Researching Solutions to the Sustainability and Environmental Challenges for Rice-based Cropping Systems in Southern Australia

Abstract

The major threat to the sustainability of irrigated agriculture in the rice growing regions of the southern Australia is secondary salinisation as a result of rising watertables. Rice growing contributes about half of the accessions to the groundwater in these regions. A range of strategies for reducing the accessions from rice are applied, including restricting rice growing to soil assessed as suitable for rice. In the past this was based on soil texture, but increasingly EM-31 survey is being used, and the inclusion of soil sodicity constraints will further improve the ability to predict suitable soils. The net evaporative demand for rice growing over the whole season is well-approximated by reference evapotranspiration (ETo), which is used to calculate the seasonal rice paddock water use limit. Potential methods for increasing rice water use efficiency and reducing recharge include shorter duration varieties and a range of water and soil management strategies. Intermittent and sprinkler irrigation can significantly reduce water use, however yields are also reduced due to cold temperature damage during early pollen microspore. Small areas of leaky soils can greatly increase total accessions to the watertable, and EM-31 surveys show that many “suitable” rice paddocks have leaky areas. Recharge from leaky areas can be reduced by puddling or by impact compaction. After rice harvest, soil water content is high, and recharge may continue, especially under the influence of winter rainfall and low evaporation. Research is underway to quantify the effect on accessions to the watertable of growing a winter crop immediately after rice harvest. Future work will investigate the conjunctive use of groundwater and surface water to promote watertable control while maximising agricultural productivity by making more water available for irrigation. The SWAGMAN (Salt Water And Groundwater MANagement) series of computer models has been developed to determine the impacts of management and climate on watertables, salinisation and yield, and the tradeoffs between environmental objectives and profitability. These models include SWAGMAN Destiny, a point scale crop model that can be run for up to 30 years of climatic data. SWAGMAN Farm is a farm scale optimisation model which predicts the most economic cropping mixes that meet specified net recharge and root zone salinity objectives, taking into account farmer preferences. Regional groundwater models have been developed to evaluate the impacts of climate and management on watertables. The development of shallow saline watertables results in the generation of saline drainage waters. Numerous evaporation basins ranging in size from a couple of hectares to a few hundred hectares have been created in recent years to receive saline drainage. Investigations into the salt and water balance of evaporation basins, the development of the model BASINMAN, and economic analyses have led to guidelines for the siting, design and management of evaporation basins. A pilot trial is also underway to investigate the feasibility of serial biological concentration, with the production of high value crops in the first 2 stages, followed by salt tolerant crops (stage 3), fish farming (4), evaporation basins (5) and a solar pond to generate energy.