The year without a summer in the North Island actually kept the South Island in power, the Electricity Authority’s weekly market brief stated this week.
Mercury generated a total of 414GWh in January from its Waikato hydro stations.
The previous high was 332GWh, nine years ago.
Southwards flows over the Cook Strait cable reached above 400MW at some points.
Only one other southward flow was higher than those seen in January 2023, and that was in January 2018, when there were also extreme weather events in the North Island in January and low hydro storage in the South Island.
In January 2023, the oceanic and atmospheric phenomenon known as La Niña, contributed to dry weather in the South Island.
Lakes Manapōuri and Te Anau experienced their lowest January inflows since 1953, while Lake Taupō recorded its fourth highest January inflow.
This “wet dry divide” impacted hydro storage and generation across the country.
Low rainfall in the south until February this year, saw hydro storage in lakes Manapōuri and Te Anau drop below their low-operating ranges.
Meridian, which operates Manapōuri power station, then had strict limits on how quickly the lake levels could drop further, which reduced its ability to generate electricity.
In contrast, heavy rain in the North Island saw Lake Taupō’s storage reach its maximum agreed limit.
Mercury had to spill water downstream into the Waikato River, in line with resource consent requirements with Waikato Regional Council. This was controlled, due to heavy rain and to prevent further flooding downstream.
During the heavy rain, Karāpiro dam outflows were above 500 cubic metres per second, resulting in some minor flooding in Hamilton.
Mercury has nine hydro generation dams on the Waikato River, with additional streams also supplying more water at various points in the chain.
Each dam has a different amount of water that can be stored directly above it. It takes about 24 hours before water released from Lake Taupō reaches the final dam at Karāpiro.
No one event may be the same in terms of how much water is flowing in from which streams.
Mercury must carefully manage these many complexities when deciding how much water to release from Lake Taupō, how much and when to generate from each dam, and how much they might need to spill at Karāpiro – all while complying with various resource consents.
