Evaluation of evapotranspiration from eddy covariance using large weighing lysimeters

Authors: Moorhead, Jerry; Marek, Gary; Gowda, Prasanna; LIN, XIAOMAO - Kansas State University; Colaizzi, Paul; Evett, Steven - Steve; KUTIKOFF, SETH - Kansas State University

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2019
Publication Date: 2/20/2019
Citation: Moorhead, J.E., Marek, G.W., Gowda, P.H., Lin, X., Colaizzi, P.D., Evett, S.R., Kutikoff, S. 2019. Evaluation of evapotranspiration from eddy covariance using large weighing lysimeters. Agronomy. 9(2):99. https://doi.org/10.3390/agronomy9020099.
DOI: https://doi.org/10.3390/agronomy9020099

Interpretive Summary: In the central U.S., the Ogallala Aquifer, one of the largest in the world, is rapidly declining. Water conservation is a key strategy to keeping this resource available for the future. In many areas over the Ogallala Aquifer, most of the water is used for irrigation. Knowing how much water agricultural crops use is important for irrigation water conservation. In dry regions, irrigation is used to supplement limited and erratic growing season rainfall to meet crop water demand. A widely used and portable instrument for estimating crop water use is eddy covariance (EC); however, past research has shown it has errors and is unable to close the energy balance. Scientists from ARS (Bushland, TX) and Kansas State University held a field experiment to test how well EC measures crop water use using sensors at different heights. The results showed error for crop water use from EC was 20 – 30% at half-hourly intervals but was reduced to 10-15% when summed to daily values. The sensor height was not a big factor for the error, rather, the growth stage of the crop had a greater effect. These results are of interest to other scientists studying crop water use and will lead to improvements in the EC measurements.

Technical Abstract: Evapotranspiration (ET) is an important component in the water budget and used extensively in water resources management, such as water planning and irrigation scheduling. In semi-arid regions, irrigation is used to supplement limited and erratic growing season rainfall to meet crop water demand. Although lysimetery is considered the most accurate method for crop water use measurements, large precision weighing lysimeters are expensive to build and operate. Alternatively, other measurement systems such as eddy covariance (EC) and scintillometery are being used to estimate crop water use. However, due to numerous explicit and implicit assumptions in the EC method, an energy balance closure problem is widely acknowledged. In this study, three EC systems were installed in a field containing a large weighing lysimeter at heights of 2.5, 4.5, and 8.5 m. Sensible heat flux (H) and ET from each EC system were evaluated against the lysimeter. Energy balance closure ranged from 64 – 67% for the three sensor heights. Results showed each EC system underestimated H and overestimated ET; however, the underestimation of H was greater than the overestimation of ET. Analysis showed accuracy of ET was greater than energy balance closure with error rates of 20 – 30% for half-hourly values. Investigating the error rates throughout the growing season showed energy balance closure and ET accuracy were greatest early in the season and more error was found after maximum plant height was reached. The greater error being associated with increased biomass may indicate energy stored in the plant canopy is one source of error. Summing the half-hourly data to a daily time-step drastically reduced error in ET to 10-15% indicating EC has potential for use in agricultural water management practices.