Water use in Rice, separating fact from hyperbole

By Cass Mutters, U.C. Cooperative Extension Advisor, Butte County

Most would agree that using some of our water supply to grow the food and fiber that we need and enjoy is a justifiable use of this limited resource. Understandably during periods of extreme drought questions are raised as to whether the crops that we grow are making efficient use of the limited water. Rice production in California is sometimes the focal point of such questions. It is true that the aquatic nature of the rice plant requires that the fields are flooded. In turn, this can lead to the perception that an inordinate of amount of water is required to grow rice. Interestingly, rice water use is comparable to many other commonly grown crops.

Before presenting those comparisons, however, a general overview of where water goes in an agricultural field is needed. There are four possible places for water to go when applied to a field: it will evaporate, be transpired by the plant, percolate (move toward the groundwater), and/or run-off from the field back into the surface water.

Percolation and run-off (drainage) are not considered “water use” because that water remains within the hydrologic system; in other words the water was not “used”, it is still there. Interestingly, the water percolation in most rice fields is extremely small due to the nature of the soil where rice is grown.

Evaporation is that portion of the water that evaporates from the soil or water surface. Transpiration is the water that moves through the plant and is lost to the atmosphere from the leaf surface. Crop water use is the combination of evaporation and transpiration, referred to as “evapotranspiration”.

In a series of studies over several years, the University of California measured the evapotranspiration from rice at locations throughout the Sacramento Valley. Results established that rice uses about 33 inches of water (2.8 acre-feet) to grow a crop from seed to harvest. Now, let’s go back to the top. How does the water use of rice compare with that of other selected crops?

Crop Seasonal Water Use (inches)
Tomato 26
Almond 38
Cotton 31
Alfalfa 42
Citrus 35
Grain Corn 28
Walnut 40

When thinking about the water used to grow a crop, it’s helpful to remember the other benefits that come from that agricultural use. In the case of rice, farmers grow premium rice that is sold domestically and internationally. This is the vital economic engine that supports numerous communities throughout the Sacramento Valley. Those same rice fields also provide habitat for hundreds of wildlife species which adds environmental value to California that goes way beyond the farm gate.

R.G. “Cass” Mutters is County Director and a Farm Advisor for the Butte County Cooperative Extension Office. He has a Ph. D. in Plant Physiology from U.C. Riverside, an M.S. in Soil Fertility from Clemson University and a B.S. in Agronomy from Colorado State University.