Rainfall intensity

  • Image, Annual maximum three-day rainfall.

    A warmer atmosphere can hold more water vapour, which then comes back to Earth as precipitation. As Earth warms, scientists expect more frequent intense rainfall. Rainfall intensity in New Zealand is variable from year to year and from location to location. However, except for Northland and Hawke’s Bay, extreme rainfall is expected to increase over most of the country, with an increase of up to 20 percent possible in the south of the South Island by the end of the century.

    Intense rainfall can result in flash floods or land slips that damage homes and property, disrupt transportation, and endanger lives. It can also interfere with recreation and increase erosion. Changes to the frequency of intense rainfall events can alter biodiversity.

    We classified Rainfall intensity as a national indicator.

    Key findings 

    For the majority of locations around New Zealand, there is no clear evidence that intense rainfall events changed between 1960 and 2016. However, intense rainfall increased at some sites and decreased at others.

    Between 1960 and 2016 at the 95 percent confidence level:

    • The proportion of annual rainfall occurring in intense events (in the 95th percentile) decreased at 4 of 30 sites (Auckland, New Plymouth, Rotorua and Taupō) and increased at two (Napier and Timaru).
    • Annual maximum one-day rainfall amounts decreased at 4 of 30 sites (Auckland, Hamilton, New Plymouth and Taupō) and increased at two (Dunedin and Timaru).
    • Decreasing trends in both measures of intense rainfall have been found at sites located in the northern and central North Island.

    Maximum 1 day rainfall and percent of annual rainfall in 95th percentile, 1960–2016 – interactive map

    Map, Maximum 1 day rainfall and percent of annual rainfall in 95th percentile trends, 1960-2016.

                                                                                                                                                                                                        Source: NIWA

    Definition and methodology

    This indicator provides information about the distribution of intense rainfall events. Rainfall intensity in New Zealand is variable from year to year and from location to location. Except Northland and Hawke’s Bay, extreme rainfall is expected to increase over most of the country, with an increase of up to 20 percent possible in the south of the South Island by the end of the century under a high emissions scenario (Ministry for the Environment, 2016).

    We used percentage of total annual rainfall in the 95th percentile and annual maximum one-day rainfall to assess rainfall intensity.

    Percentage of total annual rainfall in the 95th percentile is a measure of the proportion of annual total rain that falls in intense events. The climate normal (1981–2010 average) is used to determine the 95th percentile range. This measure provides information about the importance of intense rainfall events for total annual rainfall.

    Annual maximum one-day rainfall total is a measure of the maximum amount of rainfall that fell in a single day during a year. This measure provides information on the magnitude of intense rainfall events.

    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 for most sites are available from 1960 to 2016. However, the data record is shorter for ten sites (start dates in brackets): Auckland (1963), Gore (1972), Hokitika (1964), Kerikeri (1982), Masterton (1993), Queenstown (1969), Reefton (1961), Rotorua (1964), Taupō (1977), Whangaparaoa (1987).

    Detecting trends in rainfall intensity is difficult because of the naturally high variability and low frequency of events. The variability is caused by day-to-day weather patterns as well as inter-annual and inter-decadal climate variations to which New Zealand is particularly susceptible.

    Data quality

    We classified Rainfall intensity 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

    Ministry for the Environment (2016). Climate change projections for New Zealand: Atmosphere projections based on simulations from the IPCC 5th assessment. Retrieved from www.mfe.govt.nz.

    Archived pages

    See Annual maximum three-day rainfall (archived October 2017).

     

    Published 19 October 2017

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