Intermittency of Water Vapor Exchanges, and Their Role in ET of Vineyards in California

Uncertainties and limitations of water resources in the Western US direct attention to water consumed by agriculture. Nowhere is this more evident than in California, with the huge diversity of crops, many of which are high value. Irrigated vineyards producing wine grapes are growing rapidly in the Central Valley in a hot and dry summer climate. Optimizing irrigation of vineyards between water available and quantity and quality of product requires reliable simulations of daily ET. A current remote-sensing based mechanistic model is being tested for irrigated vineyards. But ultimate refinement the model requires understanding the biophysical processes that govern ET of vineyards. One important process is the interaction of the complex geometries of vineyards with variations in wind creating distortions in the turbulence exchange of water vapor.

Typical summer weather patterns result in periods of light winds and strongly convective conditions. When combined with the mechanical turbulence related to wind vs. row directions, the resulting water vapor exchange process (ET) becomes very transient and episodic. The objective here is to quantify the transient and intermittent behavior of venting of water vapor from vineyard canopies, and its role in affecting the hourly to daily ET values. Sonic anemometers and fast response humidity sensors are located above and within the canopy, with their measurements synchronized on a single data logger. The Pinot Noir vineyards in the study are near Lodi, CA.

Addressing the above requires first, quantifying intermittency of water vapor and temperature exchanges between inside the canopy and air above, and how such transience affects the time averaged flux values for the whole canopy. Published approaches such as “telegraphic approximation”, as well as others will be used to assess the importance of episodic events in the ET. Second, to document the phase shifts between water vapor in and above the canopy, and they connect to the total time average ET. Finally, to examine what the results imply for the flux-gradient schemes used by remote sensing of ET models such as the “two source” model.