Neutron probe aids water efficiencies in cropping

Press, 23 September 1983, Page 25

Neutron probe aids water efficiencies in cropping

Nuclear technology is helping scientists at Winchmore Irrigation Research Station in Canterbury improve their research methods.

Until recently, investigating water movement in the soil involved hours of digging to get information from sites which could not then be used again for sampling. Mr Richard Stocker is one of the scientists who has been using a neutron probe to look at water use in the soil. It does away with the need for digging, making the work easier and more efficient.

“The neutron probe is an instrument which sends out fast moving neutrons and

detects slow ones,” said Mr Stocker.

“The main thing that slows down neutrons is water, so the more slow neutrons the probe detects, the more water there is in the soil around it

“To use the probe, aluminium tubes with a diameter of about 44mm are installed vertically in the ground. The probe is lowered down each tube to take readings.

“These tubes can be returned to at any time which means we can get successive readings from one site which wasn’t possible with our previous sampling methods,” Mr Stocker said. The probes are used commercially in the United States for irrigation schedu-

ling but at $7OOO each they are not yet used for this in New Zealand. “There were a lot of problems with the calibration of the probe and with installing the access tubes in difficult stony soils such as those in Canterbury," said Mr Stocker. “The staff at Winchmore have largely overcome these problems.”

The access tubes are fragile and easily buckled. The method of installation devised at Winchmore involves placing a steel bar down the centre of the tube.

The whole assembly is then driven into the ground using a post driver. The steel bar is drawn out, and the aluminium tube is left in the ground.

“This has given us a method to reliably take successive measurements before and after irrigation,” said Mr Stocker. “The probe is used to take readings every 158 mm down the tube. These readings are taken down to 1.35 metres. “It was found that under pasture, changes in water content below 900 mm amounted to only about 2 to 5 per cent of the total water changes in the profile,” he said. It was discovered that water use from the gravelly subsoils in Canterbury is significantly greater than previously assumed. “In the past we suspected that the only significant water use came from the fine top soil, but now we have shown on a wide range of soils that another 50 per cent can be added on to the available water estimates if the stony subsoils are considered,” Mr Stocker said. “This had never been quantified before because of the problems with sampling.

“A major implication of this is that border strip irrigation systems, that were previously thought to

put on well in excess of the water holding capacity of the soil, may in fact be more efficient than assumed.”

On Lismore stony silt loam, a common soil type in Canterbury, the water holding capacity of the top 300 mm of the soil is about 60mm. It was thought that pasture plant roots were confined to this layer and that an irrigation system putting on 120 mm of water would be wasting half of it. “After neutron probe measurements, however, we found pastures were typically drawing out up to 90mm of water, of which 30mm was coming from the stony subsoil. So the wastage from flood irrigation may not be as great as previously thought on these soils,” said Mr Stocker. This finding has important implications for sprinkler irrigation systems on pasture. Usually these systems have been designed to put on about 60mm every three weeks, but the farmers find this insufficient in periods of continuous drought such as the last two summers. Sprinkler irrigated pastures have often not produced to capacity in such conditions. Mr Stocker also found that on lighter soils, no water was lost to deep drainage, provided the amount applied did not exceed the water deficit before irrigation. “It is generally thought

that if there is a deficit of 50mm, then, to be sure of filling that deficit, 60 or 70mm needs to be put on, because some will be wasted,” he said. “We found that this isn't so. As long as the input is less than the deficit, there is no water loss on light soils. “Heavy soils behave differently. They tend to become very hard with large open cracks when dry. Any water applied runs down these craclcs before wetting the rest of the soil and some may be lost to drainage.” This also has implications for the design and efficiency of irrigation schemes, particularly sprinkler systems, but as Mr Stocker said, this depends on the philosophy behind the design.

“It depends on whether you wish to spend a given water supply over a large area, or fully irrigate a smaller area with the same supply. “Maximum yields may be achieved by irrigating a small area. Yields are reduced by spreading the water over a larger area, but net return may be greater,” said Mr Stocker. “If water is in short supply then accurate application and as little wastage as possible is important. Most schemes are somewhere between these two options and it’s never a clear-cut thing. We want to provide the guidelines and demonstrate the sort of efficiency that’s available if required.”