Floods, drought and the Southern Oscillation Index
The Southern Oscillation Index
There are believed to be two main atmospheric circulations responsible for causing droughts and floods in Australia. These are El Niño events and La Nina.
El Niño is part of the Southern Oscillation, which is a huge climatic pattern that covers the Pacific Ocean and influences the weather in Australia. In a normal pattern, easterly winds flow across the Pacific Ocean (the south-easterly trades), bringing moisture and driving warm ocean currents onto the Australian Coast. These winds then rise over Australia and condense to give rain. If the trade winds are strong there are likely to be floods and tropical cyclones in Australia.
About every five years the whole pattern is disturbed. In an El Nino year, the water off the South American coast becomes warmer as the upwelling of cold water stops. This change creates lower than normal air pressure which weakens the trade winds and may even reverse them. Without trade winds to drive the circulating current it reverses, causing a 'wave' of warm water to surge eastward rather than west. With little wind picking up moisture form the cooler-than-usual sea, there is less rainfall over eastern Australia.
La Nina is the opposite of El Nino. During La Nina the seas north of Australia become slightly warmer while the eastern while the eastern Pacific Ocean becomes considerably cooler. This intensifies the easterly trade winds which pick up more moisture from the warm water. The result is often above-average rainfall over much of eastern Australia.
With this understanding of the weather patterns across the Pacific, meteorologists have been able determine that pressure changes can be used as an indication of changing weather patterns and have developed a system using an indicator known as the Southern Oscillation Index (SOI). SOI is calculated using the pressure difference between Darwin, Australia and Tahiti in the Pacific. El Nino events follow an annual pattern developing in autumn, holding through winter, spring and summer, and then either breaking down or re-establishing in the autumn of the following year. Likewise SOI is usually set by the end of May and this phase can be used to indicate rainfall patterns over the next nine months - until the start of the next autumn. Consistently positive readings in autumn usually indicates above average rainfall and consistently negative readings usually indicate lower than average rainfall. So while SOI is a useful tool for farmers, it is important to remember that it is an indication of higher or lower than average rainfall not of rainfall amounts. It is important to look at the trend or phase of the SOI as well as individual numbers and that the effects of El Nino events vary greatly between regions and with the time of the year.
Rainfall variation from the average and SOI (1965-2006)
The Southern Oscillation Index and the climate at Tocal
Statistical analysis using Australian Rainman figures shows no consistent correlation between SOI figures and the rainfall at Tocal. If you look at the graph above you will see that for significant departures from the average figures (higher or lower), the SOI does show distinct changes but not always.
Take, for example, the below-average years of 1965 and 1966. The SOI started to climb rapidly, and in 1967 above-average rainfall occurred: the drought was broken.
Again, if you look at the high rainfall year of 1988, you can see the SOI in 1987 starting to rise rapidly—the trend was positive. The same trend is evident in 1982 and is repeated in 1997.
On the other hand, from 1975 to 1977, the SOI dropped rapidly, but above-average rainfall occurred in the following years. But then, the SOI started to climb in 1977, and 1978 was another high rainfall year.
The lesson from this historical data is that the SOI is often a good predictor of seasonal conditions, but it should not be the sole basis for making decisions about the outlook for agriculture in the Paterson district.
Since European settlement, records suggest that the Paterson River floods on an irregular basis. These floods cause extensive damage to low lying areas. The floods do not follow any particular pattern and have occurred in most months. The most likely time for floods is in the late summer-autumn period.
The highest floods at Paterson were in March 1875, March 1978 and February 1990. According to local experience, the highest of these was in March 1875.
It is difficult to describe floods on the Paterson River as every flood seems to be different but the flood level on Tocal can be generally estimated by the 10m contour line. The blue areas on the diagram below are enclosed by the 10m contour and indicate areas of the property most regularly flooded. Mostly floods are more inconvenient than destructive, one notable example is the 2007 flood which caused extensive damage to Tocal and the Hunter in general.
Flooding mostly effects the rich river flats which may be covered by four metres of water in a big flood. Floods have a huge impact on the operation of the property as they make large areas of the property unusable and hinder stock movement (a problem made worse in 1911 by the establishment of the railway line). While floods cause a lot of inconvenience to farm operations the sediments left behind as the flood recedes are an important factor in the fertility of these flats.
Tocal can be flooded from three main sources:
The area in blue indicates the land that lies below the ten metre contour, this is the land most prone to flooding
- Webbers Creek and its tributaries
- Paterson River flow
- Paterson River water backed up by tides or high flows, or both, in the Hunter River.
Often, much of the property is inundated due to flooding from Webbers Creek and its tributaries before flood warnings are issued. The main indicator for floods at Tocal is the measurement gauge at Gostwyck. A level greater than 13 metres indicates that a major flood is underway.
Though the district has a high average annual rainfall by Australian standards, it is still susceptible to droughts. Records and local knowledge indicate the following years to be serious droughts: 1902, 1918, 1938-41, 1944, 1957, 1964-65, 1980, 1991 and 2002. The 1939-41 years were all below average; the total rainfall for 1939 was affected dramatically by a large fall in March of 381 mm. A similar, less dramatic fall occurred in the 1980 drought when a fall of 138 mm was received in the last days of December.
For these reasons it should be noted that droughts do not follow the calendar year and the figures should be looked at over a number of seasons to fully understand the trends. This will give an indication of how dam water, creeks, subsoil moisture levels and general paddock conditions would be. These are the conditions that affect farm production in a drought. In the Paterson district dry conditions are often exacerbated by hot westerly winds.
Droughts can be patchy in their effects. The calendar years of 1980 and 1991 at Tocal can be easily seen as droughts. Other parts of the state were not as badly affected at this time - their droughts did not coincide with droughts at Tocal. For example, the Upper Hunter Valley during 1991 was much better off than Tocal, even though it is a drought-prone area.
In the 1964-65 drought, the Allyn River stopped flowing at Halton from 9 January 1965 to July 1965; and the Paterson River stopped flowing at Mount Rivers from 11 December 1964 to 30 January 1965 and 8 March 1965 to July 1965 (Pattison 1966). In this drought, salt encroachment up the Paterson River to Paterson caused a partial or complete stop to irrigation.
Historical annual rainfall at Tocal 1965-2006
Droughts cause low levels of paddock stock water and a decline in feed production. Strong westerly winds again cause high evaporation rates. The impacts of drought are felt across the property, Tocal records indicate the impact of drought on farm costs. Paddock costs for the dairy, which includes electricity charges for irrigation and costs of fertiliser (mainly urea), averaged $15,858 per year for the non-drought years of 1987-1990, but jumped to $42,440 in 1991, a 267% increase.