Annual maximum three-day rainfall

  • Image, Annual maximum three-day rainfall.

    A three-day rainfall measurement covers a single sustained rain event or a series of shorter events over a three-day period. Such measurements help us understand and prepare for flooding or rain-induced slips that could cause damage.

    We classified Annual maximum three-day rainfall as a national indicator.

    Key findings

     Indeterminate trend

     

    Due to the large year-to-year variability and relatively short time series, no determinable trend is evident in three-day rainfall events across the country.

    • The annual maximum three-day rainfall often exceeds 100mm at the following sites in our three main population centres:
      • Auckland airport exceeded in 15 of 52 years of available data (29 percent)
      • Wellington airport site exceeded in 12 of 53 years (23 percent)
      • Christchurch airport exceeded in 10 of 64 years (16 percent).
    • Milford Sound had an average annual maximum three-day rainfall of more than 400mm for 34 years (53 percent) of the 1950–2013 period.
    • The West Coast of the South Island has a higher average than most areas.

    Figure 1

    Average and highest annual maximum three-day rainfall totals for selected sites, 1950–2013
    Site

    Data availability

    Average annual maximum three-day rainfall

    Highest annual maximum three-day rainfall

    Years

    Millimetres

    Millimetres

    Year

    Auckland

    1962–2013

    90

    181

    1967

    Christchurch

    1950–2013 

    70

    135

    1968

    Dannevirke

    1951–2013

    88

    174

    1965

    Hokitika

    1964–2013

    165

    258

    1967

    Invercargill

    1950–2013

    66

    143

    1984

    Milford Sound

    1950–2013

    430

    735

    1958

    Nelson

    1950–2013

    101

    287

    2011

    Reefton

    1961–2013

    130

    271

    1970

    Taumarunui

    1950–2013

    101

    168

    1994

    Whanganui

    1950–2013

    74

    150

    2011

    Wellington

    1960–2013

    86

    195

    1971

    Note: These sites have 49 or more years of available data from 1950 to 2013.
    Source: NIWA

    Figure 2

    Note: The sites have 49 or more years of available data from 1950 to 2013. Data cover 1962–2014 (Auckland), 1960–2014 (Wellington except 1993); 1951 and 1960–2013 (Waiouru); 1961–2013 (Reefton), 1964–2013 (Hokitika); 1951–2014 (Dannevirke and Lake Tekapo).

    Definition and methodology

    Annual maximum three-day rainfall statistics show a site’s maximum rainfall total over any three-day period for each year.

    Data are available for 30 climate stations. The stations are Auckland, Blenheim, Christchurch, Dannevirke, Dunedin, Gisbourne, Gore, Hamilton, Hokitika, Invercargill, Kerikeri, Lake Tekapo, Masterton, Milford Sound, Napier, Nelson, New Plymouth, Queenstown, Reefton, Rotorua, Tara Hills, Taumarunui, Taupo, Tauranga, Timaru, Waiouru, Wanganui, Wellington, Whangaparaoa, and Whangarei.

    However, some sites have many years of missing data for 1950–2013. Even one or two days of missing data can mean that a whole month, and therefore a year, must be discarded. Fourteen climate stations have data for 50 years or more, and the remaining 16 have data available for between 4 and 32 years. We are working with NIWA to develop infilling techniques for the missing data.

    The three-day statistics do not capture the intensity of a rain event or series of events – they do not tell us if rain fell in an hour or over three days. In future we might report on rainfall intensity. This may tell us more about the flood risk posed by different rain events, for example, surface flooding from flash floods in urban areas.

    Data quality

    We classified Annual maximum three-day rainfall as a national indicator.

    Relevance

     This national indicator is a direct measure of the ‘Rain, hail, sleet, and snow’ topic.

    Accuracy

     The accuracy of the data source is of high quality.

    See Data quality information for more detail.

     

    Published 21 October 2015

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