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

Authors: LOS, S. - Utah State University; HIPPS, L.E. - Utah State University; Alfieri, Joseph; Kustas, William - Bill; Prueger, John

Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2018
Publication Date: 1/9/2019
Publication URL: http://handle.nal.usda.gov/10113/6456689
Citation: Los, S., Hipps, L., Alfieri, J.G., Kustas, W.P., Prueger, J.H. 2019. Intermittency of water vapor fluxes from vineyards during light wind and convective conditions. Irrigation Science. 37(3):281-295. https://doi.org/10.1007/s00271-018-0617-5.
DOI: https://doi.org/10.1007/s00271-018-0617-5

Interpretive Summary: Remote sensing is the most viable method for monitoring evaporative water loss or evapotranspiration (ET) on regional scales. These data are critical to determining crop irrigation needs, informing water resource management, and ensuring vineyard sustainability. However, because of the unique structure of vineyards, many of the modeling methods that have been developed to determine ET from other crops may not accurately reflect ET from vineyards. To improve the remote sensing-based methods to determine ET, this study focuses on better understanding how moisture transported away from the plant canopy is influenced by shape of the canopy itself. Specifically, it seeks to understand how consistent ET from vineyards is over time. This is important because many remote sensing-based models assume the rate of water loss does not vary over time. Using data collected as a part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX), this study found that moisture becomes trapped beneath the vines under calm conditions and, as a result, ET from vineyards can be quite episodic with as much as 90% of the hourly ET occurring over a total of only 15 minutes. This differs from the underlying assumptions of many ET modeling systems and is a potential source of error in the model estimates of ET.

Technical Abstract: Vineyards in many semi-arid regions globally face limited water resources. Monitoring crop evapotranspiration (ET) is critical for water resource management, but remains difficult due to the complex biophysics. Both measurement and modeling methods for estimating turbulent water vapor transport rest upon 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 presents inherent factors, including canopy spacing/structure and frequent periods of light wind, unstable conditions that can create very 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). Periods of distinct and intermittent turbulence and associated episodic canopy venting were observed during periods of light winds and highly convective conditions. Power and cross spectra for intermittent periods exhibited large influence of low frequency events compared to periods of steadier exchange. Intermittent periods were found to require a much longer flux averaging period (~2 hours) than more steady periods. Episodic exchange events were isolated and summed to determine their relative contribution to the overall water vapor flux for given periods. As much as 90% of the vertical water vapor transport occurred in only about 25% of the total time during intermittency, while in more stationary periods 99% of the flux occurred across 80% of the total period. These results have implications for models, which rely on time-averaged vertical gradients, during these conditions.