Mark Thomas_A digital soil mapping approach for regolith thickness in the complex Mt Lofty Ranges regolith-landscape setting: towards a consistent Australian regolith map
This document summarizes research on modeling regolith depth in the Mt Lofty Ranges of South Australia. Regolith includes all weathered material above bedrock and plays an important role in hydrology, biology, energy transfer, biogeochemistry, land use, and more. While some regolith maps exist, coverage is limited. The researchers collected over 700 depth measurements and used environmental data like topography, climate, and geology in a regression model to predict regolith depth across the 128,000 hectare study area. Their goal is to develop a consistent national regolith map to support biophysical modeling. Future work includes testing the approach in other regions and integrating results to create a comprehensive national map.
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Mark Thomas_A digital soil mapping approach for regolith thickness in the complex Mt Lofty Ranges regolith-landscape setting: towards a consistent Australian regolith map
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
6.
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