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Drought check

http://www.gw.govt.nz/drought-check

Drought check

Updated 11 November 2020 4:53pm

This webpage provides a brief summary of climate and hydrological conditions in the region. This service is only updated during periods in which closer monitoring is required (regardless of time of the year), in recognition that we are in a dry spell of weather. It does not define an official council position on drought or drought declaration.

Situation Statement

Updated 11 November 2020
Next update due when there is a significant change of conditions, as the situation evolves 

Background

The winter rain was insufficient for a full hydrological recovery. Some areas in the south of the Region had just over half of the long-term average rainfall for the season. This, combined with higher than average temperatures, is helping promote greater evapotranspiration from soil and plants in the lead up to summer.

The Wairarapa finished the hydrological year (1 June 2019 to 31 May 2020) with a rainfall deficit ranging between 100 and 200 mm from Masterton to the eastern coast, while the high elevation areas of the Tararuas finished the year with a deficit of around 1,000 mm. Sustained rainfall is needed to restore and maintain the ‘normal’ hydrological balance.

This ‘normal’ longer-term water balance is becoming increasingly hard to maintain due to climate change, and more unreliable weather patterns (see below). 

 

Current situation

The region received widespread westerly rain in September, below average rain in October, and then widespread easterly rain on the 10th of November, with over 100mm in 24 hours on the eastern hills.

This development has provided a more stable water flow for the region, at least for now. Warm oceanic conditions, consistent with La Niña, have also largely developed.

The projections ahead (see below) support the continuation of an irregular rainfall pattern, alternating between dry spells and further easterly events (potentially heavy), as La Niña progresses. 

 

Meteorological outlook

A La Niña event has now developed, and should persist at least until the end of summer.  

A neutral Indian Ocean Dipole (IOD) and a predominantly positive Southern Annular Mode (SAM) are also expected, which are more or less typical of La Niñas. In summer, La Niñas tend to evolve to dry conditions on the west coast, with a predominant north-easterly flow bringing thunderstorms inland.

In light of these drivers, chances are enhanced for a hot and humid warm season ahead, with highly irregular rainfall pattern both on a month-to-month basis and also spatially across the region. 

 

Climate change

With climate change, droughts are expected to become more severe and frequent in the Wellington region, particularly in the Wairarapa.

Even if international climate policy efforts successfully contain global warming under 1.5-2 degrees (the Paris Agreement’s ambition), it is important that we build water resilience and be prepared for a “new normal” climate pattern, significantly drier than in the past.

We note that the warming temperatures also mean that evapotranspiration is greatly increased. There is some evidence that our soils are getting drier, and ground water storage may be decreasing, in the long-term.

See the latest national drought index state.

Browse the data

Anomaly Maps

How different has recent rainfall/soil moisture been compared with the same time in previous years?

Click on the links below to see the relevant anomaly map

Site-specific graphs

Cumulative rainfall/soil moisture totals for indicator sites compared with historical averages and other recent years

Area Rainfall Soil Moisture
Kapiti Coast (lowland) Otaki at Depot  
Kapiti Coast (high altitude) Penn Creek at McIntosh  
Porirua Horokiri Stream at Battle Hill
 
Wellington City Kaiwharawhara Stream at Karori Reservoir  
Hutt Valley (upper catchment) Hutt River at Kaitoke Headworks  
Upper Hutt Upper Hutt at Savage Park Upper Hutt at Savage Park AQ
Wainuiomata Wanuiomata River at Wainui Reservoir  
Wairarapa (high altitude) Waingawa River at Angle Knob  
Wairarapa Valley (north) Kopuaranga River at Mauriceville  
Wairarapa Valley (Masterton) Ruamahanga River at Wairarapa College Wairarapa College AQ
Wairarapa Valley (south) Tauherenikau River at Racecourse Tauherenikau River at Racecourse
Wairarapa (north-eastern hills) Whareama River at Tanawa Hut Whareama River at Tanawa Hut
Wairarapa (south-eastern hills) Waikoukou at Longbush Waikoukou at Longbush