Canterbury groundwater levels fell in February as a result of settled weather combined with average-to-dry conditions, according to monitoring by Environment Canterbury scientists.
“Most areas in Canterbury had a good start to the irrigation season as a result of an average groundwater recharge over the winter,” said Dr Howard Williams, Environment Canterbury groundwater scientist.
“Since then, however, groundwater levels have declined to below-average levels as a result of average-to-dry summer conditions combined with moderate irrigation demand.”
Rainfall in February has been average to below-average, contributing to average to below-average river and stream flows in foothills rivers and lowland streams, typical of the continued but waning El Niño weather pattern. Flows in some South Canterbury rivers were well below average.
Some areas are also experiencing low rainfalls for this irrigation year, approaching 1 in 10 year levels. Over a dozen wells have a new lowest groundwater level for this time of year.
According to the NIWA Seasonal Climate Outlook comments for coastal Canterbury, however, both rainfall and stream flows are forecast to be in the normal range for the February to April period .
“Soil moisture levels are also at close to average levels and forecast to remain in the normal range for the rest of the season,” said Dr Williams.
Temperatures should remain at near average levels for the February to April period, also according to the NIWA Seasonal Climate Outlook.
In the alpine areas stream flows were below average in February but are forecast to be above average for the coming two months as a result of higher than average rainfall on the back of the waning El Niño pattern. Soil moisture should also be above average in the alpine areas.
Irrigation restrictions
In mid-February most catchments within Canterbury had restrictions on at least some tributaries.
Two areas within the West Melton special groundwater zone near Christchurch are under restriction.
Eleven comments have been received about low groundwater levels this irrigation season, including landowners in Ladbrooks, Hawarden and Lowcliffe.
Regional detail
North Canterbury
Kaikoura groundwater levels generally remain in the average to above average range, reflecting the above average rainfalls that have fallen there since Christmas.
In Waipara, groundwater is exhibiting a wide range of levels, from high to very low, with most water levels in the average range. Some wells are displaying groundwater levels lower than previous low levels for this time of year.
Wells in the Ashley–Waimakariri zone display generally above average groundwater levels except for deep wells south of Oxford that are up-gradient of the Waimakariri Irrigation Scheme. The scheme exerts a strong positive influence on groundwater levels within and down-gradient of its command area.
Mid-Canterbury
Groundwater levels within the Christchurch–West Melton area are lower than average for this time of year. Two of the Christchurch-West Melton monitoring wells indicate groundwater levels below the first trigger level, so there are two areas under restriction. Deep wells near the coast and within the Christchurch area are displaying higher than average water levels, perhaps because of the flood events in the Waimakariri River in December and January of this summer.
In the Central Plains, both the Selwyn-Waimakariri and the Rakaia-Selwyn groundwater zones are displaying lower than average groundwater levels with some long-term monitoring wells going dry and some deep wells displaying levels at new lows for this time of year. Groundwater levels near the Rakaia River and within associated surface water-sourced irrigation schemes are holding up well. Lowland streams such as the Selwyn River, Irwell River, Kaituna River, Harts Creek and Boggy Creek are flowing at or below their minimum flows.
In the Ashburton-Rakaia zone groundwater is generally at below average levels and some Chertsey wells, such as at Acton on the south bank of the Rakaia River, continue to exhibit very low levels for this time of year.
South Canterbury
In the Mayfield-Hinds groundwater zone, many groundwater readings are below average to very low levels, especially between Hinds and the Rangitata River. Groundwater levels between the Ashburton River and Rangitata River are generally below average, with some wells displaying levels at historical lows.
South of the Rangitata River, in the vicinity of the Orari River, on the Levels Plain, and around Winchester, Temuka, Geraldine, and in coastal areas beside the Rangitata River, many wells display groundwater levels that are very low or at their lowest for the time of year.
In the Pareora-Otaio, Makikihi, Waimate, and Fairlie areas, the shallow aquifer is displaying very low groundwater levels in part because rainfall has been far below average in this portion of Canterbury. In the Waihao area, shallow groundwater is at average levels. Deeper groundwater remains at low levels as a result of summer irrigation demand.
Forecast until April 2010
According to the NIWA seasonal outlook website at: http://www.niwa.co.nz/our-science/climate/publications/all/cu/new-zealand-climate-update-128-a-february-2010/outlook---february-to-april-2010, temperatures are likely to be near or below average, seasonal rainfall is expected to be in the normal range and soil moisture levels and stream flows are most likely to be normal.
Monitoring data
The most recent monitoring data are available at: http://ecan.govt.nz/services/online-services/monitoring/groundwater-levels/Pages/Default.aspx. Note that at this webpage, any well number with recent observations can be brought up as an envelope and time series plot. If you have difficulty, call Howard Williams at the number below.
Irrigation restrictions
For an updated list of irrigation restrictions go to http://ecan.govt.nz/services/online-services/monitoring/irrigation-restrictions/pages/default.aspx.
For more information:
Ken Taylor: ECan Director Investigations and Monitoring, 027 2954 359 or
Adam Martin: ECan surface water scientist, 03 372 7350 or
Dr Howard Williams: ECan groundwater scientist, office: 03 371 7175
a/h: 027 434 2617.