Lakes Decline Sharply During August

New Zealand’s hydro-electric storage lakes declined sharply during August 2008.

At the beginning of August 2008, New Zealand’s hydro-electric storage stood at 1687 GWh (GigaWatt hours), 70% of average for the time of year but had declined to 1391 GWh, 61% of average, by month’s end. Throughout August, storage was consistently below levels recorded at the same time last year.

Inflows were above average on 15 days, mainly due to wet weather in the north which boosted North Island catchments. In the South Island, major storage lakes continued to decline. Lake Benmore languished, declining rapidly at the end of August. Lakes Aviemore and Waitaki were tightly managed while Tekapo and Pukaki declined. Lake Ohau remained critically low. Lake Manapouri was also critically low but showed a slight recovery during the last 10 days or so of the month. Lake Te Anau was below its normal lower operating level for most of the month.

In the North Island Lake Taupo, which had steadily declined during the dry period from January to May began refilling, bringing the lake level over 357 metres above sea level during August. With heavy rain affecting land adjoining the Waikato River downstream, the need to spill water from Lake Taupo to both cover for shortages in the southern hydro lakes and to prevent the lake rising above its maximum level created a headache for managers. However, while the Waikato River carried heavy flows, they were able to be kept at a level which did not exacerbate flooding of land near the river.

This was not the case back in July 1998 when Lake Taupo reached 357.45 m above sea level and it became necessary to release water into the river, prolonging a period of flooding downstream. The 1998 event was not the worst however, as Lake Taupo, which is New Zealand’s largest lake, reached a level of 357.72 metres above sea level in 1909.

The imbalance between northern and southern lakes meant that north-to-south transfers via the Cook Strait cables exceeded northbound transfers on 30 days. On the remaining day no transfers at all were made.

When figures were adjusted to match weekday for weekday, demand exceeded the same day last year on 21 days. Peak demand of 127.2 GWh occurred on August 19th. The August 2007 peak load of 123.5 GWh occurred on the 15th.

The average daily load for August 2008 was 114.7 GWh, just above the August 2007 average of 113.5 GWh.

[Compiled from data supplied by M-co.]

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