Rice Water Use Efficiency Workshop Proceedings

Abstract

For rice grown under ponded conditions in the southern Murray Darling Basin, total evapotranspiration from the paddock during the ponded period can be estimated from Epdk = 0.9 x Epan = 1 x ETo, where ETo is calculated using a locally calibrated Penman equation (Meyer 2000). The long term average ETo at Griffith is 1160 mm (11.6 ML/ha) over the rice season (Oct…Feb), while rainfall averages 160 mm. Therefore rice requires 1000 mm, on average, to meet net evaporative demand. There is less certainty in weekly or monthly estimates of evapotranspiration from ETo or pan evaporation. The available data suggest that the crop factor increases during the season, reaching a maximum around anthesis, but the data are too variable and too few to assign monthly (or weekly) crop factors with confidence. Further refinement of monthly crop factors would assist in water budgeting during the irrigation season, especially in years of lower water availability. There is also little information on evapotranspiration from draining until harvest and after harvest, and its relationship with ETo or pan evaporation. Total ETo over the rice season (Oct…Feb) at Griffith, Finley and Tullakool is similar, but it is about 10% higher for Hay. The same is true for net evaporation (ETo-rain). Seasonal variations in ETo, rain and net evaporation are large. Therefore a rice paddock water use target based on seasonal conditions was adopted by the Rice Environmental Policy Advisory Group, commencing in the 1996/97 season. This target is calculated to be equal to ETo-rain+400, where all units are in millimetres. Rice paddock water use is routinely monitored by the irrigation companies, and the purpose of the target is to detect paddocks with excessive deep drainage (“leaky” paddocks) by identifying paddocks with high water use. The biggest gains to be made in improving rice water use efficiency are by identification of leaky paddocks and their amelioration or elimination from rice growing. Accurate identification of leaky paddocks requires knowledge of the period of ponding and the pre-rice soil water content – simple information which would be easy for farmers to provide. More accurate measurement of applied irrigation water is also needed, and substantial improvement could be made by increased on-farm recording of water deliveries – however, this would require additional effort from farmers which some (many?) may be reluctant to apply for a range of reasons including pressures on time and lack of desire for this type of information. Once the technology and systems are in place for more accurate identification of leaky paddocks, then the next gains in the drive towards higher rice water use efficiency would be firstly through implementation of the policy of restricting rice to areas that meet the water use targets, and secondly to progressively lower the rice water use target to ETo4 rain+ΔSWC+100, where ΔSWC is the increase in soil water content over the rice season in the rootzone (0-1 m). Socioeconomic factors are at present a major barrier to the adoption of all of these technically simple methods for improving rice water use efficiency. Furthermore, they consider the rice enterprise in isolation from other activities on the farm. Therefore alternative approaches examining whole farm water balances are being developed such as the SWAGMAN Farm model.