Drought and rainfall

This section describes changes in drought conditions and extreme rainfall in New Zealand over time, and who might be most at risk of health impacts from these extremes.

Climate models predict more droughts for parts of New Zealand

The amount of rainfall in New Zealand varies around the country and with season. It is projected that with climate change the annual amount of rainfall and the number of days with extreme rainfalls increase in the west and south and decrease in the north and east of both islands. The number of dry days is projected to increase in the North Island and in inland South Island [1].

Dry days more common in the east

Areas with more frequent dry periods are more likely to experience droughts, whereas areas with extreme rainfalls are more likely to experience flooding. Detecting changes in rainfall patterns in New Zealand is difficult due to the high variability in rainfall from year to year and from region to region. The variability is caused by changing weather patterns and natural climate variations such as El Niño [5].

Data from 2019 show dry days (days with a soil moisture deficit) are more common in eastern regions in both the North and the South Island (Movie 1). Extreme rainfall was more common in western regions of the South Island in 2019 (Movie 2).

Our website shows how these rainfall extremes have changed year-by-year over the past 40 years (1981-2019) across territorial authorities (TAs) in New Zealand.

Movie 1: Number of days with soil moisture deficit, 1981-2019, by Territorial Authority (TA) (click here for a larger version)

Movie 1: Number of days with soil moisture deficit, 1981-2019, by Territorial Authority (TA)

Movie 2: Number of days with extreme rainfall, 1981-2019, by Territorial Authority (TA) (click here for a larger version)

 Movie 2: Number of days with extreme rainfall, 1981-2019, by Territorial Authority (TA)

Number of dry days and extreme rainfall days vary from year to year

The average number of extreme rainfall days (Figure 1) and dry days (Figure 2) in New Zealand varies from year to year. In the 30-year climate normal period from 1981-2010, New Zealand experienced 17.9 (17.7-18.1) extreme rainfall days per year. 1982 was one of the years with the least extreme rainfall days (14.3, 13.2-15.4) whereas 1995 was one of the years with the most extreme rainfall days (22.8, 21.9-23.8).

In the 30-year climate normal period from 1981-2010, New Zealand experienced 58.7 (56.7-60.7) dry days per year. 2018 was one of the years with the least dry days (31.4, 24.3-38.4) whereas 2010 was one of the years with the most dry days (95.6, 85.1-106.2).

In 2019, New Zealand experienced, on average, 65.9 (95% confidence interval: 54.7-77.2) dry days and 17.7 (16.3-19.0) days with extreme rainfall.

Figure 1: Number of extreme rainfall days in New Zealand and comparison to the 1981-2010 baseline average, 1981-2019

 Figure 1: Number of extreme rainfall days in New Zealand and comparison to the 1981-2010 baseline average, 1981-2019

Note: The baseline average period refers to the most recent Climate Normal Period, 1981-2010 (WMO 2017). 30 years of data were averaged to act as a benchmark against which current or recent observations can be compared to.

Source: National Climate Database (CliFlo), NIWA

Figure 2: Number of dry days in New Zealand and comparison to the 1981-2010 baseline average, 1981-2019

 Figure 2: Number of dry days in New Zealand and comparison to the 1981-2010 baseline average, 1981-2019

Note: The baseline average period refers to the most recent Climate Normal Period, 1981-2010 (WMO 2017). 30 years of data were averaged to act as a benchmark against which current or recent observations can be compared to.

Source: National Climate Database (CliFlo), NIWA

Drought and extreme rainfall have several effects on health

Variations in rainfall patterns can affect health in several ways [2][3][4]:

  • drinking water: Severe drought can reduce the quality and the amount of drinking water available. Flooding caused by extreme rainfall can also affect the quality of drinking water. New Zealand’s populations that rely on rainwater tanks for their drinking water supply can be particularly affected.
  • gastrointestinal infections: Rates of the gastrointestinal infections cryptosporidiosis and giardiasis are affected by rainfall patterns. Rainfall washes giardia and cryptosporidium cysts into waterways, where they can contaminate drinking water sources. Drought conditions can lead to a greater cyst concentration in groundwater and surface water sources
  • reduced food availability: Drought can reduce crop production, meaning there is less (and possibly more expensive) food available for consumption. Food from fresh water sources will also be diminished. Extreme rainfall can also damage crops.
  • mental health: Drought can have a significant effect on mental health, particularly for those in rural areas who rely on rain for their livelihoods. Similarly, extreme rainfall can lead to flooding or landslides causing damage to infrastructure.

Effects on vulnerable populations

Populations that are more vulnerable to the health effects of drought are:

Combining soil moisture deficit (Movie 1) and population data (see Population Vulnerability domain for more information) suggests, that the east coast of the North Island (i.e. Gisborne and Hawke’s Bay), are likely to be regions where people will be particularly affected by the health effects of drought.

Gisborne is a rural area with a substantial Māori population and significant socio-economic deprivation, plus a high percentage of the population is under 5 years old. Similarly, Hawke’s Bay is a rural area with a sizeable Māori population, high numbers of older people over 85 years old and children under 5 years old, and pockets of socioeconomic deprivation.

Information about the data

Number of days with soil moisture deficit

Source: CliFlo. NIWA's National Climate Database

Definition: Climate station data of the number of days per year in SMD from around New Zealand was sourced from the National Institute of Water and Atmospheric Research (NIWA). One climate station was selected per territorial authority, based on their proximity to the population-weighted centroid of each TA (2018 Census data). The number of dry days was counted for each year by TA. Only years with more than 90% of valid data were counted. Data was compared to the most recent Climate Normal Period, 1981-2010, where the 30-year average acts as a benchmark against which more recent observations can be compared to.

Annual amount of rainfall, and number of days with extreme rainfall

Source: CliFlo. NIWA's National Climate Database

Definition: Climate station data of the amount of daily rainfall (in mm) from around New Zealand was sourced from the National Institute of Water and Atmospheric Research (NIWA). One climate station was selected per territorial authority, based on their proximity to the population-weighted centroid of each TA (2018 Census data). The amount of rainfall was calculated for each year by TA. Only years with more than 90% of valid data were counted. The 95th percentile for the period 1981-2010 was calculated for each TA and number of days above this percentile was counted as extreme rainfall days. Data was compared to the most recent Climate Normal Period, 1981-2010, where the 30-year average acts as a benchmark against which more recent observations can be compared to.

References

  1. Ministry for the Environment. 2018. Climate change projections for New Zealand: Atmospheric projections based on simulations undertaken for the IPCC 5th Assessment. 2nd Edition. Wellington: Ministry for the Environment.
  2. McMichael AJ. 2013. Globalization, Climate Change, and Human Health. New England Journal of Medicine 368(14): 1335-1343. DOI: 10.1056/NEJMra1109341 (accessed 9 February 2021).
  3. Britton E, Hales S, Venugopal K, et al. 2010. Positive association between ambient temperature and salmonellosis notifications in New Zealand. Australian and New Zealand Journal of Public Health 34(2): 126-9. DOI: 10.1111/j.1753-6405.2010.00495.x (accessed 9 February 2021).
  4. Lal A, Baker MG, Hales S, et al. 2013. Potential effects of global environmental changes on cryptosporidiosis and giardiasis transmission. Trends in Parasitology 29(2): 89-90. DOI: 10.1016/j.pt.2012.10.005 (accessed 9 February 2021).
  5. Ministry for the Environment and Stats NZ. 2020. New Zealand’s Environmental Reporting Series: Our atmosphere and climate. Wellington: Ministry for the Environment and Stats NZ.

Interested in more information?

Carolin Haenfling

Phone +64 4 979 3119 (ext 63119)
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