Below Canopy Evapotranspiration in a Rain-Fed Vineyard in the Southeastern U.S.

The architecture of wine-grape vineyards is characterized by tall plants (~ 1.5 m) and widely spaced rows (~ 3 m). This wide row spacing, developed to allow sunlight interception, air flow, and field operations, creates a complex system for water and energy budgets. Because of the wide row spacing, growers must consider two distinct parts of the vineyard system for management: the vine row and the inter-rows. In vineyards where annual water scarcity is not a concern, the inter-rows are often maintained with grassed surfaces. The grass transpires soil water throughout its growth cycle; water transpired by the grass may be as much as double the water transpired by the grape vines. This may be beneficial during periods under excess water, but can be detrimental to water availability during periods when rainfall is scarce, particularly during flowering. The large quantity of water transpired below the canopy by the grass also may elevate canopy humidity, leading to enhanced disease pressures. Understanding of water and energy dynamics in the grassed inter-row, and in the bare soil below the vine, is important for establishing proper management of the vineyard. However, developing this understanding is challenging because of the complex canopy architecture. Experiments were conducted in a humid, rain-fed vineyard in North Carolina with grassed inter-rows. The general objective for these experiments is to quantify evapotranspiration from the inter-row and direct evaporation below the vines. Micro Bowen ratio systems and eddy covariance systems continuously measured energy and water fluxes below and above the grape canopy, respectively. For short-duration experiments, below-canopy potential evapotranspiration (PET) and incident radiation were monitored at ground level with micro-pans and pyranometers, respectively, spread along transects from vine row to adjacent vine row. Here, we focus on the relationships between canopy shading and PET below the canopy. Results show a clear effect of shading on below canopy PET, with distinct and different periods of high and low daytime PET at mid-inter-row, quarter-row and row positions in the N-S oriented rows. The consequences of below canopy observations for whole (vineyard) system water and energy budgets will be highlighted.