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Evapotranspiration Rates of Field Crops Determined by the Bowen Ratio Method¹

Simultaneous evapotranspiration rates were determined biweekly for the crop combinations of alfalfa and barley, alfalfa and cotton, alfalfa and sorghum, wheat and oats, and cotton from meteorological data by the Bowen ratio method. The crops were grown under irrigated conditions in the hot, arid region of south-central Arizona. Calculated evapotranspiration rates ranged from 1.0 to 1.8 times net radiation, indicating that large amounts of energy were extracted from the air mass. Alfalfa prior to cutting tended to use more water than the other crops. Water use by cotton after canopy development approached that of alfalfa. Barley, wheat, and grain sorghum appeared to require the least water.

The Bowen ratio method, as a survey technique or continuous sampling method, can be used to obtain short-period evapotranspiration rates under field conditions. It appears to be one of the few methods which will yield valid results when crops are subjected to frequent and heavy irrigation or where water tables are present.

² Research Meteorologist, U. S. Water Conservation Laboratory, Phoenix, Arizona.

Received for publication October 7, 1965.