Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2009
Publication Date: 2/15/2010
Citation: Scott, R.L. 2010. Using watershed water balance to evaluate the accuracy of eddy covariance evaporation measurements for three semiarid ecosystems. Agricultural and Forest Meteorology. 150:219-225. Interpretive Summary: Measurements of the energy and mass exchange at the land-atmosphere interface are critical for determining local, regional and global budgets, model testing, and understanding ecosystem processes. The eddy covariance technique has become the “gold-standard” to quantify these exchanges. So, there is a continual need to assess the accuracy of these measurements as opportunities arise. This study evaluates the accuracy of evaporation measurements by eddy covariance by comparing them to an evaporation estimate derived from a seasonal or annual water balance estimate over nearly twelve years and at three sites in Arizona, USA. Though there were differing results at each site, results indicate that eddy covariance estimates were highly accurate with a mean disagreement of less than 3% in comparison with the watershed estimates. This study gives confidence to the accuracy of the data collected at these sites and to the equipment and procedures used with this measurement technique.
Technical Abstract: The eddy covariance (EC) technique is a widely-used and accepted method to quantify ecosystem-scale mass and energy fluxes. Measurements of evaporation from EC are used to determine local, regional and global water budgets, calibrate and validate land surface models, and acquire understanding of ecosystem processes. This paper assesses the accuracy of EC evaporation measurements on a seasonal to annual basis by comparing them with those derived from small watershed water balances. Using nearly 12 years of data from a shrubland, grassland and savanna sites in southern Arizona USA, the two independent measures agreed to within an average of 3% annually, ranging from a -10 to +17% in any given year, when we accounted for an assumed 5% underestimation in precipitation due to gage undercatch. The agreement between the two measures was generally better in drier years and at less topographically complex sites. Despite an indication of a systematic underestimate of evaporation by a commonly-used assessment of the energy balance, the method of forcing energy balance closure on evaporation generally led to worse results.