Tamar report land Capability Survey of Tasmania k e noble 1992 Tamar Report

23

Note:

1.

Slope ranges given are the maximum slopes for the most stable soils in Tasmania

(i.e. soils on basalt).  Other less stable soils will have slope ranges  lower than these for

each capability class (see Section 10).

2.

The frequency of crop rotations will vary according to the soil type and slope of the

land.  The cropping rotations indicated are a guide to ensure that soil structure is

maintained or improved, thereby preventing degradation of the soil resource under

cropping regimes.  This applies particularly to sloping land that has the potential to be

cultivated for cropping.

3. Slope conversions.

Slope in percentage (%)

Slope in degrees (o)

5

3

12

7

18

10

30

17

40

22

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9.

Description of Area Mapped

9.1

Topography

The topography of the area covered by the Tamar map varies according to the underlying

geological formations which influence and control the landscape and landform features,

and determine their resistance to erosion.

Subdued landscapes include alluvial flood plains, flat terraces and low rolling and dissected

country, and are formed on;

a)

recent alluvial sediments e.g. Meander River, Supply River;

b)

windblown sands e.g. Beechford - George Town, Port Sorell;

c)

terraces and infill sedimentary basins of Tertiary age clays, sands and gravels e.g.

Tamar River terraces, Exeter basin, Parkham area, Birralee - Selbourne - Rosevale -

Westwood area, and Port Sorell - East Sassafras area.  Some of these areas have

undergone subsequent erosion and dissection, while areas capped by basalt have

remained relatively preserved;

d)

basalt at Thirlstane - East Sassafras, Moltema - Dunorlan; and

e)

mudstones and sandstones at Beaconsfield and Parkham.

Steeper hill country occurs on dolerite at Rubicon Hills, Wurra Wurra Hills, Black

Sugarloaf Ridge and Tippogoree Hills; and on Permian age mudstones, siltstones and

sandstones in the Frankford, Holwell and Glengarry areas.

The highest points on the map are comprised of rocks that are highly resistant to erosion,

typically dolerite and quartzite.

The majority of the steepest areas and areas of higher elevation on the map occur in

unmapped areas of Forestry Commission land and National Parks such as: Peaked Hill -

340 m; Tippogoree Hills - 350 m; Asbestos Range National Park - Point Vision 350 m; The

Tump - 450 m; Mt Careless - 460 m; The Dazzler Range - 520 m; Christmas Hills - 530 m;

and Stephens Hill - 540 m.

Some of the steepest areas mapped occur around Mt George - 250 m; Tippogoree Hills -

300 m; Brushy Rivulet - 300 m; Drys Sugarloaf - 320 m;  Rubicon Hills, Notley Hills -

350 m; Black Sugarloaf Ridge - 380 m; Stewarts Hill - 420 m; Black Sugarloaf - 500 m;

and Kellys Lookout - 550 m.  Apart from the higher peaks mentioned, the overall altitude

of the undulating low land increases from sea level at the coast to 200 m along the southern

boundary of the map around Elizabeth Town and Westwood.

The map area is dissected by the river estuaries of Port Sorell, Port Dalrymple and the

Tamar River, and by the north westerly trending Dazzler and Asbestos Ranges, Tippogoree

Hills, Rubicon Hills and Black Sugarloaf Ridge.

9.2

Climate

The area experiences a mild to cool maritime climate which is favourable for agricultural

production.  This maritime influence decreases with distance inland from the coast, and

with increasing altitude.  Average monthly maximum and minimum temperatures for

selected stations in the region are shown in Table 4.

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Seasonal variation in temperatures is greater in inland areas.  Summers are generally mild

to warm, and winters cool to cold.  Frosts can be a limiting factor for cropping in some

areas, especially inland, and at higher altitudes.  Frost information for selected stations is

shown in Table 5.

A less favourable climate for cropping occurs in the southern half of the map.  In particular,

areas with better soils that are used for cropping around Moltema, Dunorlan and Weetah,

are affected by frosts.  The area around Selbourne and Westwood also experiences a cooler

climate than areas further north, and is similarly affected by frosts.  Figure 3 shows the

average dates of first and last occurrences of air frosts in Tasmania.  On average Deloraine

experiences more than 100 frosts per year, whereas High Plains experiences around 60

frosts per year.  In contrast, Low Head experiences around 10 frosts per year.

Average annual rainfall increases from 700 mm at the coast to 1 000 mm in the central and

south western areas of the map (Holwell, Elizabeth Town) and decreases towards the south

eastern corner of the map to approximately 700 mm (at Westwood).

Because of the prevailing westerly weather flows, rainfall increases on the western side of

the Dazzler Range and the Great Western Tiers and decreases in associated rain shadow

areas on the eastern side.  Refer to Figure 4 and Table 6 for rainfall information.

Winter predominant rainfall tends to be more reliable than in other seasons.  Localised

flooding occurs during winter and spring, in particular along the Meander River.  Most of

the mapped area experiences a summer dry period, and irrigation is a common practice,

particularly in cropping and dairying areas.  Summer droughts can occur in some years.

Prevailing winds are generally from the west and north west sectors, although variations do

occur depending on the time of year, and localised orographic features.  Figure 5 shows

wind rose information for Low Head and Launceston.  During the summer months,

afternoon sea breezes are common in coastal areas.

References for Further Reading:

Australia, Bureau of Meteorology, 1980,  Climatic Survey, Tasmania.  Region 3, Northern.

Australian Government Publishing Service, Canberra.

Australia, Bureau of Meteorology, Hobart, 1986,  Mean Annual Rainfall Map, Tasmania.

Australia, Bureau of Meteorology, 1988,  Climatic Averages Australia.  Australian

Government Publishing Service, Canberra.

Australia, Bureau of Meteorology, 1990, Average and Extreme Maximum and Minimum

Temperatures, Selected Tasmanian Stations.  Commonwealth Bureau of Meteorology,

Hobart, (unpublished).

Australia, Bureau of Meteorology, 1990, Average monthly rainfall and rain days, Selected

Tasmanian Stations.  Commonwealth Bureau of Meteorology, Hobart, (unpublished).

Langford, J., 1965,  Weather and Climate, in Atlas of Tasmania.  Lands and Surveys

Department, Hobart.

Nicolls, K.D. & Aves, S.M., 1961,  Average Yearly Rainfall in Tasmania.  Commonwealth

Bureau of Meteorology, Melbourne.

Port of Launceston Authority, 1981,  Port Information, Launceston, Tasmania.

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