Intermittent Water Vapor Exchanges and Their Role in Vineyard Evapotranspiration:

Semi-arid regions globally face uncertainties and limitations on water resources which focus attention on water utilized by agriculture. This is epitomized by California’s Central Valley where a diverse set of high value crops, such as Vitus vinifera, is cultivated under high evaporative demand while stresses on water resources have mounted. Given these factors and known beneficial relationships between plant stress and fruit quality, there is a need for daily, field-scale evapotranspiration (ET) estimates for vineyard irrigation management. Modeling land-plant-atmosphere water vapor exchanges above sparsely vegetated row crops such as these, however, has proven difficult because of inherent complexities. A remotely sensed ET modeling system, developed by the USDA-ARS, has been successfully tested over a number of vegetated surfaces and is now being validated over two vineyards in the Central Valley of California through the Grape Remote sensing Atmospheric Profiling Evapotranspiration eXperiment (GRAPEX).

Unknowns remain, however, regarding biophysical processes within vineyards that govern ET. The sparse, elevated canopy architecture within vineyards causes shear generated turbulence to have complex dependencies on wind direction versus row orientation. Furthermore, growing season weather frequently consists of light wind and highly convective daytime conditions which causes turbulence to be dominated by transient, low frequency events. Combined these factors result in water vapor exchanges that become intermittent and episodic. Given these factors, it is not known how aggregated fluxes of heat and water vapor across an hour or a day are affected by intermittent exchanges. Specifically, there are questions regarding how gradient relationships in the two source energy balance approach within the ET modeling system are able to capture this intermittency.

Research is presented which analyzes GRAPEX sonic anemometer and fast response humidity sensor data from within and above the vineyard canopy. This work investigates the complexities of vineyard transport processes through three main components. The first characterizes heat and vapor exchanges between the in-canopy and above-canopy layers across the range of atmospheric conditions present during the growing season through fourier and wavelet transform methods. The second component focuses on the exchanges during light wind convective periods. This includes documenting spectra phase shifts of water vapor between in-canopy and-above canopy air, determining minimum energy eddies required to vent the canopy during intermittency, and scaling low frequency events with atmospheric boundary layer depth.  The final component is comprised of the application of published approaches from non-linear dynamics, such as recurrence plots, to gain further information on the intermittent behavior of the vineyard system.