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Title: Evapotranspiration and ecosystem water use efficiency of alfalfa under rainfed condition in the U.S. Southern Great Plains

Author
item MANJUNATHA, PRIYANKA - Oklahoma State University
item Wagle, Pradeep
item Gowda, Prasanna

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 7/31/2019
Publication Date: 12/11/2018
Citation: Manjunatha, P., Wagle, P., Gowda, P.H. 2018. Evapotranspiration and ecosystem water use efficiency of alfalfa under rainfed condition in the U.S. Southern Great Plains [abstract]. American Geophysical Union. Available at: http://adsabs.harvard.edu/abs/2018AGUFM.B33I2802M.

Interpretive Summary: Abstract only

Technical Abstract: Alfalfa (Medicago sativa L.), a perennial forage which is often called the queen of the forages, is one of the most important forage crops in the Southern Great Plains (SGP) including Oklahoma. Since alfalfa is a water-intensive crop, it is mostly grown under irrigated conditions. Although some studies have reported water use (i.e., evapotranspiration, ET) of irrigated alfalfa, annual dynamics of ET and ecosystem water use efficiency (EWUE) are lacking for rainfed alfalfa. To address this knowledge gap, eddy covariance (EC) method was used to measure ET and surface energy fluxes from April 2016 to May 2018 over a rainfed alfalfa field at the USDA-ARS Grazinglands Research Laboratory in El Reno, Oklahoma. During peak growth, maximum daily (8-day average) ET reached up to 4.77 mm in 2016, 5.64 mm in 2017, and 3.64 mm in 2018. Monthly ET ranged from 6 mm to 17 mm during winter dormancy and reached as high as 136 mm in May 2017 even though hay was harvested on May 3. Cumulative ET for the study period (April – December) in 2016 was 1.29 times of cumulative rainfall (539 mm) for the same period. Cumulative annual ET in 2017 was 0.81 times of total annual rainfall (1109 mm). In comparison, cumulative ET for the study period (January – May) in 2018 was 1.48 times of cumulative rainfall (147 mm) for the same period. The maximum 8-day composite EWUE reached ~4 g C mm-1 ET during peak growth. For the entire study period, EWUE (computed as the ratio of sums of GPP to ET) was 2.66 g C mm-1 ET. The EWUE was 2.45 g C mm-1 ET for the study period of 2016 (dry year), 2.68 g C mm-1 in 2017 (wet year), and 3.62 g C mm-1 for the study period in 2018 (very dry year). Optimum air temperature was ~22 º C and vapor pressure deficit was ~ 3.2 kPa for ET. However, ET did not decline up to the observed ranges (> 2000 µmol m-2 s-1) of photosynthetically active radiation. This study provides some insights onto the annual dynamics of ET and EWUE for alfalfa under rainfed condition in the SGP.