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Anzac rain a help but not enough

April 26, 2006

Anzac rain a help but not enough

The recent rain over the Anzac holiday reduced the need for pumped irrigation water, so some Canterbury region groundwater sites are showing a recovery in levels, Environment Canterbury scientists say.

“When it rains and farmers stop pumping groundwater, there is an immediate, corresponding recovery of groundwater levels. But this latest rainfall would not yet have recharged deeper aquifers,” says Philippa Aitchison-Earl, Environment Canterbury hydrogeologist. “These deep aquifers need an average to wet winter to get back to normal levels.”

Leading up to the past few days’ steady rain, rainfall over most of Canterbury was much lower than normal, and soil moisture deficits remain high. As a consequence, irrigation demand has been high for most of April. Record low groundwater levels continue to be set in many deep and shallow aquifer wells, and there is little sign of groundwater recovery in the inland plains. These very low regional groundwater levels are a symptom of the dry winter experienced last year, compounded by strong irrigation demand this summer.

New record all time lows continue to be set in some deep groundwater aquifers throughout the Canterbury plains between the Waimakariri and Opihi rivers. Many groundwater monitoring wells in the Canterbury Plains were reporting groundwater levels that are either still declining below March levels, or were recovering only slightly. These most affected areas have tended to fall within “red zones” which ECan’s groundwater scientists say are fully allocated.

There is no sign of longer-term groundwater level recovery within the inland and mid Central Plains. However, there is some localised recovery of groundwater (such as in the Selwyn River, Lincoln, and Bankside areas), probably due to a localised decrease in irrigation demand. Around 60% of ECan's monitoring wells exhibit declining groundwater levels, 20% exhibit slight recovery of groundwater levels, and the remaining 20% show levels approximately the same as last month. Seven of the 55 monitoring wells in the Central Plains area have been dry this last summer.

Groundwater levels in many parts of the Christchurch - West Melton area continue to be lower than this time last year, and all five of the West Melton groundwater zone monitoring bores have groundwater levels that are currently below those at which pumping restrictions are required.

Wells recharged by border dyke irrigation schemes associated with the Rangitata Diversion Race (RDR) continue to rise, such as in the Valetta and Ashburton-Lyndhurst scheme areas. Away from the schemes, water levels in shallow wells such as around Winslow-Willowby area continue to fall, or are dry. The spring-fed drains between the Ashburton and Hinds Rivers remain dry or near dry with continuing problems for irrigators and farmers who rely on the drains for stockwater. Shallow wells adjacent to the North Ashburton River have continued to decline, but remain around a metre higher than for winter 2005.

South Canterbury groundwater levels from the Opihi River south to the Waitaki River, and including those around Fairlie, are low to average for this time of year. This is due in part to continued flows (though very low in some) in the rivers contributing to these generally shallow aquifers and flow from dammed river water in the case of the Opihi. The Levels Plains Irrigation Scheme has boosted water levels in that area although they are still lower than in April last year.

North of the Waimakariri River, including Waipara and Kaikoura, groundwater levels are generally average to low, but there are a number of wells within these areas that are at record low levels.

North to south of Canterbury region groundwater breakdown:
Kaikoura: Groundwater levels at the end of March (when they were last measured in this area) varied from below average to average and most have started to rise since the end of February.

Waipara: Most water levels range from below average to low and the levels in many wells from Waipara southwards to Broomfield are still declining . In some wells between Waipara and Amberley Racecourse, water levels are at the lowest level for the five to seven year period of record. The water levels in the Glasnevin – Amberley Racecourse area are similar to the water levels recorded for the same period last year and generally lower elsewhere.

Ashley - Waimakariri Plains: Levels are now mainly average for this time of year except for some very low readings in shallow wells in the vicinity of the upper Eyre River (upstream of the Waimakariri Irrigation Scheme) where the effects of very low rainfall in the foothills are being felt. Water levels in the lower Eyre River are also declining, but are still in the normal range for this time of year.

Christchurch – West Melton: Despite the reduced water use that could be expected as conditions become cooler, groundwater levels in the Christchurch-West Melton area continue to generally be lower than they were at this time last year. In many of the deeper bores in the confined (eastern) area of the city, groundwater levels have been trending upwards in response to reduced pumping rates. To the west of the city, however, groundwater levels have remained low and all five of the West Melton groundwater zone monitoring bores display groundwater levels currently below the levels at which pumping restrictions are required.

Waimakariri – Rakaia Plains: March rainfall over the Central Plains was average to below average, and soil moisture levels were still in deficit by up to 110 mm. One consequence of this dry soil is that many groundwater monitoring wells were reporting groundwater levels that are either still declining below March levels, or were recovering only slightly. Many wells, especially deep ones, have been setting new low water level records, with April levels well down on those of April 2005.

60% of ECan's 55 monitoring wells in the Central Plains still exhibit declining groundwater levels, 20% exhibit slight recovery of groundwater levels, and the remaining 20% show levels approximately the same as last month.

As expected in the deep wells within the inland and mid Central Plains, there is no sign of groundwater level recovery. However, even in some moderately deep wells around the Selwyn River, Lincoln, and Bankside, some groundwater levels have recovered somewhat, which may be explained by a localised decrease in irrigation demand. Seven of the monitoring wells in the Central Plains area have been dry this last summer, or still are dry. These very low regional groundwater levels are a symptom of the dry winter experienced last year, compounded by strong irrigation demand this summer.

Ashburton - Rakaia Plains: The groundwater decline in some shallow wells in the Racecourse Rd area has continued with the North Ashburton River still dry near Ashburton. However, these levels are still about a metre higher than at the end of winter last year. Further upstream, groundwater in shallow wells near flowing reaches of the river or influenced by the Ashburton Lyndhurst Irrigation Scheme, or those affected by the Rakaia River, are at average to low levels for this time of year.
Most deeper aquifers continue to experience water level declines due to continued irrigation demand in response to very low rainfall. Record all time lows continue to be set in many wells.

Ashburton-Hinds Plains: Many of the shallow wells in the Winslow – Willowby area that have been monitored in the past are still dry. Levels in some others are continuing to drop, but at a lesser rate than several months ago. At this time last year levels in these wells had begun to rise in response to recharge from the Valetta irrigation scheme. Further inland, close to the Valetta irrigation scheme, levels in shallow wells continue to rise due to border dyke recharge.

The spring-fed drains between the Ashburton and Hinds Rivers remain dry or near dry with continuing problems for irrigators and farmers who rely on the drains for stockwater. Shallow wells close to the Hinds River have low levels for the time of year. Levels in deeper aquifer wells not far inland from State Highway One are starting to recover, despite continuing irrigation, due to border-dyke recharge. However levels are up to 9 m lower than at the same time last year. Further inland a slight recovery in levels is also taking place, but again the levels are significantly lower than last year. Nearer the coast levels in some wells are starting to recover, but some are still declining.

Mayfield – Hinds Plains: Levels in shallow wells within the area of the Mayfield Hinds scheme are generally normal for the time of the year. As with last month, levels in some wells close to the coast are still lower than normal, but are rising.

Levels in some deeper wells within the irrigation scheme area have continued to rise since the start of the irrigation season; some in areas of higher abstraction have dropped. For deeper wells outside the scheme area, levels have dropped.

Border dyke irrigation is still proceeding, and there are currently no restrictions on takes from the Rangitata River for the RDR, so border dyke recharge will be continuing.

Rangitata-Opihi: As has been the case in recent months, levels in shallow wells away from rivers are low – some at record lows for the time of year. There has been a slight rise in some but others continue to drop. Levels in shallow wells closer to rivers are generally within the normal range or a little lower than usual.
The level in the one deep well (K38/0013) in this area with a significant history of monitoring (since 1982) had started to recover early in April, but was still about nine metres lower than at the same time in 2005.

Levels Plain: Groundwater levels in the lower Levels Plain are low to average, and have risen in response to the Levels Plains Irrigation Scheme, but are lower than in April last year.

Opihi River: Wells recharged by the Opihi River in the Fairlie-Pleasant Point areas are generally at average levels, although there have not been significant high river flows since mid-February.

Pareora – Waitaki: Shallow groundwater levels, recharged by the foothills rivers are average to low in response to declining river flows since mid-February.

ENDS

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