Rainfall relief alpine rivers but less into spring-fed streams
The generally substantial rainfall last week has provided relief to some of Canterbury’s rivers, but irrigation restrictions continue to be applied on some groundwater and river or stream abstractions.
Adam Martin, surface water scientist at Environment Canterbury, says rainfall at some recorder sites is already at the highest level for February since records began 42 years ago.
“In the North Canterbury foothills, rainfalls are in the range of two to three times the long-term monthly average for February. Similarly, across the Canterbury Plains rainfall is at twice that expected for the month,” he says. “In the South Canterbury foothills, west of Timaru, rainfall is at one-and-a-half times that expected for February and just below the long-term monthly average, at 85 per cent, south of Timaru. Rainfall in Kaikoura is also just below the long-term monthly average at 85 per cent.”
Despite this, groundwater levels and spring-fed streams and drains are still low for the time of year in many areas. Monitoring of groundwater levels across the region last week indicated that 78 per cent of the region’s monitoring wells have below average groundwater levels.
“Groundwater levels remain serious for some users and continue to impact upon surface waterways which are fed from groundwater,” says Dr Howard Williams, ECan groundwater scientist.
While the recent rainfall has increased flows in the main alpine and foothill rivers, it has not yet been enough to increase flow sufficiently in some groundwater-fed streams. Flows in spring-fed streams also remain low in those areas where allocation of groundwater is at or close to 100 per cent. The Selwyn River at Coes Ford is at 27 per cent of its minimum flow and the Irwell River is dry, as it was for the 18 month period prior to June 2006.
Irrigation restrictions relating to spring-fed streams may change as the recharge from the recent rainfall makes its way into the groundwater system.
Soil moisture levels continue to be at about average for the time of year except in North Canterbury where heavy rain has improved conditions markedly.
Regional detail
North Canterbury
Heavy rainfall in North Canterbury has reversed the drying trend with raised river flows expected to lead to rising groundwater levels in associated shallow aquifers.
Groundwater levels in Kaikoura and Waipara are below average with many monitoring wells displaying levels at their lowest ever for the time of year. In contrast, wells in the Ashley – Waimakariri zone display generally average levels of groundwater, with only a few wells showing low levels.
Mid-Canterbury
Two of the five West Melton monitoring wells indicate groundwater levels below first trigger levels, placing part of the area on one-third restriction. Groundwater levels in another two trigger wells are below the 2nd trigger level, meaning that abstractions must be cut by two-thirds.
Within Christchurch City groundwater levels vary from much lower than average to the lowest ever for the time of year. Many are lower than levels recorded in the 2005-2006 summer.
In the Selwyn-Waimakariri groundwater zone there has been an increase from last month in the number of wells with groundwater levels at very low to lowest ever levels. Similarly, in the Rakaia-Selwyn zone, nearly all groundwater levels are below average levels with most of these at very low to lowest ever levels for the time of year.
All groundwater measurements in the Ashburton-Rakaia zone are below average levels and most are at very low to lowest ever levels for the time of year.
South Canterbury
In the Mayfield-Hinds and Orari zones, January groundwater levels were below average levels, with shallow groundwater levels at low levels with some wells at record lows for January. Similarly, in deeper aquifers, groundwater levels were very low whilst shallow wells near the Orari River were at below average levels with some wells at or near record lows for January. Shallow wells closer to the Waihi and Te Moana rivers were at average levels for January. In the Upper Waitaki, groundwater levels continue to fall with many monitored wells indicating their lowest levels since monitoring began in February 2007. In the Pareora-Waihao zones, shallow groundwater associated with the Pareora and Otaio Rivers dropped in January to low or very low levels due to a lack of river recharge. The Waihao River area fared better, with average water levels. Deeper groundwaters are at low levels for the three years of record.
The most recent monitoring data are available at:
http://www.ecan.govt.nz/Our+Environment/Water/Groundwater/Groundwater+Levels
Irrigation restrictions
For an updated list of irrigation restrictions go the Environment Canterbury homepage (www.ecan.govt.nz) and click on Irrigation restrictions, bottom right.