South Western Zone of Ancient Drainage

Posted on Jan 23, 2015


LandResourcesSouth-western Zone of Ancient Drainage

An ancient plain of low relief on weathered granites with sluggish drainage systems and uplands dominated by sands and gravels. Lateritic uplands dominated by grey sandy plain predominately with Proteaceous species.

SWCC Strategic Priority

Threats to be addressed:

  • water erosion
  • land salinization
  • soil structure decline and/or compaction
  • loss of soil carbon

DAFWA – Report Card (September 2013)

The South-western Zone of Ancient Drainage, for the purposes of DAFWA’s Report Card, is included in the Agricultural Soil Zone 8. Southern Wheatbelt (along with the South-eastern Zone of Ancient Drainage). Agricultural Soil Zones are based on repeating patterns of soil and land types. The Ag Soil Zones are used for reporting on the following themes:

  • Soil acidity
  • Wind erosion
  • Water erosion
  • Soil organic carbon
  • Soil compaction
  • Water repellence
  • Nutrient status (phophorus)

Hydrozones are based on grouping areas with similar hydrology and are used for reporting on:

  • Dryland salinity and
  • Acidification of inland waterways.

The South-western Zone of Ancient Drainage is identified on its own as a hydrozone.

Key Findings for this Zone

Soil Acidity

Soil Acidity

Summary – A large proportion of soils are below target, and the trend shows continuing deterioration.

Condition and Trend – Poor and Deteriorating (confidence in condition and trend: limited evidence or limited consensus in both).

This zone is dominated by duplex soils with deep sandy duplexes and alkaline shallow duplexes comprising over 40% of the area. Acidity in the surface soil is extreme. More than 80 and up to 89% of samples collected had pH below the recommended target for the layer. Unless surface soil pH is increased, the subsurface soil layers will continue to acidify and restrict productivity.

The subsurface layers of alkaline shallow duplexes and the saline wet soils are at lower risk. However, for the deep sandy duplexes and the gravels that make up 38% of the area, the risk of low soil pH increases, with 40% to 50% of the samples collected from the 10-20 cm layer being below the recommended target and around 30% of the samples from the 20-30 cm layer samples also below the target.

A summary of the percentage of samples below target for depths 0-10 cm, 10-20 cm and 20-30 cm for the Ag Soil Zone 8. Southern Wheatbelt.

A summary of the percentage of samples below target for depths 0-10 cm, 10-20 cm and 20-30 cm for the Ag Soil Zone 8. Southern Wheatbelt (DAFWA Report Card, 2013).

Source: Gazey C, Andrew J and Griffen E (2013). ‘Soil Acidity’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Soil Acidification in the south-west of Western Australia go here.

Wind Erosion

Wind Erosion

Summary – More reliable rain produced good crops and stable ground covers. Areas close to salt lakes were poorer.

Hazard and Trend – Low (meaning 1 yr in 4 below target hazard – yrs 2009-12) and variable (confidence in hazard: adequate high-quality evidence and high-level of consensus; confidence in trend: limited evidence or limited consensus).

Click on the image below to enlarge. Table below (from the DAFWA Report Card) shows that in 2011, there was a high average wind erosion hazard for the zone (Zone 8 – Southern Wheatbelt).

Wind Erosion Hazrad Summary for 2009-12

Wind Erosion – Hazard Summary for 2009-12 (DAFWA Report Card, 2013)

Source: Carter D and Laycock J (2013). ‘Wind Erosion’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Wind Erosion in the south-west of Western Australia go here.

Water Erosion

Water Erosion

Summary – Good condition but increasing intensity and frequency of summer storms in the east remains a concern.

Hazard and Trend – Low and deteriorating (confidence in hazard and trend: limited evidence or limited consensus in both).

Source: Galloway P and van Gool D (2013). ‘Water Erosion’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Water Erosion in the south-west of Western Australia go here

Soil Organic Carbon

Soil Organic Carbon

Summary – SOC levels declining along west to east rainfall gradient. Little data in driest areas.

Abundance and Trend – Moderate and unclear trend (confidence in abundance: limited evidence or limited consensus; confidence in trend: evidence and consensus too low to make an assessment).

Source: Griffen E, Hoyle F and Murphy D (2013). ‘Soil Organic Carbon’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Soil Organic Carbon in the south-west of Western Australia go here.

Soil Compaction

Soil Compaction

Summary – Grey clays with declining soil structure are prone to collapse and compaction.

Hazard and Trend – High (meaning soil hazard generally high and current land use likely to cause compaction) and deteriorating (confidence in hazard and in trend: limited evidence or limited consensus).

Source: Carter D, Davies S and Schoknecht N (2013). ‘Soil Compaction’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Soil Compaction in the south-west of Western Australia go here.

Water Repellence

Water Repellence

Summary – Majority of low rainfall areas have no problems. There are some problems on grey sands in the south of the zone.

Condition and Trend – Fair (meaning water repellence of low abundance) and stable (confidence in condition and in trend: limited evidence or limited consensus).

Source: Carter D, Davies S Blackwell P and Schoknecht N (2013). ‘Water repellence’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

Dryland Salinity

Dryland Salinity

Summary – Salinity is extensive and likely to to expand because groundwater levels are mostly rising.

Risk and Groundwater Trends – High (Likelihood of occurring: likely; consequence: major) and deteriorating (confidence in condition and in trend: adequate high-quality evidence and high level of consensus).

Most of the currently saline land occurs on broad valley floors. An expansion of the area affected by salinity is likely to continue, as groundwater levels continue to rise, particularly in areas of valley hazard. A major impact is expected as there are large areas within the valley hazard not currently affected. The expected timing for the development of further salinity is 50 years or more.

Source: Simons J, George R and Raper P (2013). ‘Dryland Salinity’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Dryland Salinity in the south-west of Western Australia go here.

Nutrient status (phosphorus)

Nutrient status (phosphorus)

Summary – Soil P fertility well in excess of optimal range. Ceasing P application would see P levels slowly decrease to optimal levels.

Condition and Trend – Well in excess (meaning P fertility index > 1.5) and stable (confidence in condition and in trend: limited evidence or limited consensus in both).

Source: Weaver DM and Summers RN (2013). ‘Nutrient status (phosphorus)’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Nutrient status (phosphorus) in the south-west of Western Australia go here.

Acidification of inland waterways

Acidification of inland waterways

Summary – There are areas of acid, saline groundwater and acidification of baseflow has occurred. There is insufficient data to determine trends or extent.

Condition and Trend – Good and trend unclear (confidence in condition: limited evidence or limited consensus and in trend: evidence and consensus too low to make an assessment).

The South-western Zone of Ancient Drainage is internally drained, with the western margins externally drained. About 11% of the hydrozone has an acid groundwater hazard, particularly in the valley systems. Despite the overall good rating, there is significant acidification of baseflow surface water in a limited area (Fence Road drainage system) in the south of the zone. (Seewraj 2010).

Source: Ruprecht J (2013). ‘Acidification of inland waterways’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

Projects:

SWCC has developed two videos to help you get started with soil health. Part 1 describes a process for investigating soil health and identifying what constraints to manage first. Part 2 answers some common soil health questions, such as how to manage soil organic matter and soil biology, impacts of synthetic fertilisers on soil biology, and organic soil amendments such as compost and biochar.

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Header and thumbnail images supplied by Peter Clifton.

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