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United States Department of Agriculture

Agricultural Research Service

Title: Sources of Biased Errors in Evaluating Evapotranspiration Equations

Authors
item Ritchie, Joe - MICHIGAN STATE UNIV.
item Howell, Terry
item Meyer, W - CSIRO, AUSTRALIA
item Wright, James

Submitted to: International Evapotranspiration Irrigation Scheduling Conference
Publication Type: Proceedings
Publication Acceptance Date: June 28, 1996
Publication Date: N/A

Technical Abstract: The accurate estimation of water use by crops is critical in water resource planning. To do this, accurate estimates of maximum, energy limited evapotranspiration rate for a specific crop (ETx) are a prerequisite so that differences in weather conditions between locations and seasons can be accounted for. Developing broadly applicable daily ETx equations requires accurate measurements of ETx and accompanying weather data. This paper reports on probable errors found in both these sets of measurements in available data sources from a diversity of locations. The primary biased errors were associated with apparent improper calibration of solar radiometers and with defining the effective area of a lysimeter. Solar radiation data from Davis, California, where lysimeter data are abundant, had biases that averaged 6% high for nine years. One year had little bias and others had 10% high biases. Another major source of lysimeter data is Coshocton, Ohio, where solar radiation biases of 17% low were found for th three years studied. The effective area of the Coshocton lysimeters may need adjustments by as much as 20% to compensate for rim errors and exposure errors. Carefully managed lysimeters were found to have as much as +- 10% biases that were probably caused by overlapping of vegetation from the lysimeter area with the surrounding area. Results from this study indicate that caution should be used when interpreting data from lysimeter sources for developing and calibrating equations because of these possible biases.

Last Modified: 9/10/2014
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