1.2 Climate and agriculture in the States and Territories
1.2.1 Western Australia rainfall and seasonal predictability
Western Australia spans a latitude band from about 14 to 35 degrees south and covers an area of some 2.5 million sq km. Latitude, distance from the sea, and local topography largely shape the State's wide range of climates, with different rain-producing systems important in different parts. In the winter rainfall belt near the southwest corner, and also in the monsoonal northwest Kimberley, there are areas that average over 1400 mm of rain per year. In between, a large area averages less than 250 mm of annual rainfall.
The Kimberley wet season extends on average from late December to March. The monsoon is marked by alternating active and 'break' periods, related in part to the Intraseasonal Oscillation or 30-60 day wave, which, however, affords very limited predictability. In the transition period of about three months prior to the wet season there is gradually increasing rainfall, mostly related to thunderstorms that develop inland diurnally. Seasonal rainfall predictability based on lagged correlations with the Southern Oscillation Index (SOI) is strongest in spring but mostly poor for the peak summer months. The SOI and its interannual changes are used to predict features of the WA tropical cyclone season, relating mostly to spring and autumn cyclone activity (not just for the Kimberley).
In the broad arid zone stretching approximately WNW to ESE across central WA, and also extending into less arid areas in the southwest Kimberley and the wheatbelt, a major contribution to the annual rainfall comes from Northwest Cloudbands (NWCBs). NWCB activity peaks in late autumn and early winter, May being wetter than April over most of the area. Diurnal thunderstorm activity contributes much of the area's summer rainfall, though individual tropical cyclones have been known to generate daily totals in excess of the annual mean. There are significant lagged relationships, at least over parts of the area, with Indian Ocean Sea Surface Temperature (SST) patterns as indicated by the Indian Ocean Index of Drosdowsky (1993). The correlations have opposite signs in summer and winter. Seasonal predictability based on the SOI is mainly poor, though less so for summer rainfall in the north and west Pilbara.
Towards the southwest corner, the relative influence of NWCBs decreases as cold fronts and other mid-latitudinal disturbances become predominant. Even so, a significant proportion of annual rainfall involves tropical interactions, particularly in the form of rainbands as tropical air is steered southward and lifted in advance of cold fronts in the westerlies. Over most of the southwest the rainfall peaks in June on average, with the westerlies at their farthest northward extent. The wettest six months are generally May to October, yielding over 80 per cent of the annual total over a wide area. In the north and northeast wheatbelt, however, the wettest period is earlier, Bencubbin's wettest six months being March to August. Weak but significant simultaneous correlations exist between the SOI and rainfall over southwest WA, while the SOI change from one winter to the next is significantly related to the May-October rainfall in the second year. However, seasonal predictability based on SOI lagged correlations is considerably weaker than for much of Australia and is especially limited for winter rainfall. It is strongest for autumn rain, but does not permit an accurate forecast of the date of the agriculturally important 'break of season', usually in April or May. Indian Ocean SST patterns allow some limited predictability of the rainfall, mainly for spring and summer.
In general, the seasons of maximum predictability vary according to how the predictors are used. SOI phases and trends over various periods have been shown in many cases to provide better predictive skill than simple lagged SOI-rainfall correlations. Additional skill has also been demonstrated by methods involving a combination of predictors, and particularly using SST anomaly patterns covering the Pacific and Indian Oceans. These latter patterns also allow useful predictability of seasonal temperatures.
Drosdowsky, W. (1993). Potential predictability of winter rainfall over southern and eastern Australia using Indian Ocean sea surface temperature anomalies. Australian Meteorology Magazine 42, 1-6.
1.2.2 South Australia
The climate of South Australia varies from hot and dry in the interior to the milder and wetter climates of the southern Mount Lofty Ranges and the southeast coast of South Australia. Median annual rainfall ranges from about 100 mm in the area east of Lake Eyre to more than 1000 mm on the higher parts of the Mount Lofty ranges.
During the warmer half of the year (November to April), high pressure systems (anticyclones) generally move eastward south of the Great Australian Bight. Consequently, the most frequent air stream across most of South Australia during this period is from the southeast to east. Most cold fronts during this period fail to produce much useful rain. Warm moist air occasionally moves south from the tropics during summer, and thunderstorms may develop.
During the colder half of the year (May to October) the anticyclones tend to be centred over the Great Victoria Desert and central New South Wales. The most frequent winds are from the northwest to southwest. Frontal systems associated with depressions travelling eastwards travelling across the ocean have a significant influence on the weather in southern Australia during this season.
Annual field crop production is restricted to southern districts of the State where median annual rainfall exceeds 275-300 mm, and extends up to about 550-600 mm annual rainfall. These winter crops depend almost entirely on incident rainfall received from April to October, although on some soils there is the opportunity to store limited subsoil moisture from the previous year or from heavy rain events in the summer-autumn preceding the crop.
North of the wheatbelt, annual rainfall is too low and erratic to support annual crop or pasture production. Extensive livestock grazing (sheep and cattle) is the major agricultural pursuit on these semi-arid to arid rangelands.
In the southernmost districts of the State, and in the higher regions of the Mount Lofty Ranges, rainfall is either excessive (more than 550-600 mm) and/or the topography not suited to annual cropping. Agricultural activity in these regions includes extensive livestock grazing of improved annual/perennial pastures for sheep, beef and dairy production, and intensive horticulture (annual vegetable crops, perennial tree and vine crops).
Yields of annual field crops (winter cereals, pulses and oilseeds) and pastures are largely determined by total April-October rainfall, although the distribution of this rainfall through the growing season can also influence the efficiency with which it is utilised for crop and pasture production. Timing of the autumn break (i.e. the first significant rainfall event which causes germination of winter crop, pasture and weed species) is a critical factor in determining length of growing season and hence yield. This generally occurs in May or June, but can vary greatly between years. An early break to the season (April or earlier) provides an opportunity for above average yields and the sowing of longer growing season crops, especially in lower rainfall districts. But an early break has a risk of being a false break, i.e. where sufficient rain falls to stimulate germination early in the season, but the delay to follow-up rains results in severe moisture stress and plant death. A late break (July or later) means a greater risk of low yields or even total crop failure, unless spring conditions are sufficiently favourable to compensate. Optimum time of sowing for wheat is generally accepted to be early to mid-May in low rainfall districts, ranging to late May to mid-June in higher rainfall districts where favourable spring conditions are more assured. The risk of frost damage to crops in spring in some districts causes crop sowing to be deliberately delayed to reduce the likelihood of frost at the critical crop flowering period.
The high potential for wind erosion on the lighter mallee soils of much of the State's lower rainfall districts poses an additional risk in the wheatbelt. Seasonal climatic conditions that can accentuate this risk include low rainfall/poor crop and pasture production in the previous year resulting in low residual soil cover, false breaks that promote crop sowing but subsequent poor growth or crop death, late breaks that provide little or no new season growth for soil cover, and extreme wind events with extended periods of high velocity/low humidity winds (generally from the north and north-west, preceding cold fronts, from May through to July).
Research, extension and regulatory programs conducted within Primary Industries and Resources SA (PIRSA) seek to ensure sustainable management of the State's natural resources for profitable agricultural production. This is being achieved through, inter alia, the development and promotion of farming practices and farming systems to minimise seasonal climate risks to agriculture and to capitalise on the opportunities presented by favourable seasonal conditions. Training and skills development programs to improve self-reliance in the agricultural sector through the ability to manage all manner of risks, including those presented by seasonal climate variability, are a priority within PIRSA. These include Property Management Planning and TOPCROP. There is also concern about a declining rainfall trend in recent years, particularly over the southern part of the State, this being explored through the Indian Ocean Climate Initiative with the Bureau of Meteorology, CSIRO and the Western Australia government. Where extreme climatic events do result in major loss, damage or disruption in the State's agricultural industries, PIRSA provides advice and support to the State Minister in assessing and presenting the case for Federal Government assistance under the Exceptional Circumstances provisions, and in managing the disbursement of any such funds provided.
1.2.3 Victoria
The climate of Victoria is characterised by a range of different climate zones, from the hot, dry Mallee region of the northwest to the alpine snowfields in the northeast of Victoria. To the west and north of the Great Divide the land flattens out to the dry inland plains. It is in the Mallee where the hottest temperatures in the State most commonly occur during summer, and where the annual median rainfall drops below 250 mm.
The climate changes across the State are reflected by marked changes in vegetation. Vegetation ranges from the mallee scrub country in the northwest, through irrigated plains in the Northern Country and the wetter grazing lands of the south to the forested mountainous country of eastern Victoria and the winter snow-covered alpine regions in the northeast.
North of the Great Divide there is not only a marked decline in mean annual rainfall, but also a marked increase in the variability of the amount of rainfall from year to year. It is a winter rainfall or quasi-mediterranean environment with typically dry summers. In the north of the State autumn rains that initiate the growth of the predominantly annual pastures, either native or typically annual ryegrass and subterranean clover, may occur any time from February to June. Early autumn rains that are not followed up result in the top soil drying out and seedlings dying, leading to a false autumn break. Annual pastures typically dry off in late October and November leaving standing dry feed for the summer.
Many of the farms of the Wimmera, Mallee, northern and north-east Victoria are devoted to both cereal crop and livestock production. The latter is dominated by either Merinos for wool production, or Merino ewes crossed with Border Leicester rams to produce First Cross Ewes as dams for prime lamb production in either the irrigation areas or in the south of the State. The irrigation areas are concentrated along the Murray River, and in the Goulburn Valley near Shepparton and Kyabram. Soil salinity is a major problem in many of the irrigation areas, particularly between Kerang and Swan Hill, and in dryland country where excessive tree clearing has occurred.
The mountains of the Great Divide in Victoria attain a maximum height of 1986 metres at Mt. Bogong near the town of Mt. Beauty. There are several peaks in excess of 1500 metres in the northeast of Victoria. Median annual rainfall ranges in some of the mountainous regions can be in excess of 1800mm. The Great Divide extends westwards almost to the South Australian border, with most peaks below 600 metres except in the mountainous area called the Grampians, near Stawell, where Mt. William's summit is 1167 metres.
The coastal strip, south of the ranges, is generally wetter except in the far east where the Strezlecki ranges shelter East Gippsland from the moisture laden southwesterly winds. The probability of early autumn and late spring rains is considerably higher than north of the divide. Pastures in the south-west of the State are typically perennial ryegrass and subterranean clover, whereas perennial grasses and white clover dominate in Gippsland. The south-west of the State is dominated by Merinos for wool production, crossbred sheep (e.g Merino x Border Leicester ewes to Dorset Horn rams) for prime lamb production, or cattle. Dairying is more evident in Gippsland.
1.2.4 Tasmania
Tasmania enjoys for the most part a 'temperate maritime' climate. The sea, never more than 115km distant, suppresses temperature extremes. The prevailing westerly airstream leads to a marked variation of cloudiness, rainfall and temperature. The result is a West Coast and highlands that are cool, wet and cloudy and an East Coast and lowlands that are milder, drier and sunnier.
The interaction of airstream and topography is the main factor governing rainfall in Tasmania. Consequently the annual rainfall varies markedly across the State, averaging less than 600mm in the Midlands but over 3500mm in some part of the mountainous west. The highest rainfalls occur in remote, unpopulated regions. In the highlands, above the 900 metre level, snow can occur at any time of the year. Heaviest snowfalls tend to occur in July and August.
There are three main influencing temperature regimes in Tasmania. Proximity to the sea ensures coastal locations will have a milder temperature regime than inland ones. Temperature decreases with altitude (by about 0.7°C for every 100 m), making elevated locations generally cooler than low level ones. Finally, cloudiness in the west (a result of the persistent westerly winds) suppresses daytime temperatures there.
Summers are mild, with any hot periods rarely lasting more than a few days. Rainfall is generally lower (in both amount and frequency) in summer, most notably in the west and northwest. Afternoon sea breezes are common along the coasts.
Winters are not excessively cold, especially compared to places at similar latitudes in the northern hemisphere that do not have the sea's moderating influence. Westerly winds with embedded cold fronts often cross the State, and these can bring cold weather and snow to low levels. Every so often winds will be light, the skies clear and the mornings cold and frosty.
Climate variability tends to be less than most mainland States, but is nevertheless an integral part of agriculture in Tasmania. The Tasmanian government is therefore a signatory to the National Drought Policy, and related policies that aim to improve the business and risk management skills of farm managers, and promote the identification and implementation of sustainable agricultural systems. This includes the FarmWi$e Program, part of the national program on Property Management Planning. Research aimed at meeting these policy objectives is targeted to enhance the management and decision making skills of property managers and their advisers to ensure sustainable agriculture.
1.2.5 New South Wales
New South Wales is entirely in the temperate zone. The climate is generally mild, equable and mostly free from extremes of heat and cold, but very high temperatures occur in the north-west and very cold temperatures on the southern tablelands. The Great Dividing Range, running approximately north to south in the east of New South Wales, has a large impact on the climate, creating four distinct climate zones; the coastal strip, the highlands, the western slopes and the flatter country to the west.
The climate of the coastal strip is influenced by the warm waters of the Tasman Sea, which in general keep the region free from extremes of temperature and provide moisture to increase rainfall, the annual median of which ranges from about 750 mm in the south to 2000 mm in the north. The mountains of the Great Divide attain a maximum height of 2228 metres at Mt. Kosciuszko, and there are several peaks in excess of 1500 metres, extending up to northern NSW. Travelling from east to west across the range, the elevation abruptly increases away from the coastal plain, and then west of the divide it gradually descends onto the western plains. Consequently, winter snowfalls are experienced over what are aptly called the Tablelands regions.
On the Western Slopes the rainfall gradually decreases, together with the frequency of winter snowfalls. Average maximum temperatures gradually increase as height above sea level decreases. Further to the west the land slowly flattens out to the dry inland plains, marked by cold nights. It is in the far north west where the hottest temperatures in the state most commonly occur during summer, and where the annual mean rainfall drops below 200 mm.
The way in which the climate changes across the State is reflected by marked changes in vegetation, which ranges from the sub-tropical rainforests of the northeast to the fragile alpine heath lands in the southern Alps, through the dry forests and undulating pasture lands of the Midwest to the dry plains of the northwest. The more arid west is dominated by increased variability in rainfall and native grasslands. Sub-tropical grasses dominate in northern New South Wales, whereas annual pastures and crops are widespread in the wheat-sheep country throughout the centre and south of the State. Perennial pastures are more typical in the higher rainfall areas on the south coast where more dairying occurs.
The recurrence of drought is a common feature of the NSW climate. Numerous droughts have occurred during the 20th century and had devastating effects on the rural economy of the State. The last severe drought occurred in 1982-83 when few farms remained unaffected. The droughts in the State occur because of very high variability of rainfall and very high rates of evaporation and evapotranspiration. The amount of annual evaporation always exceeds the amount of water actually available to be evaporated in most parts of the State. Evaporation is highest in the hot, dry and windy interior and least in the cool and moist south and eastern regions of the State.
The combined impact of seasonal distribution of rainfall, evaporation, temperature and frost is reflected on the land use pattern and length of growing season for crop production. About one third of eastern New South Wales has suitable climatic conditions for crop growing throughout the year. The length of the growing season shortens gradually towards the north-west primarily due to decreasing rainfall. The north-west corner of the State is unsuitable for crop production because of harsh climatic conditions.
Only about 7 per cent of the State is under crops, 6 per cent under sown pastures and 17 per cent under native pastures. Much of the remaining is grazing land having native shrubs. In the coastal and tablelands regions, agricultural land is used for intensive grazing of sheep and cattle. These regions have about half of the sown pastures of the State. Western slopes and plains have three quarters of the State's cropping area and nearly half each of sown and native pastures. In the upper and lower far western plains, most of the land is rough grazing or sparse woodland, useable as extensive and seasonal grazing but unfit for crop production.
Large areas of New South Wales have moderately fertile soils and are characterised by low cost pastures and dryland cropping. These factors give the State a competitive advantages in a variety of food and fibre products. New South Wales represents 10 per cent of the area of the country, but the gross value of its livestock and crop production represents 26 per cent of Australia's gross agricultural production. It produces 30 per cent of the meat, 41 per cent of the wheat, 99 per cent of the rice and 71 per cent of the cotton of Australia.
NSW Agriculture is committed to helping NSW food and fibre industries and rural communities to be economically viable and environmentally sustainable. Improved climate risk management is its major goal. It is developing, demonstrating and promoting best practice systems for soil, water, land resource management and vegetation management in the light of high variability of rainfall. The Department is undertaking risk analysis studies and developing strategies for managing drought, natural disasters and seasonal climate variability. It is promoting improved use of seasonal forecast information, relevant climate and weather data, and decision support tools. Climate workshops for the growers are run to train them in interpreting and using the climate data, forecast information and decision support tools in risk management at their properties.
1.2.6 Queensland
The difference that occurs in climate across an area the size of Queensland is considerable. Low rainfall and hot summers in the inland west, a monsoon season in the north, and warm temperate conditions along the coastal strip contrast with low minimum temperatures that can be experienced inland and about the southern ranges.
The climate of the coastal strip is influenced by the warm waters of the Coral and Tasman Seas, which in general keep the region free from extremes of temperature and provide moisture for rainfall. The annual median rainfall along the coastal strip is generally within the range of 1000 to 1600 mm., increasing to over 3200 mm. along parts of the North Queensland coast near Innisfail.
The mountains of the Great Divide in Queensland attain a maximum height of 1622 metres at Mt. Bartle Frere near Innisfail, and there are several peaks in excess of 1000 metres, mainly in the north and again in the far southeast. Along sections of the Great Divide, the elevation abruptly increases away from the coastal plain, and then west of the divide it gradually descends onto the western plains.
On the western side of the Great Divide, the rainfall drops quickly to an annual median of about 700 mm., and then gradually decreases further. At the same time, average maximum temperatures gradually increase with increasing distance from the coast. Further to the west the land slowly flattens out to the dry inland plains, marked by cold nights. It is in the west where the hottest temperatures in the State most commonly occur during summer, and where the annual median rainfall drops below 200 mm.
The way in which the climate changes across the State is reflected by marked changes in vegetation, which ranges from the tropical rainforests of the coastal zone of North Queensland to the cooler forests of the southern highlands, through the pastoral belt of areas such as the Darling Downs to the dry saltpans of the western inland.
Tropical Cyclones are a natural hazard from about November through to May in coastal regions. They bring with them devastating winds, heavy rain and the threat of coastal inundation from tidal surges. Whilst tropical cyclones are a threat to coastal communities, they are a major source of rain for the dry inland regions. Settlement to the west of the Great Dividing Range was made difficult by a lack of a reliable water supply. Settlement onto the open plains that flourished during years of good rainfall floundered during drought periods.
The beef cattle industry is the dominant land use in Queensland, with intensive stocking in the high rainfall tropics and extensive grazing in the arid grasslands of the south-west. Sheep are grazed on native grasslands in the semi-arid central and south-west of the State. Cereal and other crops are grown in combination with sheep and cattle in the south of the State, particularly in the Darling Downs. Sugarcane and tropical horticulture dominate much of the East Coast.
1.2.7 Northern Territory
The climate differs from that of southern Australia, and varies greatly between the Territory's northern and southern extremities. Four-fifths of the Territory lies north of the Tropic of Capricorn. The strip within about 150 km of the coast is mainly flat or undulating country up to about 200 m elevation, with extensive coastal swamps or wetlands in some parts. The interior of the "Top End" is dominated by the rocky Arnhem Land plateau. To its southwest lie the rugged hills of the southern Katherine region, while in the east the land rises generally more gently through the hilly country of the southern Roper-McArthur district to the grassy plains of the Barkly Tableland. These systems of hills divide the coastal river drainage systems from the broad but shallow inland basin, where streams are usually dry for most of the year. South again the land rises very gradually; western areas are dominated by sandy desert. Toward central Australia, the land rises more steeply into a higher plateau and rocky ranges, where a number of peaks exceed 1500 m elevation. The plateau declines steeply toward the sand dunes of the Simpson Desert in the southeast whilst the Lake Amadeus trough separates it from the lower ranges of the far southwest.
Two major atmospheric pressure systems affect the Territory: the subtropical ridge of high pressure cells (highs or anticyclones), and a broad tropical low pressure region called the monsoon trough. The subtropical highs move in a west to east direction: across southern Australia in winter, and further south in summer, usually separated by low pressure troughs or cold fronts. The highs provide the driving force behind the southeast trade winds that dominate the Territory's weather in the winter months.
The monsoon trough is a broad area of low atmospheric pressure running east-west through the tropics in the summer months. During the summer it lies for lengthy periods over north Australia, and is the source of much rainfall. Tropical cyclones can develop off the coast in the wet season, usually forming within an active monsoon trough. Heavy rain and high winds, sometimes of destructive strength, can be experienced along the coast within several hundred kilometres of the centre of a cyclone.
Northern and, to a lesser extent, central parts of the Territory experience two distinct seasons: the "wet" (October to April) and the "dry" (May to September). The change between seasons is usually gradual, with transition months of October and November (often called the "build-up") at the start of the wet, and April at its end. In central parts the contrast between wet and dry is not generally as marked as in the north.
The Alice Springs district is dry for much of the year, and has an erratic rainfall pattern, with a slight summer maximum. While zero rainfall can be experienced in all calendar months, significant totals are also possible in all months, but are more likely in summer. Winters (June to August) are cool and summers (December to February) hot; the terms "spring" and "autumn" are not usually applied to the transition seasons in between.
In the southern and central parts, weather is more variable from October to April than in the north. Sometimes decaying tropical cyclones or the monsoon trough move well south into the central regions, bringing widespread rain and thunderstorms. In general, though, east to southeasterly winds and fine conditions predominate. Temperatures can be scorching, and dust devils, whirling dust pillars raised by columns of rising hot air, are frequently seen. During October to November wildfires are fairly common - usually ignited by lightning from dry, gusty thunderstorms.
From May to September the prevailing southeasterlies bring predominantly fine conditions throughout the Territory. Rainfall in the north is low to non-existent in most areas, although light showers are common about the northeast coast and occasionally develop elsewhere over the northern Top End. Wildfires are widespread in the north during this season, particularly in later months, fuelled by the abundant wet season growth that has been dried by the prevailing southeasterlies.
Cold fronts between subtropical high cells frequently move across the Alice Springs district, particularly in the winter months, when they may also occasionally reach the Top End. Winds before a front tend to be warm to hot and, in summer, humid. The front's passage may be marked by thunderstorms or, if rainfall has been very low for a prolonged period, a wall of dust. The cool southeasterlies which follow the front generally clear the sky of cloud and are often very dry. Large areas of cloud, known as "northwest cloud bands" occasionally blow across Australia from the tropical Indian Ocean. These can bring widespread rain, and sometimes storms, to southern parts, especially if a cold front moves into the area and enhances the cloud band.
