Scientist studies how shifts in the climate affect water

NIWA Media Release 2 December 2011

Scientist studies how shifts in the climate affect our most valuable asset - water

A NIWA scientist has been looking at the effects of shifts in climate on water resources, in water-limited parts of New Zealand.

“We’re asking how these water resources might look in the future," says NIWA principal scientist Ross Woods, who is studying variability in river flows over decades and the influence of the cyclical change in the Pacific ocean-atmosphere system - the Interdecadal Pacific Oscillation (IPO).

The weather doesn’t just vary from one day to the next. It’s different from one year to the next, and one decade to the next, so water resources also vary on these time scales.

People who use and manage water supply systems need to know where sources of water are coming from and how reliable those water supplies will be. This is important for planning and design of irrigation schemes, hydropower systems and municipal water supplies.

The results of this research will be beneficial to power companies, regional councils, and anyone making water allocations decisions.

Farmers, banks and government agencies are investing hundreds of millions of dollars into irrigation. Irrigation increases the productivity of farmland, protects against droughts, and takes the uncertainty out of water flows for farmers and recreational users.

Dr Woods says, “Understanding variability in stream flow over decades can be critical for the design of water infrastructures like dams, irrigation schemes, and bridges, and to design for floods. Without this understanding, it is difficult to use river flow data from the past as a guide to the future.”

With the Interdecadal Pacific Oscillation (IPO), a characteristic circulation pattern predominates for a 20-30 year period, and then the system changes to having a different characteristic circulation pattern. These patterns are known as phases of the IPO. The IPO was in a negative phase from 1945-77 and in a positive phase from 1978-99. During the positive phase, El Nino events and westerly winds are more frequent, and rainfall in the west and south of the South Island is higher than usual. In contrast, the Bay of Plenty is drier than usual and has fewer floods. The opposite applies during the negative phase of the IPO.

“The phase we are currently in has only been going for ten years, so there is a reasonable chance that it has another ten years to go,” says Dr Woods.

In 2000, the IPO changed back to the negative phase (as for 1945-77). “The whole of South Island is drier now. If you had to make a guess about the coming 10 years, expect a slightly drier South Island. It certainly affects Canterbury and has implications for the design of irrigation and hydro power schemes,” says Dr Woods.

From a water resource management perspective, it would be wise to treat South Island flow data from the period 1978-99 as being slightly higher than the long-term average. In making forecasts of future water resources for planning purposes, it appears prudent to make allowance for the possibility that flows for the next 10-20 years could be slightly lower than the long-term average.

Dr Woods looked at long river flow records for 35 sites across New Zealand, with data from no later than 1967, through to 2010. “I calculated annual values of the mean flow, maximum flow, and 7-day low flow,” says Dr Woods.

The river flow data were first sorted according to which IPO phase they occurred in. Then statistical tests were used to assess whether there was a significant difference in flow between phases of the IPO.

So is this climate change? “Not in the popular sense of climate change, no,” says Dr Woods, “It’s not caused by increases in greenhouse gases. It’s a shift in climate that will probably shift back again in another 10 or 20 years. On top of the decadal shifts I’ve been looking at, over the next century we expect that increases in greenhouse gases will drive warmer temperatures and stronger westerly winds. The impacts of that on river flows are important too, and so we’re also working on them in the Waterscape programme.”

This research was funded by Ministry of Science and Innovation through the Waterscape programme. More detail is available in the blog at http://sciblogs.co.nz/waiology/2011/11/17/long-term-fluctuations-in-river-flow-conditions-linked-to-the-interdecadal-pacific-oscillation/

NIWA’s Ross Woods will be presenting at the 2011 NZ Hydrological Society Conference "Learning From The Past : Creating The Future", to be held at Te Papa, from 5 - 8 December.

ENDS

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