Drought and rainfall

This section describes changes in drought conditions and extreme rainfall in Aotearoa 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 climate change will cause the annual amount of rainfall and the number of days with extreme rainfall to 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 2022 show dry days (days with a soil moisture deficit) are more common in eastern regions in both the North and the South Island (Movie 2). Extreme rainfall was more common in western regions of the South Island in 2022 (Movie 1).

Our website shows how these rainfall extremes have changed year-by-year over the past 40 years (1981-2022) across territorial authorities (TAs) in New Zealand.

Movie 1: Number of days with extreme rainfall, 1981-2022, by Territorial Authority (TA) 

Movie 2: Number of days with soil moisture deficit, 1981-2022, by Territorial Authority (TA) 

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 5^th 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.