Soil moisture and drought

  • Image, Soil moisture and drought.

    Soil moisture is vital for plant growth. When plants cannot access the water they need, growth is reduced, affecting crops and food for livestock, and native biodiversity. Over a sustained period, a drought can have significant social and economic costs, particularly for rural communities.

    Potential evapotranspiration deficit (PED) can be thought of as a drought index. It is the difference between how much water could potentially be lost from the soil through evapotranspiration and how much is actually available. When PED is high, plants do not have the full amount of water available they need for growth. As our climate changes, increasing temperatures and rainfall pattern changes are expected to increase PED, and the frequency and intensity of drought, particularly in currently drought-prone regions.

    We classified Soil moisture and drought as a case study.

    Key findings

    Potential evapotranspiration deficit (PED) from the growing seasons of 1972/3 through to 2015/6 increased at 7 of 30 sites and decreased at one. Trends were assessed at the 95 percent confidence level.

    • PED increased at Whangarei, Hamilton, Tauranga, Napier, Nelson, Masterton, and Tara Hills in Otago; and decreased at Timaru from 1972/3 to 2015/6.
    • Most of New Zealand had drier than normal soils over the most recent three-year period from 2013/4 to 2015/6.
    • Average annual PED is variable across New Zealand, especially in the South Island, but most areas average between 40 and 310mm.

    Average annual potential evapotranspiration deficit (PED) trends 1972–2015 and PED anomaly 2013–2015 for 30 stations – Interactive map

    Map, Average annual potential evapotranspiration deficit trends 1972–2015 and anomaly 2013–2015.

    Source: NIWA
    Note: For PED we used growing seasons (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.

    Definition and methodology

    Evapotranspiration is the loss of water by evaporation from the soil and plant transpiration. It is measured in growing seasons (12 months from July to June) and is dependent on meteorological conditions including solar radiation, wind, temperature, and humidity, as well as soil conditions. As the growing season progresses, water loss from the soil to the atmosphere through evapotranspiration is generally greater than the water the soil receives in rainfall, leading to a shortage. This loss is measured by the potential evapotranspiration deficit (PED), which can be thought of as the amount of water that needs to go to the soil, through rainfall or irrigation, for plants to grow to their full potential (NIWA, 2007). When PED is high, plants do not have the full amount of water for growth. In general, each extra 30mm of PED means about an extra week of reduced grass growth (NIWA, 2007).

    NIWA interpolated daily Virtual Climate Station Network (VCSN) data for rainfall and potential evapotranspiration to create a regular 500m resolution grid of average annual PED for each growing year from 1972–2016 (NIWA, nd). Although the interpolations cover New Zealand’s entire land area, accuracy is lowest where station density is low and terrain is complex.

    The coordinates for each of the 30 sites (corresponding with the same 30 sites used for some of our other measures eg growing degree days) were obtained from NIWA and imported to a geographic information system. PED value from NIWA’s interpolation was obtained at each of the 30 sites for each year. Note that these values should be viewed with some caution as they are interpolated rather than measured values.

    PED anomaly is the difference between the most recent three-year average PED for 2013/14-2015/16 and the 30-year PED climate normal (1981/82–2010/11 average) VCSN interpolations. Using a three-year average accounts for inter-annual variability while providing information on recent conditions.

    Toward the end of this century, increased PED is expected to be most pronounced over the North Island’s northern and eastern regions, and the South Island’s north-eastern and central regions east of the Southern Alps (Ministry for the Environment, 2016).

    Data quality

    We classified Soil moisture and drought as a case study.


     Image, Indirect relevance.  This case study is an indirect measure of the ‘Economic impacts' topic.


     Image, High accuracy.  The accuracy of the data source is of high quality.

    See Data quality information for more detail.


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

    NIWA (nd). Virtual Climate Station data and products. Retrieved 29 May 2017 from

    NIWA (2007). Backgrounder. Retrieved from

    Archived pages

    See Soil moisture and drought (archived October 2017).


    Updated 19 October 2017

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