Improving efficiency of water use for irrigated rice in a semi-arid tropical environment

Irrigation water accounts for almost 40% of total variable production costs for rice (Oryza sativa L.) cropping in the Burdekin River Irrigation Area, northern Australia. Increasing the efficiency of water use would improve the economic viability of growers and long-term environmental benefits would also be Likely due to lower water tables and decreased salinisation in irrigation areas. The aim of these studies was to maximize grain yield by optimizing its functional components: water use, efficiency of water use for dry matter production (WUEdm) and harvest index (HI). The responses of dry matter and yield in rice (cv. Lemont) to five methods of irrigation were studied in a wet and dry season in the region. Applying a permanent flood at sowing, the 3-leaf stage (traditional) and prior to panicle initiation were compared with two unflooded methods: saturated soil culture (SSC) and intermittent irrigation at weekly intervals. Saturated soil culture consisted of growing rice on raised beds of height 0.2 m and width 1.2 m, with water maintained in the furrows (0.3 m wide) some 0.1 m below the bed surface. The results of these studies show that it is not necessary to flood rice to obtain high gain yield and quality. The trend was for yield to increase with water supply, but there was no significant difference in yield and quality between SSC and traditional flooded production, although SSC used about 32% less water in both seasons. Therefore the efficiency of water use for grain production (WUEg, g m(-2)mm(-1)) was higher in SSC than in traditional flooded production in the wet season and a similar trend existed in the dry season. There were no differences between SSC and the traditional method of irrigation in any of the grain quality components measured, indicating that this water saving method did not lower grain quality. Weed growth was generally higher in unflooded treatments, although weed populations in SSC and traditionally flooded rice were equivalent in the dry season, suggesting that weeds can be controlled in SSC. Saturated soil culture provides a viable alternative to flooded rice production for growers in semi-arid tropical environments. Substantial reductions in variable costs of production are attainable by reducing water use without reducing yield and quality.