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Hot weather in ChCh sees less water in rivers

Hot weather and easterlies: less water in rivers, groundwater levels drop

The sustained hot, dry weather in Canterbury for much of January has depleted water sources in the region, say Environment Canterbury water scientists.

Since July 2007, rainfall throughout most of Canterbury has been at a ten-year low, driven by the dominating easterly winds characteristic of the La Niña weather pattern.

“Groundwater levels are much lower than at the same time last year and are continuing their steady decline since last summer,” said Dr Howard Williams, ECan groundwater scientist.

River and stream flows have also been adversely affected by the lack of rainfall as have soil moisture levels which are as low or lower than two summers ago, he said.

“Mid January, 25 of the 34 major catchments within the Canterbury region experienced partial or full irrigation restrictions on some or all of their tributaries,” he said. Groundwater in the West Melton area near Christchurch is also under restriction.

“Substantial rainfall is needed in the next few weeks otherwise many more rivers will be running at flows where irrigation restrictions will be applied on both groundwater and river water abstractions.”

General state of river and stream water The main alpine rivers are flowing at below average levels for this time of year. The Waiau and Waimakariri Rivers are at 60 percent of average for December and the Rakaia and Rangitata Rivers are around 80 percent of their average flows.

Flows in North Canterbury for December were well below average with the Waipara River below 20 percent of expected flows. In South Canterbury flows in rivers are only marginally better, with river flows around 50 percent of the December average.

The Selwyn River at Coes Ford, Irwell River and Harts Creek in Central Canterbury are well below their minimum flow.

General state of groundwater In general, groundwater levels are significantly lower than at the same time last year. Around 85 percent of the region’s moderate to deep monitoring wells (greater than 30 metres deep) are at below average groundwater levels. Similarly, 75 percent of the region’s shallow monitoring wells (less than 30 metres deep) are at below average groundwater levels.

Regional detail North Canterbury Kaikoura and Waipara areas groundwater levels are well below average. In Kaikoura, most monitoring wells are at historic lows for the time of year. In Waipara most wells are setting new historic lows, while the remainder are at average to low levels. In contrast, wells in the Ashley – Waimakariri groundwater zone display generally average levels of groundwater, with only a few wells showing low to historic low levels. These average groundwater levels are in part buoyed up by water discharged to ground associated with the Waimakariri Irrigation Scheme.

Mid-Canterbury Groundwater levels within the Christchurch – West Melton area are generally lower than average for this time of year. In many cases they are lower than the low levels encountered in early 2006 (a previous dry period) and for some wells record lows are being recorded. Three of the Christchurch-West Melton monitoring wells indicate groundwater levels below 1st trigger levels, placing part of the area on restriction, whilst groundwater levels in the other two trigger wells are close to the 1st trigger level. Both the Selwyn-Waimakariri and the Rakaia-Selwyn groundwater zones are displaying levels far below those of last year, nearer those recorded from the summer of 2005-6. In the Selwyn-Waimakariri zone many groundwater levels are at very low to historic lows. Similarly, in the Rakaia-Selwyn zone, nearly all groundwater levels are below average levels with most of these at very low to historic low levels. In the Ashburton-Rakaia zone all groundwater measurements are below average levels and most are at very low to historic low levels.

South Canterbury In the Mayfield-Hinds groundwater zone, 73 percent of groundwater levels are below average levels. Shallow groundwater levels between the Rangitata River and Coopers Creek are at low levels with some wells at record lows for this time of year. In the deep aquifer at Orton, groundwater levels are at an all time low. Shallow wells near the Orari River are at below average levels with some wells at or near record lows for this time of year. Shallow wells closer to the Waihi and Te Moana Rivers are at average levels for this time of year. In the Upper Waitaki groundwater levels overall are decreasing, with approximately a quarter of all wells monitored exhibiting their lowest levels since monitoring began a year ago. In the Pareora-Waihao zones, shallow groundwater associated with the Pareora and Otaio Rivers continues to drop to low or very low levels, associated with a lack of river recharge. The Waihao River area fares 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.


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