Thornthwaite Moisture Index in Victoria: climate zone and Hs

The Thornthwaite Moisture Index (TMI) is the climate number that decides which AS 2870 climate zone a Victorian site sits in, and that zone sets the depth of design suction change (Hs), the depth to which the clay dries and wets each year. TMI is rainfall minus evaporation, expressed as an index. A wetter climate gives a higher TMI, a shallower Hs and smaller seasonal ground movement. A drier climate gives a lower TMI, a deeper Hs and larger movement. For a reactive clay lot, the same soil can classify one class higher in a drier part of the state than a wetter one, because Hs feeds directly into the characteristic surface movement (ys) that fixes the AS 2870 class.

That is why a desktop report for a Melbourne, Geelong, Ballarat or Bendigo address starts with the climate zone, not the soil test. The zone is knowable from published mapping before anyone drills. It tells the engineer how deep the field investigation must reach and roughly how much movement to expect.

How TMI sets the AS 2870 climate zone and Hs

AS 2870 divides Australia into six climate zones by TMI band, and each band carries a design value of Hs (Sun et al. 2017). The drier the band, the deeper the soil moves.

Zone	Description	TMI band	Hs (m)
1	Alpine / wet coastal	above +10	1.5
2	Wet temperate	-5 to +10	1.8
3	Temperate	-15 to -5	2.3
4	Dry temperate	-25 to -15	3.0
5	Semi-arid	-40 to -25	4.0
6	Arid	below -40	above 4.0

Hs is the depth over which soil suction is assumed to change over the life of the house. A deeper Hs means more of the clay profile is active, so for the same shrink-swell index (Iss, the vertical strain per unit suction change) the characteristic surface movement ys comes out larger.

Where Victoria sits on the TMI map

Victoria spans most of the useful range. Wilsons Promontory and Cape Otway, on the exposed southern coast, are wet: Sun et al. (2017) reported a TMI near 50 at Wilsons Promontory and near 30 at Cape Otway, firmly in the wet zones with shallow Hs. The far northwest is dry: Mildura returned a TMI around -41, close to the semi-arid and arid boundary, with a correspondingly deep Hs.

The populated centre of the state sits in between. Sun et al. (2017) calculated a TMI near -19 for the Melbourne Airport station over 1993 to 2013 using the Thornthwaite method, placing Melbourne in the dry temperate band with an interpolated Hs around 2.6 m. Towns such as Gisborne, Heathcote and Avoca returned TMI values in the -13 to -22 range over recent decades (Lopes & Osman 2010), the same temperate to dry temperate territory. These are not alpine or wet coastal numbers. Across most of Melbourne, Geelong, Ballarat and Bendigo the clay works to a few metres of depth, not one and a half.

Why the climate zone has been drifting drier

TMI is a long-run average, and the Victorian average has been falling. Lopes and Osman (2010) compared three twenty-year periods from 135 weather stations and found TMI lower in the most recent period at most towns. Gisborne dropped from around -1 in the 1948 to 1967 period to around -13 by 1988 to 2007, enough to move it across a zone boundary. Apollo Bay fell from the high twenties to around 12 over the same span. A shift of one zone changes Hs by several hundred millimetres and pushes ys up with it.

The effect is sharpest when a short, dry run of years is used. Karunarathne et al. (2016) worked one Melbourne site through different averaging windows and showed the class swing: a 50-year average gave an H2 result, while the drought decade of 2002 to 2012 on its own produced an Extremely reactive (E) classification, with ys rising from around 73 mm to around 95 mm. The soil did not change. The climate window did. Li and Sun (2015) linked the same drying trend to houses in Melbourne's western suburbs that were built in the dry years to 2011 and then cracked from heave when the rain returned.

What this means before you drill

The climate zone is the part of an AS 2870 classification that a desktop report can set with confidence, because it comes from published mapping rather than the lot. What it cannot do is measure the clay. Hs tells you how deep the active zone reaches; only a borehole and a shrink-swell test on this lot give the Iss that, combined with Hs, produces ys. A high TMI will not save a lot with deep, heavy basaltic clay, and a low TMI will not condemn a sandy profile. The climate zone and the soil work together, which is why the AS 2870 site classification is finished in the field, not on the desk.

Use the zone to scope the investigation. In the dry temperate parts of Victoria, Hs reaches past two metres, so the field holes and any shrink-swell sampling need to reach the full active depth rather than stopping at a shallow auger refusal. On reactive clay this is often where building on reactive clay decisions are won or lost before the first design is drawn.

Common questions

What is the Thornthwaite Moisture Index?

The Thornthwaite Moisture Index (TMI) is a climate number based on rainfall minus evaporation, used to classify how wet or dry a location is. In AS 2870 it sets the climate zone and the depth of design suction change (Hs), which in turn drives the characteristic surface movement ys that fixes a site's reactivity class.

What climate zone is Melbourne in for AS 2870?

Most of Melbourne sits in the dry temperate band (climate zone 4 or the drier edge of zone 3). Sun et al. (2017) calculated a TMI near -19 at the Melbourne Airport station over 1993 to 2013, with an interpolated Hs around 2.6 m, meaning the clay works to around two and a half metres of depth each season.

Does a lower TMI increase the site classification?

Yes. A lower TMI places the site in a drier climate zone with a deeper Hs, so more of the clay profile is active and the characteristic surface movement ys comes out larger. Karunarathne et al. (2016) showed one Melbourne site move from H2 to Extremely reactive when a drought decade was used in place of a 50-year average.

To see the climate zone, likely Hs and mapped geology for a specific Victorian address, generate a desktop report for the address, or see a sample report first.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria and South Australia.

Sources

• D. Lopes and N. Y. Osman (2010). Changes of Thornthwaite’s Total Moisture Indices in Victoria from 1948-2007 and the effect on seasonal foundation movements. Australian Geomechanics, Volume 45, Number 1 (Mar 2010). geomechanics.org.au
• A.M.A.N. Karunarathne, E.F Gad, M.M. Disfani, S. Sivanerupan and J.L. Wilson (2016). Review of calculation procedures of Thornthwaite Moisture Index and its impact on footing design. Australian Geomechanics, Volume 51, Number 1 (Mar 2016). geomechanics.org.au
• Xi Sun, Jie Li and Annan Zhou (2017). Evaluation and comparison of methods for calculating Thornthwaite Moisture Index. Australian Geomechanics, Volume 52, Number 2 (Jun 2017). geomechanics.org.au
• Jie Li and Xi Sun (2015). Evaluation of changes of the Thornthwaite Moisture Index in Victoria. Australian Geomechanics, Volume 50, Number 3 (Sep 2015). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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