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The Association between Free-Water Evaporation and Evapotranspiration of Spring Wheat Under the Prevailing Climatic Conditions of the Plains Area of Montana¹

ABSTRACT

In a study involving over 25 years of crop and weather data at 2 stations in the Northern Plains of Montana, evapotranspiration of dryland spring wheat was negatively correlated with free-water evaporation. In years of high temperatures and low rainfall, i.e., high free-water evaporation, reduced crop growth and an inadequate supply of soil moisture resulted in low evapotranspiration.

Cultural practice and geographic location influenced the amount of evapotranspiration per unit of free-water evaporation. A greater amount of evapotranspiration per inch of free-water evaporation always resulted under fallow than under continuous wheat.

When adjustments were made for variation in size of plants (evapotranspiration/total dry weight ratio), the correlation between free-water evaporation and evapotranspiration per unit of dry matter produced was positive. However, the degree of association between free-water evaporation and the evapotranspiration ratio was apparently reduced by limited soil moisture supply.

The effects of cultural practice, geographic location, and limited moisture supplies on the magnitude of evapotranspiration per unit of free-water evaporation detract considerably from the value of free-water evaporation measurements as an indication of water use by wheat plants under semiarid conditions.

¹ Contribution from the U. S. Department of Agriculture, A.R.S., and the Montana Agr. Exp. Sta. cooperating. Published with the approval of the Director as Paper No. 388, Journal Series, Montana Agr. Exp. Sta. Presented before Div. I, Soil Science Society of America, Nov. 15, 1956, at Cincinnati, Ohio.

² Soil Scientist, Western Soil and Water Management Section, Soil and Water Conservation Research Division, A.R.S., U.S.D.A.; and Professor of Mathematics and Statistics, Montana State College, respectively.

The authors wish to acknowledge the capable technical assistance of past and present personnel of the Huntley and Northern Montana Branch Experiment Stations where the data used in this article were collected. The council of Dr. J. C. Hide throughout this study was greatly appreciated.

Received for publication September 24, 1956. Accepted for publication May 8, 1957.