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-1980’s, 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.