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Foggy Tussock

An 18-month study may soon answer whether tussock contributes a significant amount of water to runoff through fog interception.

Snow-tussock lysimeter studies have shown that tussock catches 10-36 times the water caught in standard rain gauges. This high water yield has been attributed to fog interception. Other studies have suggested that tussock's low evaporation rate is a more significant factor.

Dunedin relies on Otago's tussock country for more than half of its daily water requirements, so an understanding of the factors involved is important.

The current study was set up to determine, quantitatively, the contribution of fog to water yield, using a lysimeter at Glendu, in East Otago. The lysimeter is a large block of tussock encased in a steel cylinder, and measures precipitation by weight changes. The amount of intercepted water can be compared with rainfall, measured by rain gauges on and around the lysimeter. Leaf wetness sensors help to distinguish between rain and fog.

A second study compared water yield of tussock and pine catchments. Results from the Glendu study showed that although tussock produces more water than pine forests, only 1% of the tussock water was due to fog interception.

The lysimeter was then moved to a more fog-prone site, Swampy Summit, outside Dunedin. Results are expected by the end of the year, although it may take some time for the tussock/fog interception debate to be answered.

Janine Griffin

Janine Griffin is a freelance journalist specialising in science issues.