Kevin Long has analysed recent rain fall patterns in Central Victoria and argues that possible cyclical changes in weather are behind what could be an extended period of drier weather in southern Australia.
As a Central Victorian resident for 52 years, I have had a life-long interest in weather patterns and climate variations in this area. I have seen three distinct flood periods and three distinct drought periods, one drought period on average every 18 years.
To get a more complete understanding of what’s been happening with our weather, I have gathered numerous sets of rainfall figures for this area and analysed them carefully. The consistencies that have been revealed over the last 200 years are undeniable.
In the Central Victoria region, the period starting 1830 delivered 40 years of above-average rainfall, followed by 72 years of below-average rainfall.
This pattern appears to be repeating itself, with approximately 44 years of wet from 1952, followed to this point, by a drier period since the early 1990s.
This suggests that a period of below-average rainfall will continue for the next 60 years with small respites of wet weather occurring approximately every 18 years. The swing from wet to dry climate has on previous occasions been spread over 10 years with about a 20 per cent drop of average rain.
This 20 per cent drop in rainfall equates to about 50 per cent drop of the inflows to Central Victorian reservoirs. In the dry zone of 1880-1952, significant flows to reservoirs occurred only in the La Nina years, usually one year in four. In wet periods, however, (1952-1996), reservoirs received significant inflows approximately every second year.
During the recent wet period (1952-1996), water authorities evolved a “water allocation system” that only allows our reservoirs to securely deliver water rights for three years. If there had not been any inflow during those years there would be no water for the fourth year.
This system commits close to 50 per cent of the reservoirs’ capacity to above-quota sales plus supplementary deliveries in the first two seasons, as well as, water rights.
This leaves approximately 27 per cent of reservoirs’ capacity to provide for third, fourth, fifth or sixth year if necessary.
It’s this high percentage of above-quota sales that destroys the security of water supply in the system. The current allocation and compulsory payment of water rights places all risk in the hands of the irrigator.
Water authorities hide behind a shield of compulsory payment for water right even when water is not delivered.
Central Victorians were saved from catastrophic disaster by one three-week rain event in October/November, 2000. Without that 180mm (Bendigo’s figures) of rain, little irrigation would have been possible, from then to now.
Only during August did above-average rain fall last year, yielding barely enough water to supply water rights.
With our reservoirs now nearly empty and this year’s rainfall well below average, adequate water for irrigation is unlikely to be available for the next two years. As last year was a peak rain year, we are unlikely to see another peak rain year till 2006-2008.
With this cycle dominating our expected weather in the future, it is imperative that Central Victorian reservoirs be managed appropriately to deliver secure water rights over four years, and only half water rights if the reservoirs drop below 30 per cent full.
The selling of water, when the reservoirs are less then 70 per cent full, will greatly expose the irrigators and rural businesses to devastation due to zero water allocation for periods of one to three years.
For this drought year, the water horse has bolted. For everyone’s sake, don’t let us be exposed to this unacceptable risk of relying on this year’s rainfall to supply next year’s water.