SOIL AND CANOPY ENERGY BALANCES IN A WEST TEXAS VINEYARD

Water use in vineyards is controlled by energy absorbed by plants and the soil surface. An 8 day field experiment was conducted in a commerc ial vineyard near Lamesa, TX, to evaluate soil and canopy energy balan ces, and to examine energy exchange between canopy and soil. Grapevine s in the vineyard were wrapped tightly to trellis wires, creating comp act hedgerows that were 3 m apart and of 1.6 m height and 0.4 m width, with little foliage below 1 m above the soil surface. The Bowen ratio method was used to measure the vineyard energy balance including tota l latent heat flux (lambda E). Latent heat flux from the canopy (lambd a E(c)) was determined from sap flow measurements of transpiration. So il latent heat flux (lambda E(s)) was calculated as the difference bet ween lambda E and lambda E(c). These measurements were combined with m easurements of soil net irradiance to partition the vineyard energy ba lance into soil and canopy components. During the study, lambda E(s) a ccounted for 44-68% of lambda E. Unstable conditions predominated duri ng the study, with the soil generating sensible heat that was transfer red to the canopy, producing values of lambda E(c) that were greater t han canopy net irradiance. Within-row advection of sensible heat was 1 7-36% of lambda E(c). Although the canopy was cooler than within- and above-canopy air, it was not a strong enough sink for sensible heat to produce stable conditions above the canopy. The narrow hedgerows crea ted an unusual diurnal pattern of canopy net irradiance, having midmor ning and midafternoon peaks, and a low midday plateau. Morning and aft ernoon peaks occurred during times of maximum direct beam irradiance o n east and west sides of the hedgerows. Results also showed that withi n canopy wind speed and air temperature were affected by wind directio n.