Reduced Water Use and Methane Emissions from Rice Grown Using Multiple Inlet plus Intermittent Irrigation
J. H. Massey, M.C. Smith
By 2020, global rice (Oryza sativa) production must increase ca. 30% above current levels to meet the increased demands of a growing, more affluent population. Many rice-producing countries consume all of their rice in-country. In contrast, the U.S. currently exports ca. 40% of its rice crop. Thus, one might expect that increased production in the U.S. will play a key role in meeting increased global demand for rice. To meet this need, however, U.S. rice producers must address several issues that threaten the long-term sustainability of current production practices. First among these issues is groundwater depletion, as current rice production methods require seasonal use of ca. 2.5 acre-feet of water or >five times that used in producing corn, cotton, or soybeans.† The use of groundwater for irrigation purposes has contributed to aquifer declines averaging ca. 1 ft (Mississippi) to 3 ft (Arkansas) per year in the Mississippi River delta where >80% of U.S. rice is grown. Beginning in the mid-1980ís, rice-growing regions in Asia, particularly China, developed water conservation practices to balance agricultural, urban, and industrial demands for limited water resources. Water savings of up to 50% over that of continuously-flooded rice paddies have occurred in fields using intermittent irrigation where floodwaters are allowed to naturally subside prior to each flood reestablishment.† It is not certain, however, that this same approach would transfer to much the larger U.S. rice production fields that can be up to 300 A in size. Concerns surrounding the use of intermittent rice irrigation in the U.S. include potential negative impacts on pest control, fertility management, grain quality, and yield. Our on-going project is determining the feasibility of growing rice using intermittent irrigation in production-scale fields. We have coupled intermittent flood management with multiple-inlet flood distribution using plastic poly-pipe so that the cyclical floods can be quickly reestablished across large rice fields typical of the Mississippi River delta. Results to date from six field sites (typical size ~ 40 A) indicate that water use may be reduced by ca. 30% with no decrease in rice yield; greater savings are expected as producers become more comfortable with this approach. If intermittent rice flooding proves to be agronomically viable, its adoption could also impact another issue facing rice production: Globally, flooded rice culture is a significant source of methane. As a result, practices that reduce methane emissions have been sought. Wide-spread adoption of intermittent rice irrigation has reportedly reduced methane emissions in Asia. Although methane produced by U.S. rice represents <0.5% anthropogenic sources, reductions in methane as an indirect benefit of water-saving rice irrigation practices would still be welcomed. Our preliminary, static-chamber results agree with those of others that indicate that significant reductions in methane flux (up to 70%) may occur when rice is grown using intermittent flooding rather than continuous flooding currently practiced in the U.S.