In an extremely challenging season for irrigation, Abby Kellett finds out about the mechanics of water capture and supply in one of New Zealand’s most productive regions.
Irrigation is now at the heart of many farming operations in New Zealand following a push to double the country’s food exports by 2025. In South Island’s Canterbury plains, it is difficult to find a view that does not feature irrigation of some sort, whether it be a hard hose gun or a 1.6km long centre pivot.
In many cases, investments in irrigation have allowed livestock farmers to boost stock numbers by up to 800 per cent and have enabled arable farmers to extend their cropping options and boost productivity.
This season, extreme weather has meant careful water management has been crucial in ensuring crops received a regular supply of water. New Zealand has officially experienced its hottest summer on record, with average summer temperatures of 18.8degC. Conversely in late-February, cyclone Gita hit the country, dumping more than 300mm (11.8 inches) of rain in some regions.
The changeable weather has proved a challenge for irrigation companies around the country, including New Zealand’s biggest manmade irrigation site, Rangitata South Irrigation Scheme (RSIS), owned by Rooney Earthmoving. It is responsible for capturing water from the Rangitata River, storing it and distributing it to on-farm ponds as required.
Irrigation scheme manager, Rowan Clarke says: “This summer, there was only a 36-hour period when there was water available to take from the river, which only provided us with four days-worth of water supply in two-and-a-half months – so this year has been terrible.”
Following cyclone Gita, storage pond levels at the site increased by 11 per cent in only 48 hours, however for more efficient water capture, Mr Clarke says it is better to have less intense rainfall, but more frequent.
He says: “The main inlet of the scheme only allows me to take in 20cu.m of water per second, so even when the rain did come, I could only capture a proportion of it.
“If we have loads of rain and the rivers are running full, I can’t get enough out of the river. If the river just ran at 140cu.m per second and stayed at that rate for a week it would be fantastic, but instead the river runs at 500cu.m/second and stays there for only two days.”
Recognising the demand for a regulated water supply in the area, Rooney Earthmoving set about building the irrigation scheme eight years ago, with the site finally completed in 2014.
The site spans over 400ha (988 acres) and consists of seven storage ponds. The storage ponds supply 42 on-farm ponds which irrigate about 14,000ha (34,594 acres) of farmland.
Each pond on the irrigation scheme has the capacity to hold about 2.3m cu.m of water, while on-farm ponds vary widely in size from 20,000cu.m to about 400,000cu.m.
The movement of water both into the scheme, between ponds and out to farmers is all controlled via a phone or laptop using telemetered automation or if required, via manual operation from a control hut.
To ensure the scheme consistently has capacity to take in more water if required, while also making sure the ponds do not run dry, Mr Clarke has to analyse data on river capacities, river speed and weather forecasts to help inform his decisions.
“The trick of the game is to give farmers as much water as you can, without running out of water. But I do not want my ponds at full capacity either because if we then get a flood, I have missed out on an opportunity to capture more water, which would make me a bad water farmer. It is a real juggling act.”
Despite being able to store such a large volume of water at the scheme, Mr Clarke says in a drought year, high demand for water means ponds can empty very quickly.
“At full capacity, all seven ponds combined hold about 16.5m cu.m of water, but at maximum output when water is leaving the site at 7,000 litres per second, the ponds could be emptied in only 30 days.
“Farmers are required to have 10 days worth of water storage, so between us we should be able to go 40 days without rain and still be able to irrigate. However, this year, we went over 50 days without rain and so only the farmers who were irrigating at lower rates were able to irrigate throughout the dry spell.”
For rainfall to occur in Canterbury, the region relies on hot, moist air being blown in from Australia. But the wind which blows the rain to the east of the county, is also what dries land out, explains Mr Clarke.
“We need the wind to bring moisture in from the west coast but that same wind is also capable of sucking about 5mm of rain out of the ground per day. Typically, irrigators are applying about 5mm of water per day and so there is a danger that the wind cancels out the irrigation.
“To avoid this, I encourage my clients to only apply 10mm of water during a 12-hour period at nights when losses are less as oppose to 5mm during a 24-hour period.
“Ultimately, it is about responsible water management by both myself and farmers – if either of us are irresponsible with that limited resource, then we both run into problems,” says Mr Clarke.
As part of the scheme, the company has constructed a 500m long purpose-built salmon spawning race which runs alongside its storage ponds, with the aim of increasing fish populations.
The race, which is the first of its kind in New Zealand, enables salmon to spawn and rear their young before returning to the Rangitata River Fishery.
The gravels used in the race provide the perfect safe habitat for salmon to bury their eggs. When they hatch the salmon return as adults to continue the breeding cycle, says Mr Clarke.
A salmon hatchery has also been set up to supply the race with egg-filled fish.
To prevent fish from entering the scheme, the company use behavioural screens. Mr Clarke says: “Studies have shown that fish are less likely to swim in the dark crevices near the edge of the river and so we have manipulated the inlet so that very few fish enter the scheme.
“Unlike mechanical mesh fish screens which trap fish in grills, this method does not harm the fish and had a 95 per cent success rate.”