Backgrounder

Backgrounder

Pacific ENSO signal

The El Niño event has passed its peak and is now waning. Equatorial subsurface temperature anomalies have weakened east of the dateline and westerly zonal wind anomalies have reduced in horizontal extent to a region near the dateline. The area of suppressed convection in the west still affects eastern and central Australia, the Coral Sea, and the north Tasman Sea.

Almost all forecast models are predicting a return to neutral conditions by the end of autumn 2003.

Mean sea surface temperature departures from normal.

Update on the SOI

The mean Southern Oscillation Index (SOI) for January was -0.2, with the three month average now at -0.8. The present moderate El Niño has passed its peak, and is now waning. Further general information on El Niño is available on the World Meteorological Organization web site, www.wmo.ch

The Southern Oscillation Index (SOI).

Online climate graphics

Climate maps and line plots of climate site observations are updated each week on the Climate Now website.

The New Zealand hydrological network dates from 1905, when measurements of levels and outflows started at Lake Taupo to investigate hydroelectric potential. In the 1930s, measurements for irrigation investigation began, and later, catchment boards monitored rivers for soil conservation and river control. In the 1960s, a national "representative basin" network was established to provide information about water resources, flood sizes, and effects of land use changes such as afforestation. Today more than 400 streamflow stations are operated, mainly by regional/district council or NIWA staff.

Rainfall is measured with standard recording raingauges. River water level is monitored by water level recorders housed in concrete or steel towers adjacent to the river. Levels are converted to flow using a relationship known as a rating curve. River flow is commonly expressed in cubic metres per second (m3/s or cumecs), or litres per second (L/s) for low flows. The flow can be at a particular time such as flood peak, or it can be as an average of hours, days, months, or years.

The extensive, and continually growing, archives of streamflow data for the stations shown on the map, from NIWA’s National Hydrometric Network, are used for a wide range of investigations, such as hydropower, irrigation, flood studies, and drought assessments. They are stored in databases of the Water Resources and Climate Archive.

More recently, a soil moisture network has been established (see figure for location of stations). The data from this network will provide ground-truth checking for the soil moisture maps produced in Soil moisture.

River level and water quantity monitoring network.

Soil moisture monitoring network.


Measuring river flow rate in the Hutt River. Observational data like this supplement the National Hydrometric Network to provide a comprehensive picture of the nation's water resources. Photograph: Alan Blacklock