Water supply under threat
Alexandra resident Roger Browne shares his concerns about how irrigation will be affected in a warmer world.
Central Otago’s horticulture and viticulture industries are critically dependent on a reliable supply of irrigation water.
Much of this water is supplied by dams, which are replenished by the spring thaw.
In winter, snow accumulates on the surrounding mountains such as the Hawkduns and the Old Man Range.
Unlike the South Island’s glacially sculptured main alpine chain, our local mountains are called block mountains because of their relatively flat tops.
During winter, extensive snow fields cover these tops and the spring thaw feeds streams as well as elevated wetlands and string bogs.
Water continues to flow from the wetlands and bogs right through the summer.
One result of this is that some of the water that fell in the form of snow in the winter will still be flowing into the irrigation dams throughout summer.
From the point of view of the irrigation companies, the winter snow pack is nature’s storage system for irrigation water.
So what would be the impact of an altered climate? Between now and the end of this century average temperatures are expected to rise, and the snow line with it.
In addition, westerly winds are expected to increase in winter and spring.
This would accelerate the melting of any snow on Central Otago’s block mountains. The combined result is likely to be more run-off in the winter months and less water available for irrigation in summer, when it is most needed.
How will existing gravity-fed irrigation schemes be able to fulfil the needs of the fruit and grape growers in the latter decades of this century?
More storage capacity will be needed to make up for the loss of winter snow. Some existing irrigation dams can be extended, such as is proposed for Falls Dam on the Manuherikia.
Elsewhere storage dams may need to be built from scratch.
Whatever the solution, it will be an additional expense for irrigators.