Annual and seasonal rainfall

  • Image, Annual rainfall.

    Rain is vital for life – it supplies the water we need to drink and to grow our food, keeps our ecosystems healthy, and supplies our electricity. New Zealand’s mountainous terrain and location in the roaring forties mean rainfall varies across the country. Changes in rainfall amount or timing can significantly affect agriculture, energy, recreation, and the environment. For example, an increase or decrease of rainfall in spring can have marked effects on crops or fish populations.

    Climate models project rainfall to increase during winter and spring in the west of both the North and South islands, with drier conditions in the east and north. In summer, wetter conditions are projected in the north and east of both islands.

    We classified Annual and seasonal rainfall as a national indicator.

    Key findings

    Some areas in New Zealand have experienced changes in seasonal rainfall since 1960.

    • At the 95 percent confidence level, between 1960 and 2016, 8 of 30 locations experienced changes in the amount of spring rainfall. Five sites in the northern and central North Island had decreased rainfall (Kerikeri, Whangarei, Tauranga, Taupo, Rotorua). In contrast, two sites in the southwest of the North Island (Whanganui, Waiouru) and one in the south of the South Island (Invercargill) had increased rainfall.
    • Between 1960 and 2016, at the 95 percent confidence level, 4 of 30 locations showed a trend in winter rainfall (New Plymouth, Wellington, and Whangarei decreased and Milford Sound increased), and three locations showed trends in summer rainfall (Dunedin and Kerikeri increased while Dannevirke decreased). No trend was apparent in autumn rainfall for any location.
    • On average, winter is the wettest season for most of the country except for the southern half of the South Island, where most rain falls during summer.  
    • In general, the South Island experiences more consistent average rainfall across seasons, but greater variability in annual totals, as the wettest and driest areas of the country are both in the South Island.

    Figure 1

    Seasonal trends (1960-2016) and seasonal average rainfall (1981-2010) – interactive map

    Map, Seasonal trends 1960-2016 and seasonal average rainfall 1981-2010.  

    Source: NIWA

    Figure 2

    Average total annual rainfall, 1972–2016 – interactive map

    Map, Seasonal trends 1960-2016 and seasonal average rainfall 1981-2010.

    Source: NIWA

    Definition and methodology

    Annual rainfall is the total accumulated rainfall in a year, using daily rainfall estimates from rain gauges at 30 locations across New Zealand. We calculate average seasonal rainfall by aggregating daily rainfall estimates by site for each meteorological season of the year: summer = December (of previous year)-January-February, autumn = March-April-May, winter = June-July-August, and spring = September-October-November. For the 30-year period 1981–2010, rainfall normals (averages) for each season were calculated for comparison purposes. Annual and seasonal rainfall is highly variable and depends on short-term weather patterns and long-term climate oscillations such as the El Niño Southern Oscillation, Interdecadal Pacific Oscillation, and Southern Annular Mode, which may all be influenced by a warming climate.

    NIWA interpolated annual and seasonal rainfall totals, measured at climate station locations across the country, to create a regular 500m resolution grid of average seasonal rainfall from 1981 to 2010 and annual rainfall for each year from 1972 to 2016. Missing data were infilled using Virtual Climate Station Network data (NIWA, nd). Although the interpolations cover the entire land area of New Zealand, accuracy is lowest where station density is low and terrain is complex (Tait et al, 2006; Tait et al, 2012; Tait & Turner, 2005).

    NIWA also provided daily rainfall data from 1960 to 2016 for each of 30 regionally representative climate stations (note that the time series for nine stations started later than 1960: Auckland, 1962; Gore, 1971; Hokitika, 1963; Kerikeri, 1981; Masterton, 1992; Queenstown, 1968; Rotorua, 1963; Taupo, 1976; Whangaparaoa, 1986). These data were used to assess seasonal rainfall trends. Criteria used to select the 30 regionally representative climate stations are that the station must currently be open and be likely to remain open for the foreseeable future; have a long record of reliable, good-quality data; be located near a large city (eg at an airport site) so is representative of the climate where many people in the region live. One station per region is to be selected, but if deemed necessary, two or three stations may be selected to represent a large region.

    Data quality

    We classified Annual and seasonal rainfall as a national indicator.

    Relevance

       This national indicator is a direct measure of the ‘Climate' topic.

    Accuracy

       The accuracy of the data source is of high quality.

    See Data quality information for more detail.

    References

    NIWA (nd). Virtual climate station data and products. Retrieved 29 May 2017 from www.niwa.co.nz.

    Tait, A, Henderson, R, Turner, R, & Zheng, X (2006). Thin plate smoothing spline interpolation of daily rainfall for New Zealand using a climatological rainfall surface. International Journal of Climatology, 26(May 2006), 2097–2115.

    Tait, A, Sturman, J, & Clark, M (2012). An assessment of the accuracy of interpolated daily rainfall for New Zealand. Journal of Hydrology New Zealand, 51(1), 25–44.

    Tait, A & Turner, R (2005). Generating multiyear gridded daily rainfall over New Zealand. Journal of Applied Meteorology, 44(9), 1315–1323. http://doi.org/10.1175/JAM2279.1.

    Archived pages

    See Annual rainfall (archived October 2017).

    Updated 30 January 2018

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