Intermittency of water vapor fluxes from vineyards during light wind and convective conditions

• Published in: Irrigation science Vol. 37; no. 3; pp. 281 - 295
• Main Authors: Los, Sebastian A., Hipps, Lawrence E., Alfieri, Joseph G., Kustas, William P., Prueger, John H.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-05-2019; Springer Nature B.V
• Subjects: Agriculture; Aquatic Pollution; Arid climates; Arid regions; Arid zones; Atmospheric models; Biomedical and Life Sciences; Biophysics; Canopies; Canopy; Climate Change; Covariance; Environment; Evapotranspiration; Exchanging; Fluxes; Life Sciences; Light; Modelling; Original Paper; Remote sensing; Resource management; Semi arid areas; Semiarid climates; Semiarid lands; Sustainable Development; Transport; Turbulence; Vineyards; Waste Water Technology; Water Industry/Water Technologies; Water Management; Water Pollution Control; Water resources; Water resources management; Water vapor; Water vapour; Wind; Winds; Wineries & vineyards
• ISSN: 0342-7188; 1432-1319
• DOI: 10.1007/s00271-018-0617-5

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.