|Kustas, William - Bill|
|Evett, Steven - Steve|
|NEALE, C - Utah State University|
|CHAVES, J - Colorado State University|
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 12/13/2013
Publication Date: 4/7/2014
Citation: Alfieri, J.G., Kustas, W.P., Prueger, J.H., Evett, S.R., Neale, C., Chaves, J.L. 2014. The factors influencing field-scale measurements of evapotranspiration [abstract]. International Symposium on Evapotranspiration. 2014 CD-ROM.
Technical Abstract: Accurate estimates of evapotranspiration (ET) are critical to maximizing the efficient use of water for agricultural production which, particularly in arid and semi-arid environments, is the largest consumptive user of fresh water. Although numerous techniques are available for determining ET in-situ, the resulting measurements often differ significantly. Focusing on three methods – eddy covariance (EC), lysimetry (LY), and neutron probe/mass balance (NP) – this study sought to identify the main factors causing discrepancies in the ET measurements. The data used in this study were collected over adjacent irrigated cotton fields in the Texas High Plains as a part of the 2008 Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX08). Each field was instrumented with a pair of EC systems, a large weighing lysimeter, and four access tubes for neutron probe measurements. Initial comparisons of the ET measurements by the EC and LY showed differences of up to 0.30 mm hr-1 (200 W m-2). Three key factors contributing to these differences were identified: i) the imperfect closure of the surface energy budget by the EC system; ii) the failure of the EC system to fully capture advective effects; and, ii) the inability of LY to represent the surface conditions of the fields as a whole. After accounting for these effects, the discrepancy between the EC and LY measurements was typically 0.04 mm hr-1 (25 W m-2). Additional analyses of the cumulative water balance from EC (corrected for closure issues), LY, and NP showed that LY tended to overestimate cumulative ET by 10 to 35 mm compared to the EC and NP measurements. These differences were directly linked to variations in the vegetation density within the footprint of the different measurement techniques. The results of this study demonstrated the impacts of fine-scale spatial variability on measurements of ET while underscoring the limitations of each of these measurement techniques.