SOIL AND CANOPY ENERGY BALANCES IN A WEST TEXAS VINEYARD
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Water use in vineyards is controlled by energy absorbed by plants and the soil surface. An 8 day field experiment was conducted in a commercial vineyard near Lamesa, TX, to evaluate soil and canopy energy balances, and to examine energy exchange between canopy and soil. Grapevines in the vineyard were wrapped tightly to trellis wires, creating compact 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 total latent heat flux (E). Latent heat flux from the canopy (EC) was determined from sap flow measurements of transpiration. Soil latent heat flux (ES) was calculated as the difference between E and EC. These measurements were combined with measurements of soil net irradiance to partition the vineyard energy balance into soil and canopy components. During the study, ES accounted for 44-68% of E. Unstable conditions predominated during the study, with the soil generating sensible heat that was transferred to the canopy, producing values of EC that were greater than canopy net irradiance. Within-row advection of sensible heat was 17-36% of EC. 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 created an unusual diurnal pattern of canopy net irradiance, having midmorning and midafternoon peaks, and a low midday plateau. Morning and afternoon peaks occurred during times of maximum direct beam irradiance on east and west sides of the hedgerows. Results also showed that within-canopy wind speed and air temperature were affected by wind direction. 1994.
