Intermittency of water vapor fluxes from vineyards during light wind and convective conditions
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Vineyards in many semi-arid regions globally face limited water resources. Monitoring evapotranspiration (ET) of vineyards is critical for water resource management, but remains difficult due to the complex biophysics of the surfaces. Both measurement and modeling approaches for estimating turbulent water vapor transport rely on implicit assumptions that exchanges occur in a reasonably regular fashion over the time scales generally used for averaging. However, heterogeneous vegetation in semi-arid climates, such as many vineyards, presents inherent factors, including canopy row/row space structure and frequent periods of light wind, unstable conditions, that can create episodic transport characteristics. Eddy covariance data were collected above and within the canopy of two vineyards in the Central Valley of California during the Grape Remote sensing Atmospheric Profile & Evapotranspiration eXperiment (GRAPEX). The goal was to document and quantify the existence of intermittent turbulence transport of water vapor, and associated episodic canopy venting. These effects were found to correlate with periods light winds and highly unstable/convective conditions. Power and cross-spectra for intermittent periods documented enhancement of low-frequency water vapor exchange events compared to more steady periods, and diminished time scale correlation between humidity within the canopy and above the canopy. Analyses show that intermittent cases can necessitate longer flux-averaging periods (up to 2 h) than more steady conditions. Episodic exchange events were isolated and summed to determine their relative contribution to the overall water vapor flux. Since light wind, unstable conditions are relatively common in many arid vineyard regions, these findings have implications for mechanistic ET models that rely on time-averaged vertical gradients, which implies reasonably steady transport.
Funding was provided by Utah Agriculture Experiment Station Project UTAO 1186 and NASA Grant #NNX17AF51G. Much of the data collection during GRAPEX IOPs was made possible through funding provided by E.&J. Gallo Winery. In addition, we would like to thank the staff of the Viticulture, Chemistry and Enology Division of E.&J. Gallo Winery for their logistical support as part of the GRAPEX project. Finally, this effort would not have been possible without the cooperation of Mr. Ernie Dosio of Pacific Agri Lands Management, along with the Borden vineyard staff, for logistical support of GRAPEX field and research activities. USDA is an equal opportunity provider and employer.
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Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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