Science Centres: Energy

Low hydro inflows: it's La Niña

Summer inflows range widely, but are generally lower and have a narrower range in La Niña conditions.

Tekapo Canal (Photo: Elliot Tuck, NIWA)

Monitoring of inflows to New Zealand’s hydroelectric lakes stretches back to the 1920s. With the benefit of such a long time series, NIWA can show that the flow into South Island hydro lakes in La Niña years is considerably lower than the flow for other years.

Effect of the El Niño Southern Oscillation (ENSO)

During El Niño conditions, more westerly and southwesterly wind than usual occurs over the country, typically bringing rain into western and southern parts of the country. Conversely, La Niña conditions, which prevailed over the summer of 2007–08, bring more northeasterly winds than usual. These bring moisture into the north and east of the North Island, but relatively little rain into the main hydroelectric power catchments.

This summer’s low lake inflows were normal for a La Niña year

As the plots right show, summer inflows range widely, but are generally lower and have a narrower range in La Niña conditions. The summer inflow for 2007–08 (2756 MW) was close to the median for La Niña summers (2830 MW) so far from being ‘low’, this summer’s inflows were normal for La Niña conditions.

Why summer inflows matter

Typically about 30% of inflow to hydroelectric lakes occurs in the summer months of December, January, and February when power demand is least. Much of the water is, therefore, retained in controlled storage in Lakes Taupo, Tekapo, Pukaki, and Hawea for generation later in the year when power demand increases and inflows are less. If these lakes do not fill in summer, we tend to see high wholesale electricity market prices in autumn and winter because more costly forms of generation must be used to make up for hydrogeneration shortfalls.

What can the energy industry do?

Modern global climate models can predict the ENSO state for several months ahead. The likelihood of La Niña conditions in the summer of 2007–08 was first flagged in NIWA’s Climate Update in May 2007. During the latter part of 2007, the indications strengthened. For example, the November 2007 Climate Update reported ‘La Niña to stay for summer’. This information is useful for both planning and operations in the electricity industry.

Planning for a higher level of renewable energy requires increasing sophistication in handling all sources of variability, including a good understanding of climate related risks. It is a demanding task to handle electricity operations when falling lake levels and river flows approach consent limits. Better understanding of natural variability by the general public and decision makers will also be important for confidence in future national energy security.

For more information, contact:

Dr Alistair McKerchar, NIWA hydrologist, a.mckerchar@niwa.co.nz

Dr James Renwick, NIWA climate scientist, j.renwick@niwa.co.nz