Rainfall improves river and groundwater levels

Rainfall in February and at the beginning of March gave a much needed boost to groundwater levels and river flows in the Canterbury region. However recent Environment Canterbury monitoring shows that in many areas these levels are still below average for the time of year and irrigation restrictions still apply on some groundwater and river water users.

“Recent rainfall has increased groundwater levels in shallow aquifers, especially those close to foothill rivers,” says Dr Howard Williams, ECan groundwater scientist. “While some groundwater users have benefited, rivers and streams fed from groundwater in some areas are still affected by the lack of rainfall.”

While February saw record rainfall in some areas of Canterbury, rainfall in North and South Canterbury for the first half of March is below the long-term monthly average at 40 per cent and 30 per cent respectively. These figures, along with the low rainfall in the region since July 2007, continue to contribute to the depleted state of groundwater and river water levels.

“Monitoring across the region last week indicated that most of the region’s monitoring wells have average to below average groundwater levels,” says Dr Williams. “Most deep wells have yet to respond to the summer rainfall and any recovery in their water levels is likely to be due to decreased demand from irrigators.”

Similarly, flows in spring-fed streams remain low where allocation of groundwater is at or close to 100 per cent. Increased flows in the main alpine and foothill rivers as a result of the recent rainfall were short-lived with flows having returned to their former levels.

Soil moisture levels are now generally at average or above average levels for the time of year in all areas.

Regional detail
North Canterbury
Rainfall in North Canterbury boosted river flows and has led to raised groundwater levels in associated shallow aquifers. Groundwater levels in Waipara have recovered slightly; with many monitoring wells displaying average to below average levels, recovering from the lowest ever levels for the time of year recorded last month. In contrast, wells in the Ashley – Waimakariri groundwater zone display generally average to above average groundwater levels, with only a few wells showing low levels and some showing signs of enhanced recharge from irrigation with river water.

Mid and Central Canterbury
Within Christchurch City, groundwater levels have improved from much lower than average to at or below average for the time of year.

Three of the five West Melton monitoring wells exhibit levels indicating no restriction. One well has a groundwater level close to the second trigger level, placing part of the area almost at two-thirds restriction. The remaining well is below the first trigger level, meaning that abstractions must be cut by one-third.

In the Selwyn-Waimakariri zone there has been a slight increase in groundwater levels from last month with levels now generally recovered in shallow wells. Levels in deep wells have not recovered to the same extent, except those close to foothill rivers. Similarly, in the Rakaia-Selwyn zone, nearly all groundwater levels in shallow wells are at average to below average levels with some deep wells still recording their lowest levels for the time of year.

Groundwater levels in the Ashburton-Rakaia zone showed a slight recovery over February levels but many are still at below average for the time of year.

South Canterbury
In the Mayfield-Hinds zone, February groundwater levels in shallow wells are at or below average levels. In deeper aquifers, groundwater levels are very low. Further south in the Rangitata to Opihi area, groundwater levels in the majority of shallow wells between Coopers Creek and the Rangitata River have risen since February and are now at average to below average levels for this time of year. Groundwater levels between Coopers Creek and the Waihi River have also increased since last month and are at average levels for the time of year. Groundwater levels in the deeper aquifers remain at average levels.

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 to the Environment Canterbury homepage (www.ecan.govt.nz ) and click on Irrigation restrictions, bottom right.