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Title: On the discrepancy between eddy covariance and lysimetry-based surface flux measurements under strongly advective conditions

Author
item Alfieri, Joseph
item Kustas, William - Bill
item Prueger, John
item HIPPS, L - Utah State University
item Evett, Steven - Steve
item BASARA, J - University Of Oklahoma
item NEALE, C - Utah State University
item French, Andrew
item Colaizzi, Paul
item AGAM, N - Collaborator
item CHAVEZ, J - Colorado State University
item Howell, Terry

Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2012
Publication Date: 12/1/2012
Citation: Alfieri, J.G., Kustas, W.P., Prueger, J.H., Hipps, L.E., Evett, S.R., Basara, J., Neale, C., French, A.N., Colaizzi, P.D., Agam, N., Chavez, J., Howell, T.A. 2012. On the discrepancy between eddy covariance and lysimetry-based surface flux measurements under strongly advective conditions. Advances in Water Resources. 50:62-78.

Interpretive Summary: The task of ensuring there is sufficient water to meet the needs of all end users will become very difficult in the near future as the demand for freshwater increases with a growing population and changing climate. Since irrigation is the largest user of freshwater resources representing between 70% and 80% of total freshwater withdrawals and 90% of consumptive water use globally during the last century, it has become a critical focus of water conservation efforts. Improving the effectiveness of irrigation requires accurate estimates of both the current water needs of the crops and the evaporative losses from the fields. Increasingly, these estimates are obtained using remote sensing-based models particularly when evapotranspiration (ET) estimates are needed on a regional scale. These remote sensing-based methods are typically validated and calibrated using in-situ ET measurements. Although there are a variety of techniques for measuring ET, Eddy Covariance (EC) and Lysimetery (LY) are two of the most common methods used. The EC method determines ET as a function of the covariance between vertical wind speed and water vapor density while LY estimates ET as a function of the change in weight of soil monolith over time. Each of these methods has their own advantages and disadvantages related to their theoretical underpinning and in-field application. Using data collected primarily over a pair of irrigated cotton fields as a part of the Bushland Evapotranspiration and Remote Sensing Experiment (BEAREX08), ET measurements collected with EC and LY systems were compared and substantial differences were found. Daytime mean differences in the ET measurements from the two techniques were found to be in excess of nearly 0.3 mm/hr under strongly advective conditions. Causes for this disparity were related to failure of the EC method to fully balance the surface energy budget, local advection of warm, dry air over the irrigated cotton fields, and the failure of the LY systems to accurately represent the surface properties of the cotton fields as a whole. Regardless of the underlying cause, the discrepancy among the ET measurement techniques underscores the difficulty in collecting reliable measurements under strongly advective conditions. It also raises awareness of the uncertainty associated with in-situ techniques and the need for caution when using such data for model validation.

Technical Abstract: Discrepancies can arise among surface flux measurements collected using disparate techniques due to differences in both the instrumentation and theoretical underpinnings of the different measurement methods. Using data collected primarily over a pair of irrigated cotton fields as a part of the Bushland Evapotranspiration and Remote Sensing Experiment (BEAREX08), flux measurements collected with two commonly-used methods, eddy covariance (EC) and lysimetry (LY), were compared and substantial differences were found. Daytime mean differences in the flux measurements from the two techniques were in excess of 200 W/m^2 under strongly advective conditions. Three causes for this disparity were found: (i) the failure of the eddy covariance systems to fully balance the surface energy budget, (ii), flux divergence due to the local advection of warm, dry air over the irrigated cotton fields, and (iii) the failure of lysimeters to accurately represent the surface properties of the cotton fields as a whole. Regardless of the underlying cause, the discrepancy among the flux measurements underscores the difficulty in collecting these measurements under strongly advective conditions. It also raises awareness of the uncertainty associated with in-situ micrometeorological measurements and the need for caution when using such data for model validation or as observational evidence to definitively support or refute scientific hypotheses.