Minimal snow effect on groundwater levels

Thursday, 29 September 2005

Groundwater levels throughout much of the Canterbury Plains continue to be at very low, and in many cases, record low, levels. The recent snowfall on September 19 has had almost no impact on groundwater recharge, says Environment Canterbury groundwater scientist Russel Sanders.

“The pre-existing, low soil moisture levels mean that the snow melt was largely absorbed by the soils with little left over for shallow groundwater. However, the cooling and dampening of the soils associated with the snow is probably partly responsible for a delay in more widespread irrigation turn-on. This in turn has delayed the more rapid decline in groundwater levels that occur in many areas at the start of the irrigation season.

“Levels are generally between one and three metres lower than they were at the same time last year, and very few are rising at all,” he says. “Most aquifers have levelled or are now declining. Those that were declining before are generally continuing to decline.”

The drying up of some shallow domestic and irrigation wells near Ashburton continues.

Direct groundwater recharge data are collected by ECan scientists using buried instruments called lysimeters. The lysimeters measure how much rainfall moves through the soil to groundwater aquifers.

Results from data collected in the past few days confirm that the winter rainfall recharge has been extremely low, ranging between less than 1% of the winter average (at Lincoln) to a maximum of almost 28% of average (in Christchurch). These compare with 14% of average at Winchmore near Ashburton and 27% at Hororata.

“These very low winter recharge figures, in association with low flows in many of our foothills rivers, are the dominant factors responsible for the current very low groundwater levels,” said Russel Sanders. “In some areas this is compounded by the fact that groundwater levels at the end of last summer were very low due to increased irrigation development in recent years.”

There has also been a trend towards more efficient border dyke or spray irrigation in some surface water irrigation schemes which has lessened irrigation recharge in those areas, he says.

Canterbury’s alpine rivers are low with the Rangitata on part restriction, says ECan water resources scientist, Graeme Horrell. Foothill streams are also at very low levels with the North Branch of the Ashburton River and the Ashburton River at SH1 both on full restriction. The outlook is bleak for the lowland streams with the Selwyn River at Coe’s Ford already on full restriction having missed out on any recharge over winter.

North to south of Canterbury region breakdown:
Kaikoura: Groundwater levels are about average for this time of year, although levels are now declining in some wells.

Waipara: Water levels are average to low with most wells now levelled off or started to decline at levels generally comparable with the same time last year.

Ashley - Waimakariri Plains: Levels are average to low except for some very low readings in shallow wells in the vicinity of the Eyre River and near the Ashley River in the vicinity of Rangiora. A number of wells between Oxford and Horrellville have risen due to recharge from Waimakariri River water to the dry Eyre River bed via the Waimakariri Irrigation Scheme. This has been a three-week trial, now ended, to test the potential benefits of artificial recharge.

Christchurch – West Melton: Groundwater levels in the Christchurch - West Melton zone are almost everywhere lower than average; particularly to the west of the city where levels are the lowest on record for the time of year. Three of the five West Melton - Yaldhurst trigger wells continue to be below the first trigger level and irrigation restrictions are in place for the relevant sub-zones. Groundwater levels in the deeper confined aquifers are generally closer to average levels.

Waimakariri – Rakaia Plains: Groundwater levels in the inland Central plains are all at record lows. For deep aquifers, levels leading into the irrigation season are consistently two metres lower than last season, reflecting a combination of lack of winter rainfall recharge and cumulative impacts of increased pumping. Declining levels are already occurring in response to intermittent irrigation beginning in September. It is expected that levels will continue to decline to record lows throughout the irrigation season.

Shallow aquifers associated with the Selwyn River are also at record lows, with the Selwyn not having any significant rainfall recharge since January this year. The shallow aquifers in the vicinity of Lake Ellesmere/Te Waihora are at average to low levels and most wells are now starting to decline.

Ashburton - Rakaia Plains: Many shallow wells are at all time record lows with a number being reported as going dry, while ECan continues to receive applications to deepen such wells. Many are in the vicinity of the North Ashburton River which continues to be dry since before the winter. Some of these wells have shown a slight response to a fresh in the river early in September, but this flow was too short-lived to have any significant impact, ECan groundwater scientists say.

Deeper aquifers are at very low or record low levels for this time of year. Levels in most areas have stopped rising or are starting to decline. Levels are one to two metres lower than this time last year, even before the onset of widespread irrigation pumping, which usually results in a more rapid decline in water levels.

Ashburton-Hinds Plains: Levels in first aquifer wells are very low and some are now at a record lows for September. Some are a metre or more lower than at the same time last year. Levels in some shallow wells are likely to continue dropping until February when they may rise due to border dyke recharge. However, the trend towards more efficient border dyke or spray irrigation in some surface water irrigation schemes has lessened groundwater recharge in those areas.

Levels in deeper wells are also very low, most at record lows for the time of year. At least one has even been reported as being more than three metres lower than in September 2004. Levels have generally been dropping since early winter.

Groundwater irrigation in the area has only been intermittent so far this season. If border dyke irrigation is less than usual this year, due to restrictions from low flows in the Rangitata River, further problems could be experienced with shallow wells in the area - and perhaps also deeper wells.

Mayfield – Hinds Plains: Shallow wells are exhibiting all-time record low levels but are beginning to rise now that the border dyke irrigation season has started. Levels are low due to the very low winter rainfall. As in other places, the trend towards more efficient border dyke or spray irrigation within the Mayfield – Hinds Irrigation Scheme means there is less water available to recharge groundwater.

Levels in deeper wells are declining, but at a much slower rate than the shallow ones.

Rangitata-Opihi: Very low to record low water levels for September are being recorded in the Upper Orari, partly as a consequence of low flows in the Orari River. Normal levels are recorded in the coastal end of the system. Deep groundwater has recovered to normal levels for the time of year.

Levels Plain: Levels in the lower Levels Plain are very low, but are expected to rise when the Levels Plains Irrigation Scheme recommences.

Opihi River: Wells recharged by the Opihi River; in the Fairlie-Pleasant Point areas are low, presumably as there have been no significant high flow events since April or significant recent rainfall.

For further information on irrigation restrictions: http://www.ecan.govt.nz/Our+Environment/Water/Rivers/Irrigation+Restrictions/