Long-term fluctuations in river flow conditions linked to the Interdecadal Pacific Oscillation


How will water resources in the water-limited parts of New Zealand, such as Canterbury and Hawkes Bay, look in the future?

The issue

As part of the Waterscape research programme, NIWA is conducting a number of case studies on the potential effects of climate change on water resources, focusing on water-limited parts of New Zealand, such as Canterbury and Hawkes Bay.

In addition to variations due to climate change, water resources may also vary for other reasons. The weather is different from one year to the next, and one decade to the next, so water resources also vary across these various time scales. Understanding these variations will allow us to view any potential future changes in the context of the year-to-year changes that water managers already deal with.

This research looked at decadal-scale variability in river flows. Understanding decadal variability in streamflow can be critical for the design of water infrastructure for hydropower, irrigation and water supply. For example, if future river flows are smaller than past data would suggest, then a hydropower dam will not produce as much electricity as its owners planned for when they invested in it. Without a good understanding of decadal river flow variability, it is difficult to use river flow data from the past as a guide for planning water infrastructure.


The Interdecadal Pacific Oscillation (IPO) is a cyclical change in the Pacific ocean-atmosphere system which affects New Zealand's climate. 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 intense than usual, and rainfall in the west and south of the South Island is higher than usual. The opposite applies during the negative phase.

Does the IPO affect river flows?

McKerchar and Henderson (2003) showed that there was a significant difference between streamflow statistics for the periods 1945-77 and 1978-99, especially in the west and south of the South Island. For example, the mean flow in the Clutha River at Balclutha over 1978-99 was 14% larger than during 1956-77.

So why look at the IPO again?

In 2000, the IPO changed back to the negative phase (i.e. like 1945-77), though the magnitude of the recent IPO is closer to zero (neutral). So now the research team is asking, in the ten years since 2000, have the river flow statistics changed back to the values they had for the negative phase (1945-77). If they have changed back, that will change our outlook for river flows over the coming decade.

Analysis and results

The research team examined long-term river flow records for 35 sites across New Zealand. They calculated annual values of the mean flow, maximum flow, and 7-day low flow. The figure to the right (click to enlarge) shows an example of the results, for floods, mean flows and low flows in the Buller River at Te Kuha, on the West Coast of the South Island. The mean annual flood over the period 2000-09 was 15% lower than the corresponding value for 1978-99. Mean flows and mean annual low flows were also lower in 2000-09 than in 1978-99, by about 10%.

Results for 15 other rivers in the South Island also showed this general result: that these three flow statistics were lower in 2000-09 than they had been in 1978-99. Not all the differences were statistically significant, but in almost every case the flows for 2000-09 were lower than for 1978-99. The research team was only expecting that rivers draining the south and west of the South Island would show this effect. Results for North Island rivers were more mixed, and didn't display a consistent pattern of change. More investigation is needed to understand the reasons behind both results.


Flow data from 2000 onwards in the South Island support the idea that flows in those rivers are lower during the negative phase of the IPO. The data suggest that the post-2000 reduction in flow has been of the order of 10%. It is unclear for how long the IPO will remain negative, but previous IPO phases have lasted 20-30 years, so the current negative phase may last another 10-20 years. Similarly, it is still unclear whether the observed correlation between flow and IPO will continue.

Viewed from a water resource manager's perspective, it may be best to treat South Island flow statistics from the period 1978-99 as being slightly higher than the long-term average. As a result, when making forecasts of future water resources for planning purposes, it is sensible to make allowance for the possibility that flows for the next 10-20 years could be lower than the long-term average. Towards the end of that time horizon (i.e. 2020-2030), we must begin to consider the potential effects of climate change, which is expected to increase precipitation in the South and West of the South Island.

Further reading

McKerchar, Alistair I; Renwick, James A and Schmidt, Jochen. Diminishing Streamflows on the East Coast of the South Island New Zealand and Linkage to Climate Variability and Change. Journal of Hydrology (New Zealand), Vol. 49, No. 1, 2010: 1-14.

Effects of IPO on river flow

McKerchar, A.I.; Henderson, R.D. (2003). Shifts in flood and low-flow regimes in New Zealand due to interdecadal climate variations. Hydrological Sciences Journal 48(4): 637-654.

Effects of El Nino and La Nina on river flow

McKerchar, A.I.; Pearson, C.P.; Fitzharris, B.B. 1998: Dependency of summer lake inflows and precipitation on spring SOI. Journal of Hydrology 205 : 66-80.

Mosley, M.P. (2000). Regional differences in the effects of El Nino and La Nina on low flows and floods. Hydrological Sciences Journal 45(2): 249-267.

Effects of Climate change on river flow

McMillan, H.K.; Jackson, B.; Poyck, S. 2010a: Flood risk under climate change: A framework for assessing the impacts of climate change on river flow and floods, using dynamicallydownscaled climate scenarios. NIWA Client Report CHC2010-033 for Ministry of Agriculture and Forestry.

McKerchar A, Mullan B (2004) Seasonal inflow distributions for New Zealand hydroelectric power stations. NIWA Client Report: CHC2004-131. Report for Ministry of Economic Development, Wellington, NZ.

Ministry of Agriculture and Forestry 2011: Projected effects of climate change on water supply reliability in Mid-Canterbury. MAF Technical Paper 2011/12, prepared by Aqualinc Research Limited, 43 p.

Poyck, S.; Hendrikx, J.; McMillan, H.; Hreinsson, E.Ö.; Woods, R.A. (2011). Combined snow- and streamflow modelling to estimate impacts of climate change on water resources in the Clutha River, New Zealand. Journal of Hydrology (NZ) 50(2): 293-312.

Srinivasan, M.S.; Schmidt, J.; Poyck, S.; Hreinsson, E.Ö. (2011). Irrigation Reliability Under Climate Change Scenarios: A Modeling Investigation in a River-Based Irrigation Scheme in New Zealand. Journal of the American Water Resources Association 47(6): 1261-1274.

Zammit, C.; Woods, R.A. (2011). Projected climate and river flow for the Waimakariri catchment for 2040s and 2090s. NIWA Client Report No. CHC2011-025. 52 p. 

Page last updated: 
14 March 2017
River flow measurement site on the Matukituki River, a tributary of Lake Wanaka. Credit: Steve Le Gal