Irrigation risk management strategies to reduce water use and maximize profitability; a paradigm shift in performance to $ per unit of water : Final report CTA038

Abstract

The Australian sugar industry is predisposed to maximize the benefits of irrigation because of its geographic and climatic location. However mistakes of older irrigation schemes elsewhere in Australia, need to be avoided. Performance criteria such as $ produced per unit of water used 'which have the long term aim of very closely matching plant water use with water applied, will be the single most important factor ensuring longevity of irrigation areas' (Meyer, 1997). Matching plant water use and irrigation, requires knowledge of climatic demand for water, soil water supply, and crop response to water deficits.

This project aimed to develop accurate estimates of water requirements for sugarcane with the use of state-of-the-art technology capable of monitoring the interaction of the atmosphere and the cropped surface. This interaction has been captured in a mathematical equation known as the Penman-Monteith equation as well as in several sugarcane specific models and equations. The research aimed to test and improve these equations and models so that they can be applied reliably to meet the aim of matching crop water supply and demand.

An additional aim of the project was to determine how yield-building processes are affected when water supply cannot meet demand. Given that the aim of irrigating sugarcane is to improve sucrose yield and CCS it is important that the process of sucrose accumulation is targeted when managing irrigation. A third aim was to develop recommendations on ways to reduce water use with minimum impact on productivity and maximum enhancement of profitability using new knowledge of crop water requirements and crop response to water stress.

Two divergent approaches were adopted for the project to meet the dual but complementary aims of defining water requirements and investigating water stress physiology of sugarcane. The one was based on sophisticated measurements of energy used to evaporate water from the cropped surface. The other approach was based on more conventional field plot designs in order to establish responses of sugarcane to water stress and irrigation scheduling treatments. The APSIM-Sugarcane model was used extensively in this research.

The research provided sound guidelines for irrigation of sugarcane in full irrigation schemes particularly in the Burdekin. An erect stand of sugarcane with a complete canopy and an adequate amount of water will use 25% more water than a well-kept lawn (reference evaporation) as determined by the Penman-Monteith equation. Irrigation applied to meet this demand is the maximum irrigation required for sugarcane. The experiments identified two important conditions under which irrigation could be applied at less than maximum rates with minimum risk to crop yield. The one situation is when the soil has been saturated by rain (as often occurs in the tropics) and a large amount of water can be supplied from the soil rather than directly from irrigation. The other situation is when the crop has lodged after the wet season. Lodging generally reduces growth rate and water use.

The new findings have been captured in a spreadsheet model now being tested by some growers in the Burdekin in order to improve scheduling of irrigation. This simple computer model has already identified some deficiencies in current irrigation practices where irrigation frequency was too high at times and too low at other times. The scheduling software is an improvement on existing recommendations which are aimed at irrigating when stalk elongation is reduced to 50% of potential. The computerized scheduling method deals with a wider range of conditions including all growth stages, variable soil wetness and burnt or trashed surfaces and it can be applied to any irrigation system. This scheduling system has the potential to save water and enhance yields in all full irrigation schemes. The new findings have also been used to alter irrigation scheduling tables in the Ord. Longer irrigation intervals in Spring and Summer are now recommended and this should further reduce irrigation use in the Ord and reduce the impact of irrigation on rising water tables.

Knowledge of water stress physiology has been thoroughly upgraded in this project. Processes (cell division and elongation) responsible for leaf extension and leaf emergence degrade relatively soon after irrigation is withheld. Leaf and stalk extension rates are thus highly sensitive indicators of crop water status. Irrigation is often recommended when stalk elongation rate is reduced 50% by water stress. However research in this project showed that biomass yield would not be reduced until stalk elongation rate is reduced to 30% of the potential rate. Under conditions of low evaporative demand (winter), stalk elongation can remain at low levels (<30%) for a long time without a loss in sucrose yield.

The project has delivered other simple crop stress criteria than can be used to schedule limited irrigation and to schedule drying off and harvesting operations. For example a loss of 3 to 4 green leaves would indicate the best time to harvest in order to benefit from increased sucrose storage during drying off. In one experiment that benefit amounted to an additional 3.6 t ha-1 of sucrose.

The results have been published in a number of conference and journal papers. The project contributed substantially to the Sugar CRC short course on irrigation and the results have been presented at numerous workshops. The project has contributed to a new extension project in the Ord (CSE007) where the energy balance technique, mastered in this project is now being used. Irrigation use in the Ord has reduced substantially as result of CSR022 and could be reduced further with the new technology developed in this project. There is continued commitment to promote the scheduling technology arising from this project

The research has and will benefit international scientific knowledge particularly regarding sugarcane evaporation standards as well as sugarcane water stress physiology. The sugar industry now has internationally accepted standards for determining the water requirement for sugarcane and these may be required to defend water use in the sugar industry in future. The work has strengthened links between Australian, South African and Swaziland sugarcane research bodies because of the collaborative nature of the work.