Evaluation of sensible heat flux and evapotranspiration using a surface layer scintillometer and a large weighing lysimeter

Authors: Moorhead, Jerry; Marek, Gary; Colaizzi, Paul; Gowda, Prasanna; Evett, Steven - Steve; MAREK, THOMAS - Texas A&M Agrilife; PORTER, DANA - Texas A&M Agrilife; Brauer, David

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 2/21/2019
Publication Date: 2/25/2019
Citation: Moorhead, J.E., Marek, G.W., Colaizzi, P.D., Gowda, P.H., Evett, S.R., Marek, T.H., Porter, D.O., Brauer, D.K. 2019. Evaluation of sensible heat flux and evapotranspiration using a surface layer scintillometer and a large weighing lysimeter. In: Lakshmi M., editor. Top 5 Contributions in Sensor and Biosensor Technology. 3rd Edition. Hyderabad, India: Avid Science. p. 1-40.

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 providing daily estimates of crop water use with an error of 13%. These results show there is potential to use the laser scintillometer for estimating crop water use in agricultural water management.

Technical Abstract: Accurate estimates of actual crop evapotranspiration (ETa) are required for optimal irrigation water management, especially in semi-arid regions. Common ETa measurement methods include Bowen Ratio, Eddy Covariance (EC), weighing lysimeters, and scintillometers. Although large, representative lysimeters are considered the ultimate standard for measurement of ET, but they are expensive to install and costly to maintain. Measurement systems such as EC and scintillometers are much simpler and cost effective alternatives for ET estimation; however, uncertainty about their accuracy is an important and concerning issue. In previous scintillometer studies, EC data were used for validation, which raises concern, as EC systems have a widely acknowledged and significant energy balance closure problem. In this study, a Surface Layer Scintillometer (SLS) was evaluated for accuracy in determining ET, as well as sensible and latent heat flux, as compared to a large weighing lysimeter in Bushland, TX. The SLS was installed over irrigated grain sorghum (Sorghum bicolor (L.) Moench) for the period of July 29 – August 17, 2015 and over grain corn (Zea mays L.) for the period of June 23 – October 2, 2016. Results showed poor correlation for sensible heat flux, but good correlation for ET, with R-squared values of 0.87 and 0.89 for hourly and daily ET, respectively. The accuracy of the SLS was comparable to other ET measurement instruments with an RMSE of 0.10 mm per hour (40%) for hourly ET; however, summing hourly values to a daily time step reduced the actual ET error to 14% (0.75 mm per day). This level of accuracy indicates that potential exists for the SLS to be used in some water management applications. As few studies have been conducted evaluating the SLS for ET purposes, or in combination with lysimetric data, further evaluations would be beneficial to investigate the applicability of the SLS in water resources management.