Mr Pendergast is a Queensland Department of Primary Industries and Fisheries rural water use efficiency development extension officer based at Emerald, and is finalising his PhD research examining the potential for SDI oxygation technology.

The 5.2 hectare sub-surface drip irrigation trial site on Tony Ronnfeldt's Emerald Irrigation Area farm, Nyang, was established seven years ago by the Department of Natural Resources and Water.

This project was to evaluate water use efficiency and levels of herbicide, pesticide and fertiliser chemicals in irrigation runoff of SDI compared with conventional furrow irrigation.

The site has 12 individually irrigated experimental blocks with SDI lines buried 300mm under the soil surface which are scheduled to deliver the precise volume of water to maintain optimum soil moisture in the crop root zone.

With a question mark over the below-expectation yield performance of cotton crops irrigated by SDI, Mr Pendergast's PhD project was to see if promising glasshouse experimental results using oxygation translated to the field.

The trials overseen by Central Queensland University's Professor David Midmore have examined the potential benefits of oxygation – a technique that involves entraining air into irrigation water delivered via sub-surface drip lines.

"Because of the high moisture holding capacity of the heavy soil, it was determined that the cotton plants were being subjected to episodic water logging events after each irrigation," Mr Pedergast said.

"Although each event was short term, the cumulated effect incurred a final yield penalty preventing SDI irrigated cotton from achieving its full potential."

Mr Pendergast began field trialling oxygation technology three years ago using Mazzi injectors that were adjusted to deliver a 12pc air by volume mix into the water lines to alleviate the root zone water-logging.

When comparing the crop performance of sub-surface drip between oxygated and non-oxygated blocks, there was a significant yield increase achieved through oxygation for the 2004-05 and 2005-06 trial crops (27pc and 16pc respectively).

"The oxygation trials show that we can achieve significant increases in both yield and water use efficiency using this technique," Mr Pendergast said.

"When we add up the water saving advantages and improved yield of oxygated sub-surface drip, growers who are prepared to adopt and manage the technology are in a better position to justify the high SDI capital cost of around $3500 to $4500/ha."