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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Hydrology and Remote Sensing Laboratory » Research » Publications at this Location » Publication #274806

Title: How representative is a point? The spatial variability of surface energy fluxes across short distances in a sand-sagebrush ecosystem

item Alfieri, Joseph
item BLANKEN, P - University Of Colorado

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2012
Publication Date: 12/1/2012
Citation: Alfieri, J.G., Blanken, P.D. 2012. How representative is a point? The spatial variability of surface energy fluxes across short distances in a sand-sagebrush ecosystem. Journal of Arid Environments. 87:42-49.

Interpretive Summary: Often, when the numerical and remote sensing-based models that are used to estimate evapotranspiration (ET) are evaluated, they are tested against micrometeorological measurements that are assumed to accurately represent the larger region. Understanding the degree to which this assumption is true is important because the accuracy, and thus usefulness, of these models can be limited by uncertainty in the “ground truth” data. In turn, this can negatively impact agricultural water use and irrigation management decisions that rely on information from these models. In order to ascertain the validity of this assumption, ET data collected in a sand-sagebrush ecosystem was used. By comparing measurements collected at a series of locations within 64 m of one another, it was found that statistically significant differences in ET, as well as the exchange of other quantities, can occur. It was also found that these variations in ET are linked to differences in surface properties such as vegetation density and soil moisture. These results indicate that micrometeorological measurements are not necessarily representative of the larger surrounding region, underscoring the need to develop a robust suite of tools for assessing how well field measurements reflect conditions in the surrounding landscape.

Technical Abstract: During the summer of 2001, the spatial variation of the surface energy fluxes across short distances, 16 m to 32 m, was examined over a sagebrush steppe ecosystem in the northeastern Colorado. Two eddy covariance micrometeorological stations were used to test the hypothesis that fine-scale variations in the physical properties of the site result in significant variation in the surface energy balance. Through a comparative analysis of the flux measurement, it was found that statistically significant variations in the sensible, latent and soil heat fluxes were present at the study site. These variations were linked to small changes in the near-surface soil moisture content and leaf area index. The results of this study suggest there may be substantial uncertainty surrounding a single point measurement when it is used to represent the exchange of heat and moisture over a large area. This uncertainty must be considered when using in-situ measurements to evaluate remote sensing products or numerical models.