Rangitata Diversion Race

A look at New Zealands largest irrigation scheme - 67km long irrigation scheme extending from an intake on the Rangitata River at Klondyke to a discharge at Highbank on the Rakaia River. It can carry a flow of approximately 30 cubic metres of water per second. A second intake on the South Ashburton River can also divert approx 7 cubic metres per second of water (when available) into the RDR.

The Rangitata Diversion Race was a dream in the minds of farmers in Mid Canterbury when the 750,000 acre ( 303,514ha ) plain was first farmed in the mid-19th Century. Regarded as the largest area of nearly flat land in New Zealand, all development was closely linked to water problems water races to supply stock on light soils and water courses to drain heavy swamp lands.

Work began on the scheme on April 2nd 1937 and was completed in November 1944. Race water first generated electric power at Highbank power station on June 8th 1945. Expected to cost 1.5 million pounds, the scheme costs grew to 2 million pounds.

Since 1945 there have been a number of amendments to refine the efficiency of the RDR. Montalto, the second hydroelectric power station was built in 1981 and started producing electricity a year later. The Sandtrap was designed and built at a similar time to remove much of the suspended sediment from the water by reducing the velocity of the flow. The sediment causes wear and tear on the hydro electric turbines and can cause gastric problems for sheep and lambs when washed out onto paddocks. Conversely the sediment is a good bonding agent that can help to plug microscopic holes in the race lining.

Consents:The RDR Resource Consents were renewed by Environment Canterbury in 2008. Part of the consent conditions required the RDR to install fish diversions to reduce the number of native and exotic fish from becoming entrained in the Race. 2Kms downstream from the Rangitata River intake a behaviourial screen called a Bioacoutic Fish Fence that uses bubbles and underwater sounds to divert fish to a channel returning them to the river, was installed in 2007. At the South Ashburton River, a physical rock gallery was installed in 2008 together with a bypass to return fish to the river.

There are three Community Irrigation Schemes, two hydroelectric power stations, Ashburton District Council stockwater race system and various private stockwater and irrigation schemes which are supplied by the race.

The irrigation schemes are:

- Mayfield Hinds - Contract area 32,000 hectares | Scheme allocation 16.5 cubic m/sec.

- Valetta - Contract area 7,300 hectares | Scheme allocation 4.4 cubic m/sec.

- Ashburton Lyndhurst - Contract area 25,000 hectares | Scheme allocation 13.0 cubic m/sec.

In all 66,000 ha are irrigated and water demand sits at about 33.95 cubic m/s

Irrigation has priority of supply during the irrigation season extending from September 10th to May 9th each year.

Ben Curry says the method of irrigation has changed over the years. AT the start it was wild flooding then farmers moved to border dyke guns and now centre pivots.

Ben Curry says most of the farmers on the scheme have some kind of storage as a buffer for a few days but he says large scale storage is a critical issue for the scheme. The obvious problem is cost . The issue was first looked at 15 years ago but fell apart due to cost and consents. At the moment the RDR is looking to buy a farm to create large scale storage and may have some information on this by the time we get to film.

During the rest of the year, Highbank power station located at the Rakaia end of the race has priority of supply.

The other power station, Montalto, is located 1.5kms downstream from the Mayfield-Hinds Irrigation Scheme. Major control structures built along the race include the Rangitata intake and sandtrap, checkgates, spillways and siphons passing under rivers and streams which cross the race.

Ben Curry says that water use has become an extremely hot topic in the region since it became clear there wasnt enough of the stuff to go round. Consents had been given willy nilly in the past. Farmers had built businesses around extracting water from the ground for example and now ECanz was saying that they had to stop taking it from the ground water and tap into an above ground scheme.

The CWMS is an attempt to put all the players in the same tent greenies, farmers, irrigators, generators and environmentalists.

Ben says that in the past the first thing people did when there was a water dispute was go to the lawyer.

John van Polanen is chairman of the Ashburton Lindhurst part of the scheme. He runs 640 cows on 185 ha. He is in his 22nd season and has a lower order sharemilker on the property.

He used to use border-dyke irrigation on the farm but found he was always short of water and needed more. These days he has mostly centre pivot with low droppers and finds he gets on what he needs.

On Johns part of the scheme there has recently been some work piping water to around 3800 ha of the Ash/Linds 25,000ha . John says it has worked really well.

It is gravity fed needs no pump. They are now talking about doing the rest of the scheme.

Farmers in the Ash Lindhurst scheme like others on the RDR get their water via a supply contract with RDR. Each farmer is entitled to a maximum of 2 shares per ha.

Farmers can trade or sell shares. He says as a scheme they regulate themselves and have some self auditing systems in play.

The water for the RDR is only used for irrigation. Water for the dairy comes from a bore.