Further drop in Waikato River levels expected

Environment Waikato is keeping a close watch on declining Waikato River levels which are being affected by the region’s ongoing drought.

The drought has resulted in lower than usual water levels in Lake Taupo and reduced flows in the river.

The surface of Lake Taupo - at 356.03 metres above sea level (masl) as of noon 9 April - was still high enough to be within its consented operating range of between 355.85 masl to 357.25 masl.

But the level is steadily falling at the rate of about eight millimetres a day and, in the absence of significant rainfall, this drop off is likely to continue for some time.

To help protect future water reserves, electricity company Mighty River Power has, since November, operated in “water conservation” mode. This month, it has been able to further reduce the minimum flow from Lake Karapiro from at least 148 cubic metres a second to at least 140 cubic metres a second. This initiative helps keep the Taupo lake level within the normal operating range as long as possible whilst maintaining flow from Karapiro to the lower river.

However, despite these actions, unless significant rainfall happens soon Lake Taupo could fall below 355.85 masl sometime next month.

When the 355.85 masl level is reached, Mighty River Power, under its resource consent, is obliged to only allow as much water out of the lake as is coming in from the surrounding catchment. Under these drought conditions, this will result in the flows in the Waikato River downstream of Karapiro dropping to a more natural level.

This will particularly effect river flows below the dam at Lake Karapiro and this may start to pose problems for river users.

“The main effects are likely to be on the ability of water supply intakes to operate at maximum efficiency, but there may also be impacts on recreational users,” said resource use group manager Dennis Crequer.

However, at this stage the situation is not expected to impact noticeably on water levels in the hydro electricity dams between Taupo and Karapiro, Mr Crequer said.

He believed that until Lake Taupo drops below the 355.85 masl level there is no major cause for concern regarding river levels but Environment Waikato will keep significant water users up to date about the situation.

“Environment Waikato will continue to monitor the situation and keep stakeholders informed of developments,” Mr Crequer said.

He pointed out that low lake levels have been recorded several times a decade over the past twenty or so years.

“However, one of the key differences this time is that we are less confident of rain in the near future to alleviate the situation.”

The lowest lake level ever recorded by NIWA since December 1978 was in 2001 when the lake dropped to 355.91 masl.