Snowmelt: a seasonal boost to hydro lake levels

In New Zealand, between 80-95% of electricity is generated by renewable sources, with the majority produced by hydro dams. Most dams are in the South Island and their electricity is ferried to the North Island via large cables (called the High-Voltage Direct Current or HVDC).

The additional water used in dams (hydro storage) is refilled by both rain and snow, with snow having a particularly strong seasonal impact on storage. Hydro lake levels typically drop during autumn and winter, as hydro generation runs hard and rain falls as snow on the slopes of the Southern Alps. Then snowmelt flows into the hydro lakes in spring and summer. This snowmelt contributes to between 20-70% of spring and summer water inflows¹ in the South Island’s hydro lakes.

There are three main water catchments in the Southern Alps, the:

• Waitaki river, which feeds hydro dams like Benmore and Aviemore;
• Clutha river, which runs through the Clyde and Roxburgh dams; and
• Waiau river, which flows through the Manapōuri Power Station.

Snow typically accumulates at elevations above 1,000 metres, so the first two catchments experience more snowmelt as their mountains are higher.

New Zealand experienced its wettest winter on record in 2022. In late October 2022, national hydro storage was 23% percent above the historic mean ie, the lakes were high, when typically, they would be low. Hence, snowmelt - combined with already high lake levels - could see high hydro storage persist at lakes Pūkaki, Hawea and Takapō throughout the remainder of the year².