Measuring Crop Water Use
(ETc) in CA Rice-01 



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Project Leader and Principal UC Investigators

Richard L. Snyder, biometeorologist, Department of Land, Air and Water Resources, UC Davis


The goal of this project is to develop accurate rice crop water use information that can be used by industry, water purveyors and policy makers. Previous estimates of rice crop water use were based on studies from the 1960s. Much has changed since then, including the advent of short-statured rice varieties and methods to estimate crop water use.

WaterUse.JPG (58505 bytes)A study was conducted at a California Irrigation Management Information System (CIMIS) station at Nicolaus and a nearby rice field. An experiment was designed to measure rice crop evapotranspiration and to determine crop coefficient values relative to a reference point. Researchers used the “surface renewal” method, which uses high frequency temperature data and energy balance measurements of net radiation and heat flux (soil and water).

Total estimated seasonal water use was 37.3 inches, considerably lower than the 42 inches typically reported for the Sacramento Valley. Crop coefficient values started at about 1.40, decreased to about 1.03 by early July and dropped to 0.80 by season’s end. This trend was similar to that observed in the previous year’s experiments.

The results of this experiment show that a later planting might reduce seasonal water usage. This is because evapotranspiration rates are lower in the fall than in spring and canopy development is faster when rice is planted later. Later planting dates might be a good strategy to reduce consumptive use during drought years.


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