Growing degree days

  • Image, Growing degree days.

    Growing degree days (GDD) measures the amount of warmth available for plant and insect growth and can be used to predict when flowers will bloom and crops and insects will mature. GDD counts the total number of degrees Celsius each day is above a threshold of 10 degrees Celsius. Increased GDD means that plants and insects reach maturity faster, provided that other conditions necessary for growth are favourable, such as sufficient moisture and nutrients. As a measure of temperature, GDD experiences short-term changes in response to climate variations, such as El Niño, and in the longer term is affected by our warming climate.

    We classified Growing degree days as a national indicator.

    Key findings

     Increasing trend

    At a national level, GDD increased across New Zealand, as well as over the North and South islands, over the 44 growing season years from 1972/3 to 2015/6.

    Of 30 sites across New Zealand from 1972/3 to 2015/6 (unless stated otherwise), at the 95 percent confidence level:

    • GDD increased at 16 sites; of these sites, three had data for shorter timeframes (start dates ranging from 1982/3 to 2000/1)
    • no trend could be determined for 14 sites; of these sites, one had data starting from 1976/77.
    Figure 1

    Trends for annual growing degree days (10 °C base threshold), 1972–2015 – Interactive map

    Map, Trends for annual growing degree days, 10 degree Celsius base threshold, 1973–2016.

                                                                                                                                                                                                    Source: NIWA
    Note: A year is one growing season (1 July–30 June of the following year), defined in the map by the start year. For example, the year 1972 in the map represents the growing season 1 July 1972–30 June 1973.
    Not all sites have data for 1972–2015. The data record is shorter for four sites (start dates in brackets): Kerikeri (1982), Masterton (1993), Taupo (1976), and Whangaparaoa (2000).
    Figure 2
    Note: Data are from the 26 sites which had data for the whole time-series: Auckland, Blenheim, Christchurch, Dannevirke, Dunedin, Gisborne, Gore, Hamilton, Hokitika, Invercargill, Lake Tekapo, Milford Sound, Napier, Nelson, New Plymouth, Queenstown, Reefton, Rotorua, Tara Hills, Taumarunui, Tauranga, Timaru, Waiouru, Wellington, Whanganui, and Whangarei.
    A year is one growing season (1 July–30 June of the following year). For example, the period 1972/3 in the graph represents the growing season 1 July 1972–30 June 1973.

    Definition and methodology

    Growing degree days (GDD) counts the number of days that are warmer than a threshold temperature (Tbase) in a year. GDD is calculated by subtracting the Tbase from the average daily temperature (maximum plus minimum temperature divided by two). If the average daily temperature is less than Tbase the GDD for that day is assigned a value of zero.

      

    We use 10 degrees Celsius as the Tbase threshold because it relates to the threshold for good pasture growth, particularly clover growth (Hutchinson et al, 2000). This is an important consideration for New Zealand dairy, beef, and sheep farmers. It is also a common reference threshold in other countries.

    Annual data is available up to the end of 2016 for a total of 30 sites: 26 sites from January 1972 and four sites between 1976 and 2000. Missing data were infilled where records were not available for all days (Macara & Tait, 2015).

    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.

    A year is one growing season (1 July–30 June of the following year).

    GDD is not the sole predictor of growth in a growing season. Other factors also influence growth, such as soil moisture, wind, and nutrient supply.

    Data quality

    We classified Growing degree days 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

    Hutchinson, GK, Richards, K, & Risk, WH (2000). Aspects of accumulated heat patterns (growing degree-days) and pasture growth in Southland (PDF, 38kB). Proceedings of the New Zealand Grassland Association 62, 81–85. Retrieved from www.grassland.org.nz.

    Macara, G, & Tait, A (2015). Infilling of missing climate data for the 2015 Environmental Synthesis Report: Temperature, rainfall and wind. Client report no. WLG2015-33. Prepared for the Ministry for the Environment, 37p. Wellington: NIWA.

    Archived pages

    See Growing degree days (archived October 2017).

    Updated 19 October 2017

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