Below-Canopy Water and Energy Transfer in a North Carolina Vineyard.

HOLLAND, STEPHEN. Below-Canopy Water and Energy Transfer in a North Carolina Vineyard. (Under the direction of Joshua Heitman). Producing European grape varieties for wine making is a burgeoning, but challenging industry in the southeastern U.S. High precipitation and soil with poor internal drainage lead to high humidity that, in turn, creates ideal conditions for fungal diseases. Without adaptations to management practices to reduce canopy humidity, fungal diseases will continue to limit the local wine grape industry. Altering inter-row management may offer a solution. We hypothesized that inter-row crops, typically fescue in southeastern vineyards, serve as a pump extracting water from the soil more efficiently than evaporation from bare surface conditions. This increases below canopy humidity because of limited mixing between the near-surface microclimate and the atmosphere above the canopy. The following objectives seek to test this hypothesis; 1) develop methodology to continuously measure water fluxes from the vineyard inter-row, 2) estimate and compare surface water vapor flux under i) bare soil conditions and ii) fescue cover crop conditions in the inter-row, and 3) determine effects of surface water vapor flux on canopy humidity. Measurements of below canopy water vapor flux were collected based on surface energy balance with a novel microBowen ratio (MBR) system in a commercial vineyard. ET rates were well correlated between MBR systems and micro-lysimeters (R 2 = 0.87, slope = 1.13) and between the MBR systems and eddy covariance measurements (R 2 = 0.80, slope = 0.80). MBR results indicate that the vapor flux is greater in the fescue inter-row compared to the bare soil inter-row (e.g. 32% greater in May). As a result of greater inter-row evapotranspiration (ET), the soil water content was lower in the fescue inter-row. The soil water content was not, however, depleted sufficiently to beneficially compete with the grapevines for soil water. This was further supported with grapevine water stress measurements, which indicated no stress for grapevines with fescue inter-rows. Direct measurements of relative humidity (RH) and a simple, one-dimensional model were used to determine the effects of the surface vapor flux on the canopy humidity. The measured and modeled RH were significantly correlated and both showed an increase in RH above the fescue inter-row compared to bare soil (2% and 5%, respectively, during the growing season), due to greater ET. Model results were assumed to be a better representation of actual effects of ground cover because of constraints imposed by plot size on measurements. The increased RH above the fescue inter-row suggests that fescue may impose a greater risk to grapevine health. Altering management practices to decrease the amount of water entering the canopy atmosphere, such as changing the width of the grass inter-row or using grass with lower ET, may lead to a more productive vineyard. Below-Canopy Water and Energy Transfer in a North Carolina Vineyard by Stephen Edwin Holland A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science