Climate Update 66 - December 2004

Backgrounder
Measuring drought

Irrigating near Luggate, Central Otago.

Potential evapotranspiration deficit (mm) that is likely to be equalled or exceeded once every 20 years.

Drought is often hard to quantify, and its severity and duration affect different enterprises in different ways.
One way of measuring drought is to calculate the amount of water typically available for a particular purpose, and then estimate the reduction in water available for that purpose during low rainfall seasons.
For example, we can estimate the amount of water that is 'consumed' during normal pasture growth

Climate Update is a summary each month of New Zealand’s climate, including soil moisture and river flows.
December 2004 – Number 66
November’s climate: November air temperatures higher than average overall.
November’s river flows: Low streamflows in the north and east of the North Island.
Soil moisture levels: Low rainfall in the North Island and north of the South Island.
Three-month outlook: A tendency for drier conditions in the north and east of the country ranging to wetter in the west and southwest.

The outlook for December 2004 to February 2005
Lower than normal mean sea-level pressures are expected southeast of New Zealand, with stronger than normal west to southwest wind flow over the country.

Soil moisture: dry spots developing
End of November soil moisture levels were lower than normal in much of Northland, and in Hawke’s Bay, Marlborough, and Kaikoura. Elsewhere, moisture conditions were near normal.
 
Soil moisture deficit in the pasture root zone at the end of November (right) compared with the deficit at the same time last year (centre) and the long-term end of November average (left). The analysis is done for an average soil type where the available water capacity is taken to be 150 mm.