1. Modelling regolith depth in the Mt Lofty
Ranges, SA: towards an Australian regolith map
Mark Thomas, CSIRO Land and Water
John Wilford, Geoscience Australia
John Gallant, CSIRO Land and Water

2. Regolith – what is it, why is it
important?
• “Soil” is often only thought of as the upper 0.3 – sometimes to 1.5 m - of ground surface
• Traditionally a strong agricultural focus
• Regolith is all weathered bedrock material mantling hard bedrock, including soil
• Has critical biophysical function, moderating interactions of
• Hydrology: storage, movement, quality ….
• Biology: fauna, flora, biodiversity ….
• Energy: absorption, emission, transfer
• Biogeochemistry: C, N, P, trace elements, gaseous exchange (H2O, O2, CO2, NOx …) …..
• Cultural: land use patterns, farming, urban, conservation
• These are integrated functions
• We need a broader understanding of regolith - beyond just soil, beyond agriculture focus - for
biophysical modelling
• Regolith mapping is in short supply
• Only small % of Australia covered at 1:250,000 scale or better
• Most maps show nature and composition, rarely depth
• Mapping is inconsistent
• We’re making a start towards a consistent national coverage by researching how to map regolith depth

3. Study area: Mount Lofty Ranges
• Controls on regolith depth:
• Complex geology
• Complex weathering, long
landscape history
 Recently exposed (fresh, shallow)
 residual mantles (Tertiary,
ancient and deep)
• Neotectonic history
• Range of landforms
• MLR contains many analogues of
southern Australian landscapes, so a
good place to start

4. Site data, depth to hard bedrock
• Strong reliance on existing, “legacy” regolith data
• Borehole logs
• Mineral exploration, groundwater, geotechnical
• > 14,000
• New site observations
• Road cuttings
• Creek beds to exposed bedrock
• Issue: biased landscape positions
• New drilling
• Focused on areas where our field knowledge was
poor/incomplete
• Drilled in clusters, along toposequences
• < 9 m, 60 mm in tact cores removed
• 714 useable depth records
• 128,000 ha study area
• ~ 1 observation per 180 ha

5. Modelling regolith depth
• An environmental correlation approach
• Using readily available environmental parameters (digital grid
maps) to predict other, less easy to measure environmental
parameters
• The environmental parameters correlate to soil properties, e.g.
relationships between elevation and hillslope soil depth
• Environmental parameters from national data infrastructure,
including
• Topographic parameters: ref John Gallant’s talk, including
MrVBF, topographic wetness index, aspect, slope …
• Climatic parameters: rainfall, evaporation, ….
• Geochemical parameters: geology, gamma radiometrics,
weathering intensity index, ….
• 28 used
• Grids standardised to 30 m ground resolution
• Piecewise, multi linear regression modelling (“Cubist” software)
• Model produces a decision tree, consisting of
 Branches, the rules/conditions (“piecewise”)
 Leaves , the linear regression models
• Cubist makes models explicit for expert evaluation
• Model result
• R2 0.64

7. Next?
• Working towards a seamless, consistent national regolith map
• An enduring piece of national data infrastructure for all Australian/international biophysical
modellers
• With estimates of uncertainty
• Presented /served via
• Discoverable through the TERN Data Discovery Portal http://portal.tern.org.au
• Visible through the TERN Soil Portal http://www.asris.csiro.au/viewer/tern
• Downloadable from CSIRO Data Access Portal https://data/csiro/dap
• Phase 2 - testing the approach in the Burdekin catchment (on-going)
• New biophysical region with new national analogues
• Complements the spatial disaggregation approach (Nathan Odgers’ talk), which will be used for
method benchmarking
• Multiple lines of evidence, testing multiple approaches towards the best national data product(s)
• National approaches may vary, depending on
• Biophysical regions, landscape histories
• Quality, density of useable site observations
• The local “power” of specific environmental parameters
• National regolith map: a stitched patchwork of multiple biophysical regolith models/maps (?)
• New regolith map themes
• Whole-of-profile or layer-by-layer stocks
• Plant available water
• C, N, P, trace elements ….
• clay content, salinity, pH …
• …

8. Acknowledgements
• CRC Forestry
• SA Government (DWLBC)
Thank you
Dr Mark Thomas
CSIRO Land and Water
mark.thomas@csiro.au
08 8303 8471