More efficiency possible in Mid-Canty irrigation

Press, 21 October 1983, Page 29

More efficiency possible in Mid-Canty irrigation

Greater efficiencies within existing borderdyke irrigation schemes could mean a reduction in demand on such resources as the Rakaia River, according to research by the Ministry of Agriculture and Fisheries in Ashburton.

An agricultural engineering adviser with the M.A.F. in Ashburton, Mr Robert van Bentum, has produced a feasibility study on the Ashburton-Lyndhurst scheme which says that almost one cubic metre a second (cumec) of water could be saved there.

Although the study deals solely with the AshburtonLyndhurst scheme, in his conclusions Mr van Bentum says that by upgrading the whole area of community borderdyke schemes in MidCanterbury just over three cumecs could be made available at the “farm gate” for further irrigation. The “farm gate" 3.1 cumecs is equivalent to 3.6 cumecs of water from the Rangitata River after allowing for losses. According to Mr van Bentum, the estimated cost for upgrading the three schemes is $1.5 million, while to take 3.6 cumecs from the Rakaia River would “conservatively cost” three times that amount.

The saving would effectively water some 8870 ha of intensive cropping land, he said. That, too, could well be conservative because of bigger possible savings through the use of water budgeting to stretch the allocation for the Valetta and Mayfield-Hinds schemes. The feasibility study said that the thrust of the project was to look at the improvement possible from increasing on-farm flows and it was based on trial work from the Winchmore Research Station by Dr Anthony Taylor. There was no contention that management could provide some improvement in efficiency, however upgrading off-farm aspects of the scheme could provide an immediate and bigger improvement over all, it said. The scheme covered 27,000 ha and originally its design irrigable area was 16,250 ha. The most recent

information from the Ministry of Works and Development showed that that was already being exceeded with 18,500 ha being irrigated (15,000 ha borderdyke, 1500 ha wild flooded, 2000 ha spray). The scheme’s allocation of water correlates to 12 cumecs. Of that, losses in the Rangitata Diversion and supply races are estimated at 2 cumecs. A further 15 per cent is estimated to be lost in on-farm supply races and “dead water” for a paddock application equivalent to 7.82 cumecs.

The two problem areas in the scheme are the fluctuations in flows received at the turnout and below design on-farm flows. From trial work by Dr Taylor it was concluded that at one property the average turnout flow was below the normal (230 litres a second) and over a period

of time it varied from 130 to 270 litres a second.

The suggestions made in the study to eliminate the fluctuations and to deliver a constant flow r!ate are the use of labyrinth weirs and bladder valves. The flow rate would be lifted to 300 litres a second which meant some supply races, road crossings and culvert turnouts would have to be enlarged, the study says. However, the amount of on-farm upgrading considered necessary is minimal . , • The reasoning for increasing the flow rate comes from Dr Taylor’s trials which found that a turnout of 230 litres a second required an application depth of 88mm while at 300 litres a second only 76mm was required to gain the same result.

Quantified, the 12mm saving allows for an extra 0.96

cumecs and the study gives a series of options for that using both borderdyke and spray, either within the existing scheme or outside it.

The capital cost of upgrading the scheme is set at $500,000, based on estimates from local M.O.W. representatives. Where the water is to be used outside the scheme, the cost of reticulation is estimated at $500,000 for a total cost of $1 million.

The study also shows that the scheme is under-utilised early in ‘ the irrigation season. Demand peaked at 10 cumecs in January last season, which was one of the driest on record.

During September demand was only 9 per cent, in October it was 16 per cent and in November it rose to 59 per cent of the available water, '