Strategic use of limited irrigation offers the opportunity for vertical expansion in the
Australian sugar industry, particularly where rainfall is moderate to high. Responses to
limited water in supplementary irrigation systems vary greatly due to climatic
variability, and it is difficult to know whether investment of time and money in
supplementary irrigation is worthwhile. From theoretical considerations, it appears that
benefits from supplementary irrigation in regions like the Herbert could be as large as
30 tonnes of cane per megalitre (ML) in some years, although the average annual
rainfall exceeds 1500 mm. In this paper, a distinction is made between response to
irrigation or irrigation water use efficiency (IWUE) and overall water use efficiency
which, for the Australian sugar industry, is about 12 tonnes cane per ML effective
water. An experiment began in 1995 on the farm of Mr Roy Pace at Bambaroo to
determine if theoretical IWUE could be matched by measured IWUE. Responses of up
to 27 tonnes cane per ML migation were obtained in the experiment, and sucrose
content was reduced only when insufficient time was allowed for drying-off. The
variation in IWUE was explained satisfactorily by theory embodied in the APSIMSugarcane model, but the model underestimated large IWUE values because it failed to account fully for carry-over effects of irrigation. With the model's conservative
tendencies in mind, the long-term benefits of irrigation with limited water were
determined. Yield responses of 40 t/ha from 2 ML!ha could be expected in 90% of years
at Bambaroo on the Molonga soil series of the experimental site. The paper points to
issues that require further research before limited water can be exploited fully with
minimum risk.