Groundwater levels improve slightly thanks to rain
May 22, 2006
Groundwater levels improve slightly thanks to rainfall and no irrigation
Recent rainfall and enhanced river flows over most of Canterbury has raised groundwater levels in shallow and river-fed systems, say Environment Canterbury scientists. However in deeper aquifers groundwater levels remain low. Some groundwater aquifers are still at record lows, despite some limited recovery due to irrigators being turned off, says Philippa Aitchison-Earl, ECan hydrogeologist.
Throughout the Canterbury plains, between the Waimakariri and Opihi rivers, groundwater in many deep aquifer wells remains at very low levels, says Ms Aitchison-Earl. “Wells monitored this month are showing groundwater levels that have recovered slightly since April, but are still lower than ever recorded before for May and up to five metres lower than in May 2005. In some inland regions such as Darfield-Te Pirita-Hororata, no recovery has been measured and groundwater levels are still declining to record all time lows.”
Flows in spring-fed streams in the Central Canterbury Ellesmere catchment have recovered. However the Selwyn River and Harts Creek remain below their minimum flow levels, and the Irwell River remains dry and last flowed in February 2005.
Groundwater levels in the Christchurch - West Melton area have generally been rising over the last month. But they are still lower than at this time last year. Groundwater levels in all five of the West Melton Groundwater Zone monitoring bores continue to be below the levels at which pumping restrictions are required.
Wells recharged by border dyke irrigation schemes, such as in the Valetta and Ashburton-Lyndhurst scheme area, continue to rise. Away from these schemes, water levels in shallow wells such as around Winslow-Willowby area show signs of recovery, but remain at low levels. With irrigation stopping for the season, flows in the spring-fed drains between the Ashburton and Hinds Rivers remains low, but is improving. Flows in the Ashburton and Hinds rivers have improved and water levels are generally rising. However, shallow wells close to both of these rivers remain seasonally low.
Shallow groundwater levels remain average for wells within the Waimakariri-Irrigation scheme, but there are low levels in the area up-gradient of the scheme (upper Eyre River) and in deeper aquifers.
South Canterbury groundwater levels from the Opihi River south to the Waitaki River, and including those around Fairlie, are average to high, thanks to increased foothills river flow having recharged the predominantly shallow aquifers.
North of the Waimakariri
North of the Waimakariri River, including Waipara and Kaikoura, groundwater levels are generally average to low, but there is a number of wells within these areas that remain at record low levels.
North to south of Canterbury region groundwater breakdown:
Kaikoura: Groundwater levels are generally average, but lower than for the same period last year. Most recorded levels have continued to rise since the end of February.
Waipara: Most water levels range from below average to low for this time of year. While the levels in some wells near Waipara township have continued to decline, levels in most other areas have stabilised or are now beginning to recover. In some wells between Waipara and Amberley Racecourse, water levels are at the lowest level for the five to seven year period of record. Most water levels are lower than the levels recorded for the same period last year.
Ashley - Waimakariri Plains: Shallow groundwater levels are mostly average for this time of year except for some low readings in shallow wells in the vicinity of the upper Eyre River (upstream of the Waimakariri Irrigation Scheme) where the effects of continued low rainfall in the foothills are being felt. Water levels in the deeper aquifers are very low for this time of year.
Christchurch – West Melton: Groundwater levels in the Christchurch - West Melton area have generally been rising over the last month - particularly within the confined aquifer area under much of the city. Nevertheless, the majority of wells have groundwater levels that are lower than at this time last year. Groundwater levels in all five of the West Melton Groundwater Zone monitoring bores continue to be below the levels at which pumping restrictions are required.
Waimakariri – Rakaia Plains:
Most, if not all, irrigation has now ceased for the farming year. As a result, groundwater levels in many shallow and intermediate depth wells are showing a slight rebound as a result of this and the effects of increased seasonal land surface recharge. Despite this recovery, 27 out of 64 (42%) monitoring wells are setting low level records for this time of the year. Whilst water levels are recovering, it is significant that many are consistently deeper than this time last year.
Groundwater levels are average in 17 wells that are close to sources of recharge such as the Rakaia and Waimakariri rivers. The level in one well at Southbridge also declined because of the drop-off in irrigation within the adjacent Northbank scheme, which had been recharging that well.
Despite the rainfall recharge in the last month, groundwater levels in deep aquifer wells in the Darfield-Hororata-Te Pirita area are still declining, said Ms Aitchison-Earl.
Rainfall in April was generally average, with inland areas receiving a little more than average; coastal areas, a little less. Rainfall for the year starting July 2005 to date is 100 mm below average for Lincoln and Winchmore, and about 150 mm below average at Darfield.
Flows in spring-fed streams such as Harts Creek (810 litres/second), Boggy Creek (25 litres/second), and the Selwyn River (400 litres/second) are recovering. However, flows for the Selwyn River and Harts Creek are still below their respective minimum flows. The Irwell River is not showing a measurable flow.
Ashburton - Rakaia Plains: The groundwater levels in most shallow wells have shown slight recovery, although are at the lowest range for this time of year.
Most deeper aquifers continue to experience water level declines. Record lows continue to be set in these deep wells.
Ashburton-Hinds Plains: Groundwater levels around both the Ashburton and Hinds rivers remain below average for this time of year. While some wells show a recovery in water levels, others show no recovery or a continuing, slight, decline.
Water levels remain seasonally low with some wells still at record low levels. South of the Ashburton River water levels are generally recovering slowly, with no monitoring bores showing a significant decline.
Mayfield – Hinds Plains: Groundwater levels continue to recover, with most monitoring wells at average levels for this time of year.
Rangitata-Opihi: The majority of wells are now recovering, however water levels across this groundwater area remain slightly below average for this time of year.
Levels Plain: Groundwater levels in the lower Levels Plain are currently at average levels.
Opihi River: Shallow wells recharged by the Opihi River in the Fairlie-Pleasant Point areas have risen in response to increased river flows in May.
Pareora – Waitaki: Shallow groundwater levels, recharged by the foothills rivers are average to high in response to increased river flows this month.