NZFarmer : January 13th 2009
12 Straight Furrow • January 13, 2009 Water Resources Storage ponds are appearing in large numbers as farmers move to secure reliable water sources IG water storage ponds are appearing in large numbers on South Island farms as farmers seek more secure and reliable sources of irrigation water. Development has been swift, in fact, most of it has happened over the past two years. Now hundreds of artificial lakes dot the east of the South Island from North Otago to North Canterbury, but concentrated in Mid Canterbury. They average about 6ha in area. by Howard Keene Their development has been dri- ven by a big expansion of dairying at a time when it has become increas- ingly difficulty to obtain surface and ground water consents as resources are deemed fully allocated. Farmer and natural resources engi- neer Keri Johnston has had a big role to play in this movement as a con- sulting engineer. She reckons she has worked on 60 to 70 new dam pro- jects in the past two years, and is a little relieved that her workload is now beginning to slow as economic conditions tighten. “There has been a huge shift to ponds and away from bordedykes,” she said. Included in that work has been a 17ha lake on the 700ha property that she and husband Craig manage as a run-off block for corporate farmer Dairy Holdings Ltd. The potential for reliable irrigation water has been apparent since they moved on to the property, near Stavely on the upper plains, four years ago. “I’ve been wanting to do it on here for a long time,” she said. The dam and associated irrigation infrastructure are just about com- plete, in time for a big influx of young dairy stock in the next few months. Dairy Holdings has 58 farms in the South Island this year, milking over 45,000 cows. The property the Johnstons manage specifically raises young stock for company’s milking operations. At the peak time from January to May they will carry 8000 to 10,000 calves and heifers on the farm. It is also the driest time. “Hence the need for reliable storage,” she said. Weaner calves are carried through until they are in-calf heifers, and so are on the farm for just over two years. “We are the young-stock block for the group.” They have consent to take water for irrigation from Taylors Stream, but that is a smallish foothills stream, and restrictions on water take are usually imposed during summer. Their previous irrigation was a borderdyke system. Last summer was warm and dry, and the farm had only 42% reliability of water supply, Keri said. Restrictions started on Boxing Day and didn’t come off again. “We need- ed to buffer the times when restric- tions were on. We’ve got pretty poor reliability of supply and we need a lot of storage.” This year with the new storage pond they hope to have close to 100% reliability, and to make better use of the available water they have, the old borderdyke system is gone and they have installed three centre pivot irrigators. Because their water supply was unreliable, their irrigation pond is one of the biggest around, storing 300,000cu m of water. Most ponds created so far average around 100,000cu m, but they usually have a more reliable water supply. The 17ha pond was supposed to have been created in two months, but the wet winter meant it took much longer, and it is only just being completed now. Making use of the slope of the Canterbury Plains (the western end of the property is 21m higher than the eastern end), the bund holding in the 5m deep lake is 5m high at the eastern end, decreasing to virtually nothing at the western end where water from Taylors Stream flows in under gravity. The pond is divided into two by a wall, allowing the damming bund to be 5m high instead of 8m if it was just a single pond. It also reduces the water pressure on the holding embankment. After the top soil was excavated, material was laid down on the base of the pond basin in 150-200mm lay- www.straightfurrow.co.nz Dam! They’re popping up all over the place B Engineer Keri Johnston on the embankment dividing a new water-storage pond she has designed. It is on the property she and her husband, Craig, manage near Staveley, in Mid Canterbury. ers and compacted to make it as impermeable as possible. The underlying material is gravel, sand and silt, and there will be water losses through the base of about 10-20% at first. However, that loss was less than from evapotranspiration losses, and Keri said the pond bottom should seal itself more over time. There’s no way these small lakes will fill naturally, and so farmers constructing them need to be on an irrigation scheme or, like the Johnstons, need to have resource consent to take from a natural supply. Keri said that 90% of the ponds she had been involved with were on farms on existing irrigation schemes. The logical way to go for others in future might be to seek con- sent to take water from rivers and streams for ponds only dur- ing high flow periods. That is already starting to happen. She said resource consent for ponds is not too difficult to obtain. In the Ashburton District, a consent is needed to dam, and rarely a building consent is need- ed because of the height of the dam. “We’ve been working pretty closely with Environment Canterbury on a set of conditions so the onus comes back on the contractor or engineer. That ensures good building.” A compacted embankment holds the pond water in. ONE FARM ALL YEAR ROUND ACCESS BRIDGES AND STOCK UNDERPASSES She said the chances of these low dams failing were extremely small and the consequences would not be catastrophic. The costs of installing a pond were comparable to the costs for putting down a 200m well, she said. The costs of constructing a pond equates to about $2 per cu m of water stored. “Long term, it’s cost effective. EMMETTS CIVIL CONSTRUCTION LTD Contact the people who have developed practical solutions to every possible access problem. They’re the North Island specialists in farm access bridges and stock underpasses. 400 Heads Road, Wanganui Phone 06-349-1788 Fax 06-344-2406 A/hrs 06-345-0082 or 06-345-6100 Est 1948 NZ1094056 From an operational perspective it’s incredibly cost effective for us. Last January, we had nearly 8000 stock on, and no water and no grass. It was like a desert and we had to feed out. “It’s more sustainable for a farm like this, and more sustain- able for Taylors Stream too as we will take less of our allocation.” So what are the environmental downsides? “I don’t know of any. The biggest downside to the owner is the loss of land.” She said there was some con- cern from Fish and Game that more ponds would attract more pest bird species such as Canada geese. Left: A new centre pivot irrigator waiting to be connected up. Above:The intake channel for the new water storage pond.
January 20th 2009