No alarm as lake levels fall
Lake levels in the South Island are below average but there is no worry as the risk of shortage in the next two months is assessed to be low.
Storage for the Waitaki catchment is 79 per cent of average, while the South Island is 75 per cent of average, a Meridian Energy spokesperson said.
"Storage in our two South Island catchments is currently below the historical average for this time of year. Lake Pukaki is currently at 72 per cent of average for example.
"We're carefully managing storage levels by generating more conservatively, which is our normal response to these conditions," he said.
According to Transpower, hydro storage is divided into two categories - controlled and contingent storage - for security of supply purposes.
Generators can use controlled storage at any time, but contingent storage may only be used during defined periods of shortage or risk of shortage.
The South Island represents about 85 per cent of New Zealand's controlled storage capacity and is often used as a key measure.
During sustained dry periods, controlled and contingent storage are important indicators of overall supply risks.
Storage is expressed in gigawatt-hours - GWh (a measure of the energy that can be produced using the water).
Controlled storage for the South Island as of April 20, was 1947GWh, about 59 per cent of the maximum South Island controlled storage.
This amount of controlled storage places it in the "security normal" range which means the risk of shortage is less than 1 per cent, barring major unexpected equipment failures.
Nationwide, hydroelectric generation contributes about 60 per cent of total electricity supply, with many generators of widely varying sizes distributed throughout the country.
Inflows (rainfall and snowmelt) can be stored in hydro lakes until needed. The most important are Lakes Tekapo, Pukaki, Hawea, Manapouri, Te Anau, and Taupo.
The lakes have quite limited operating ranges - for technical and resource consent reasons, each lake's level cannot be lowered below a certain point.
- The Timaru Herald