The Impacts of Canopy Structure on the Turbulent Fluxes over Vineyards

Grapes are not only the largest specialty in California, where there are more than 325,000 ha of vineyard, they are also a commodity with substantial economic importance. California wineries, for example, employ more than 300,000 people and contribute in excess of $60B to the state's economy each year. Grape production is also one of the largest agricultural consumers of scarce water resources. Understanding the factors impacting evaporative water loss in vineyards is critical in order to develop the evaluation tools and monitoring techniques needed to assess the water needs of grape growers and ensure water is managed effectively. Using a combination of eddy covariance and other data collected at two adjacent vineyards located in northern California, this study investigated the role of the unique canopy structure of vineyards on turbulent transport and exchange processes including evapotranspiration. While the ambient environmental conditions were similar at the two sites, one of the vineyards contained mature pinot grapes while the other had much younger vines. Both contained a cover crop in the inter-row. With the exception of the periods following rain events and prior to vine leaf emergence , a comparison of the turbulent energy fluxes at the two sites indicated that the latent heat flux (λE) and evaporative fraction (E_f) were consistently greater at the mature site than in the young vineyard. On average, the daytime λE at the mature vineyard was 35 W m^-2 or 27% greater than the flux at the young vineyard; correspondingly, E_f was 0.63 at the mature vineyard compared 0.53 at the young vineyard. Commensurate differences were observed in both the sensible heat and carbon dioxide fluxes. These differences appear to be linked to the greater vegetation density of the mature vineyard which facilitates increased transpiration while sheltering the underlying surface. This sheltering is likely to hinder both soil evaporation and transpiration from the cover crop while impeding the transport of moisture away from the surface.