Science Centres: Energy

How will future climate affect hydro storage?

A switch in a long-term climate cycle may signal a 20–30 year stretch of more frequent dry years

Annual flows recorded for the Clutha River at Balclutha, 1 October 1947–30 September 1999. The mean flow is substantially higher during the recent positive phase of the Interdecadal Pacific Oscillation. This large-scale pattern of climate variability may now have switched to a negative phase which is likely to bring lower flows again.

‘Recent low inflows to New Zealand hydro lakes are actually rather mild compared to the dry period of the mid 1960s,’ says NIWA scientist Dr Jim Renwick. ‘NIWA research shows that the period 1977–98 was relatively wet on the South Island west coast and in the headwaters of key southern lakes, compared to the preceding 30 years. Such long term variability in lake inflows and rainfall is related to alternating phases of the Interdecadal Pacific Oscillation.’

The Interdecadal Pacific Oscillation, or IPO, is a large-scale pattern of climate variation affecting the whole Pacific basin. It sets the stage for more or fewer El Niño or La Niña events. During a positive phase of the IPO (for example, the late 1970s to late 1990s), we see more frequent El Niño events. These make for cooler, windier weather with stronger westerlies bringing more rain, high river levels, and greater inflows into the southern hydro lakes. In a negative phase of the IPO (for example, the late 1940s to mid 1970s), we see more frequent La Niña events, with milder, less windy conditions, and lower hydro inflows.

Dr Renwick says the IPO may be changing phase again, back to the pattern observed before 1977. ‘If this occurs, low inflow periods for South Island lakes will be more frequent, and possibly more severe, over the next 20–30 years than in recent times.’

NIWA Science

In the longer term, says Dr Renwick, ‘climate change is predicted to mean stronger westerly winds over New Zealand, more rain in western and alpine regions, and more reliable flows into our hydro lakes.’

Currently, the South Island hydro lakes rely on spring snowmelt to recharge. Warmer temperatures may mean that in the winter over the Alps more precipitation falls as rain rather than snow, compared with now, providing higher inflows when we need them most. NIWA scientists are working to quantify how snow storage and melt is likely to change in the future.