Performance of atmometers in estimating reference evapotranspiration in a semi-arid environment

• Published in: Agricultural water management Vol. 130; pp. 27 - 35
• Main Authors: Gleason, D.J., Andales, A.A., Bauder, T.A., Chávez, J.L.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.12.2013; Elsevier
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; air temperature; alfalfa; Atmometer; Biological and medical sciences; Calibration; Evapotranspiration; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; irrigation scheduling; linear models; meteorological data; Water balance and requirements. Evapotranspiration; wind; Atmometer; Evapotranspiration; Calibration; Measuring instrument; Arid environment; Reference; Water engineering; Semi arid climate; Performance
• ISSN: 0378-3774; 1873-2283
• DOI: 10.1016/j.agwat.2013.08.008

•Atmometer measured ET correlates well with the ASCE ETrs predicted values.•Atmometer performed best when alfalfa was at reference condition.•Atmometer underperformed on windy and cool days.•Localized calibration increased performance. Evapotranspiration (ET) based irrigation scheduling requires accurate measurements of reference evapotranspiration. Mathematical formulas with localized weather data can be used to accurately predict alfalfa reference ET rates (mmh−1 and mmd−1). When local meteorological data are unavailable, a physical measurement of ET can be taken with an atmometer. The objective of this study was to determine if a Model E atmometer (ETgage Company, Loveland, CO), equipped with a canvas #54 cover, could be used to effectively estimate alfalfa reference ET. The ASCE Standardized Alfalfa Reference ET Equation (ASCE ETrs) was used as the standard for comparison of atmometer ET values to determine atmometer performance. Four years of alfalfa ET, as determined by an atmometer (ETgage), were compared to ASCE ETrs. Daily as well as 2, 3, 5, and 7 day sums of daily ETgage and ASCE ETrs were compared using simple least-squares linear regression. Coefficients of determination (R2) between daily ETgage and ASCE ETrs for all years were greater than or equal to 0.80. Throughout the study, the atmometer tended to underestimate ASCE ETrs. Average seasonal underestimation of ASCE ETrs measured by the atmometer ranged from 9.06% to 18.9%. Root Mean Square Error (RMSE) and Mean Bias Error (MBE) ranged from 1.14 to 1.82mmd−1 and −0.66 to −1.51mmd−1, respectively. The atmometer underestimated daily ASCE ETrs 88% of the time, with an average underestimation of 1.30mmd−1. Underestimation of ASCE ETrs measured by the atmometer occurred most often on days when mean daily horizontal wind speeds were greater than 2ms−1 and/or when mean daily air temperatures were below 20°C. The atmometer performed best when the alfalfa was at reference condition. Localized calibration equations for reference and non-reference conditions with a temperature correction were developed to improve accuracy, with average magnitude of MBE reduced from −0.97mmd−1 to 0.13mmd−1.