One risk of climate change is the possibility of a greater intensity, duration and frequency of dry spells and droughts across New Zealand. If poorly matched to soils, irrigating agricultural land to mitigate drought could increase soil drainage and therefore leaching of agricultural contaminants (such as nitrogen, phosphorus, sediment and faecal microbes) from land into waterways.
Sources and Flows researchers in the Our Land and Water National Science Challenge looked to see if the severity and risk of drought has changed, and if that information could be used to decease soil drainage. They then looked at precisely matching irrigation to soil and soil moisture characteristics to prevent leaching of nitrogen and phosphorus.
Data from 6 sites representing contrasting regions were examined. A severity index combined intensity and duration to provide a quantitative assessment of the impact of drought on regional hydrological processes. The severity and frequency of droughts varied across the country. Perhaps surprisingly, the 3 North Island sites experienced longer and more severe droughts than the 3 South Island sites.
In Invercargill, a drought with a duration of 10 months and a severity index of 8 (10 being the highest impact) occurred once in every 10 years, whereas a drought of the same duration and severity occurred once in 6 years at Winchmore, once in 8 years at Nelson, once in 7 years at Wallaceville, once in 5 years at Napier, and once in 6 years at Kaitaia.
Perhaps surprisingly, the 3 North Island sites experienced longer and more severe droughts than the 3 South Island sites
Modelling of soil drainage at these sites indicated 35% to 80% more drainage peaks than described by parameters based on detailed physical description of soil. This suggests better monitoring of soil moisture and scheduling of irrigation is required.
The final stage of research used precision irrigation technologies, timed and matched to soil type at a location in Central Otago, to show that the loss of nitrogen and phosphorus in drainage was reduced by 70% to 80% compared to standard irrigation practice.
Because of this research, the Northland Regional Council is refining drought intensity, duration and frequency curves for Northland. These curves will be used to guide regional irrigation schemes and the allocation of water in the region.
For more information about this and related research, see:
- Sources and Flows
- A copula-based analysis of severity-duration-frequency of droughts in six climatic regions of New Zealand in Journal of Hydrology, February 2017
- Soil hydraulic modelling outcomes with four parameterization methods: comparing soil description and data inversion approaches in Vadose Zone Journal, March 2018