A new irrigation scheme for central Canterbury – do we need one?
- Of all the land in New Zealand that is irrigated, over 70% is in Canterbury. However these farms rely heavily on water sourced from deep wells – an estimated 50% of farmers within the proposed Central Plains Water scheme area currently pump from underground aquifers, many of which are suffering from over-abstraction.
Other farmers that irrigate rely on what is termed ‘run of river’ water – they hold or apply for individual resource consents to take water directly from lowland streams and the Waimakariri and Rakaia rivers. The downside of this situation is their water supply is not reliable – their consents do not allow them to take water if river levels drop for extended periods, particularly in droughts during January and February.
- A comprehensive irrigation scheme for central Canterbury was first proposed in 1883. However in the last 120 years, other regional schemes successfully attracted central Government funding (including the Rangitata Diversion Race in South Canterbury built in the 1930s and 40s, the Lower Waitaki and Morven Glenavy schemes in South Canterbury, and the Amuri schemes in North Canterbury) but the majority of the prime agricultural land closest to Christchurch missed out.
- Across the area of the scheme, the central Canterbury plains have an even slope of around 1:150. This makes the region ideally suited to a large scale irrigation project – with the exception of some pumping to get water up from the supply races to the higher levels of plains, the system proposed by CPWL will mostly rely on gravity. The scheme’s very low energy requirements future-proofs it from likely increases in electricity costs.
- Many international experts have noted the extremely favourable natural conditions for irrigation on the central Canterbury plains – flat land, no drainage or potential salinity problems which bedevil many overseas schemes, potential to improve access to groundwater systems, and a highly developed and skilled agriculture sector.
- The 2005 ‘State of the Canterbury Region Water Resource’ report by Environment Canterbury includes the following conclusions:
- over most of the Canterbury Plains, groundwater levels particularly in deep aquifers are currently 1 to 3 metres lower than the same time last year
- record groundwater lows have been recorded in many areas
- lowland streams that depend highly on groundwater supply are very low
- it is expected that record groundwater lows will be recorded across the Central Plains as a consequence of lower rainfall over winter and the anticipated summer draw down for irrigation. This may lead to some well owners having to deepen pumping for underground water and, towards Te Waihora / Lake Ellesmere, many wells will lose artesian pressure as the season progresses.
ECan’s prediction (above) has proved to be correct and has been reported extensively by Canterbury media, eg. “…about two dozen shallow wells in central Canterbury have run dry, leaving people without water to drink or for showers” (The Press, 01/02/06), “I’ve never seen so many people ringing up saying their well has gone dry … I’ve had irrigators ringing up who have been on to drillers to deepen their wells” (ECan groundwater scientist Russel Sanders reported in The Press 31/01/06)