Rice and water

The Comprehensive Assessment of Water Management in Agriculture (CA) seeks answers to the question of how freshwater resources can be developed and managed to feed the world's population and reduce poverty, while at the same time promoting environmental security. The CA pays particular attention to rice as this crop is the most common staple food of the largest number of people on Earth (about 3 billion people) while receiving an estimated 24-30% of the world's developed freshwater resources. Rice environments also provide unique-but as yet poorly understood-ecosystem services such as the regulation of water and the preservation of aquatic and terrestrial biodiversity. Rice production under flooded conditions is highly sustainable. In comparison with other field crops, flooded rice fields produce more of the greenhouse gas methane but less nitrous oxide, have no to very little nitrate pollution of the groundwater, and use relatively little to no herbicides. Flooded rice can locally raise groundwater tables with subsequent risk of salinization if the groundwater carries salts, but is also an effective restoration crop to leach accumulated salts from the soil in combination with drainage. The production of rice needs to increase in the coming decades to meet the food demand of growing populations. To meet the dual challenges of producing enough food and alleviating poverty, more rice needs to be produced at a low cost per kilogram grain (ensuring reasonable profits for producers) so that prices can be kept low for poor consumers. This increase in rice production needs to be accomplished under increasing scarcity of water, which threatens the sustainability and capability to provide ecosystem services of current production systems. Water scarcity is expected to shift rice production to more water-abundant delta areas, and to lead to crop diversification and more aerobic (nonflooded) soil conditions in rice fields in water-short areas. In these latter areas, investments should target the adoption of water-saving technologies, the reuse of drainage and percolation water, and the improvement of irrigation supply systems. A suite of water-saving technologies can help farmers reduce percolation, drainage, and evaporation losses from their fields by 15-20% without a yield decline. However, greater understanding of the adverse effects of increasingly aerobic field conditions on the sustainability of rice production, environment, and ecosystem services is needed. In drought-, salinity-, and flood-prone environments, the combination of improved varieties with specific management packages has the potential to increase on-farm yields by 50-100% in the coming 10 years, provided that investment in research and extension is intensified. (c) 2007, Elsevier Inc.