Natural hazards

Drought

Drought occurs due to a deficiency of precipitation over an extended period of time, usually a season or more. Drought should be considered relative to some long-term average condition of balance between precipitation and evapotranspiration (i.e., evaporation + transpiration) in a particular area, a condition often perceived as "normal." It is also related to the timing (i.e., principal season of occurrence, delays in the start of the rainy season, occurrence of rains in relation to principal crop growth stages) and the effectiveness of the rains (i.e., rainfall intensity, number of rainfall events).Other climatic factors such as high temperature, high wind, and low relative humidity are often associated with it in many regions of the world and can significantly aggravate its severity.(Source: National Drought Mitigation Centre.)

Auckland is more likely to experience drought conditions during El Nino periods of the Southern Oscillation.The summer of 1997/98 was associated with an El Nino pattern.This was also the summer of a water crisis for the Auckland region.The summer of 2002/03 will again be under El Nino conditions which suggests that drought is more likely than in other non-El Nino (La Nina) periods.To view the ARC commissioned NIWA report on this summer's predicted climate click here.

Agricultural vs Water Supply/Hydrological Drought

Drought can be defined in various ways.An 'agricultural drought' is a period when the soil is estimated to be 'moisture deficit'.A 'hydrological drought' is when the effects of low precipitation affect hydrological systems and is usefully defined as a recurrence value of 'one-in-x-years' for a fixed period such as a season.A hydrological drought can result in a water supply shortage, although storage capacity and demand are also important factors.

Agricultural Drought

A significant agricultural drought can impact on Auckland's agricultural and horticultural industries, in particular:

• reduced number breeding sheep and cattle and resultant economic losses
• $10's of millions of loss of production to the horticultural industry
• increased risk of losses due to fire.
   

Water Supply Drought

In the summer period between late 1994 and early 1995, the Auckland region was subjected to a water supply drought.The return period for this event, based on 12 month cumulative rainfall totals, was 1:25 year event (Auckland Engineering Lifelines Project, 1997 order here).

By the end of 2002, Auckland's water supply will have a designed reliability to meet a drought event with a recurrence of 200 years.Drought events with a recurrence exceeding 200 years will result in Water supply restrictions with implications for sanitation and business activities.