climate can have deep or shallow soils depending on the underlying geology; and the soil depth on a given rock type in a particular climate can vary substantially with slope. Vegetation also has a profound effect on soil development and erosion, but at this stage we are assuming that its effect can be captured by geology, climate and terrain at least for the undisturbed condition under which most extant soils developed.

An initial model based upon these three effects was posed as a preliminary hypothesis in the form of an index with values greater than 1 indicating transport-limited conditions and less than 1 indicating supply-limited conditions. The topographic component is represented using local slope. The climate component is limited to water balance in the first instance using the ratio of precipitation to potential evaporation computed from solar radiation and temperature (both dependent on topography – known as the Prescott index). Weatherability of rock was assessed from descriptions in a geology map. The three factors are multiplied together to create the index.

We spent a week at the Green Hills site (see also Caroline Mohammed's report ) and surrounding area digging holes and inspecting road cuttings to test this model and found that the general concept of the three controlling factors was valid but the construction of the model was inadequate. Part of this was due to errors in assessing rock weatherability and the simplistic manner in which the factors were combined. Slope, for instance, appears to have a dominant influence on the steepest slopes but is less important on moderate slopes. The use of a simple polygon map of geology also caused problems, with clear variations due to compositional changes in granodiorites mapped as a single unit.

The weathering model is now being revised based upon the Green Hills experience. To date, the model has only been developed for south-east New South Wales (NSW) and around Canberra. The next version should be a significant improvement and suitable for testing over wider areas: into Victoria, other areas of NSW and Tasmania. We intend to do some validation work at a field site in north-east Tasmania in October.

Reliable discrimination between transport-limited and supply-limited hillslopes will support mapping of soil thickness over broad areas, at least in south-eastern Australia. Application in other areas may require further extensions to the model to account for other controlling factors, including temperature and more complicated soil-formation processes. We are also looking into investigating the validity of soil thickness models in Western Australia.