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Fraction of Net Radiation Utilized in Evapotranspiration from a Corn Crop¹

ABSTRACT

The ratio of daily evapotranspiration to net radiation was determined at various stages in the development of a corn crop. Measurements, made near the center of a 266- by 340-foot corn plot near Guelph, Ontario are reported for 16 days of observation during the summer of 1958. Soil moisture in the plot was maintained at a level exceeding 75% available. There was good agreement between the daily evapotranspiration determined by a floating lysimeter and the daily evapotranspiration computed hourly from the vertical heat budget method. The ratio of evapotranspiration to net radiation (E/R_n) averaged 0.81 ± .09 for the daytime period on days following rain when a large area was equally moist. Low values of E/R_n were associated with days of heavy cloud and high humidity. Values of E/R_n greater than unity during the daytime were obtained on some days when the corn plot was irrigated and the surroundings were dry. Freezing of the crop reduced E/R_n to 0.52. The stage of crop development prior to freezing did not have any effect on E/R_n under the soil moisture conditions of this experiment.

¹ Contribution from the Department of Soils, Ontario Agr. Coll., Guelph, Can. Presented before Div. I and VI, Soil Science Society of America, Cincinnati, Ohio, Nov. 18, 1959. Part of a thesis submitted in partial fulfillment of the M.S.A. degree at the Ontario Agr. Coll. by the senior author.

² Graduate Assistant and Assistant Professor of Soils, respectively. The senior author is now Research Assistant, Dept. of Agronomy, Cornell University, Ithaca, N. Y.

Received for publication May 19, 1960. Accepted for publication July 25, 1960.