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 Journal
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 Journal. 9(2):99. https://doi.org/10.3390/agronomy9020099.
DOI: https://doi.org/10.3390/agronomy9020099

Interpretive Summary: With water resources continually declining, conserving water is essential. Water conservation in agriculture is mostly done through irrigation management. For the best irrigation management, accurate estimates of crop water use are required. Many instruments are available to estimate crop water use; however, there are benefits and disadvantages to each one. The more simple and easier to use instruments do not have good enough accuracy. A relatively unused instrument, the laser scintillometer, has not been tested for use in measuring crop water use. Scientists from ARS (Bushland, TX) and Texas A&M AgriLife Research and Extension Service held a field experiment to test how well the laser scintillometer measures crop water use. It was found that the laser scintillometer was relatively accurate with and error of 40% for hourly time steps, but much better for daily time steps with an error of 13%. The daily error is much lower than other, more common instruments. These results show there is potential to use the laser scintillometer for estimating crop water use in agricultural water management.

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, high-precision weighing lysimeters are expensive to build and operate. Alternatively, other measurement systems such as eddy covariance (EC) 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% to 67% for the three sensor heights. Results showed that all three EC systems underestimated H and consequently overestimated ET; however, the underestimation of H was greater in magnitude 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. Further analysis of error rates throughout the growing season showed that energy balance closure and ET accuracy were greatest early in the season and larger error was found after plants reached their maximum height. Therefore, large errors associated with increased biomass may indicate unaccounted-for energy stored in the plant canopy as one source of error. Summing the half-hourly data to a daily time-step drastically reduced error in ET to 10%–15%, indicating that EC has potential for use in agricultural water management.