Apparent Errors in Sap Flow Measurements at High Transpiration Rates by Modified Heat-balance Gauges on Woody Vine (Vitis) Stems.

摘要

Heat-balance sap flow gauges were configured to produce a more thermally uniform stem cross-section under high flow rates. On mature grapevines (Vitis labruscana; leaf area per vine < or = 27 m2) either undisturbed in the field or transplanted to large containers (ca. 1 m3 volume) that were monitored on a balance, custom-built gauges (stem diam. < or = 46 mm) under variable power were run simultaneously with a whole-plant gas exchange system. Transpiration (Tr) varied from about 80 g h1 overnight to daily maxima approaching 2100 g h1 (container vines) and 2700 g h1 (field vines). In container vines, instantaneous estimates of Tr (6.00 to 20.00 local standard time [HR]) agreed most closely between gas exchange and gravimetry (Spearman Rank R = 0.96; p<0.05). Estimates of Tr were correlated significantly between sap flow and gas exchange (R = 0.94; p<0.05), and between sap flow and gravimetry (R = 0.88; p<0.05). On average, cumulative estimates of Tr over 24h (17 to 19 L d1) agreed within 5% (gravimetry vs. gas exchange) and within 17% (sap gauge vs. gas exchange or gravimetry). Instantaneous estimates of Tr for field vines were correlated significantly (p<0.05) between sap flow and gas exchange (R = 0.87). Cumulative water use per vine was 18 to 22 L d-1; estimation methods differed on average by 15%. In both container and field vines, sap flow gauges underestimated high mid-day rates of Tr consistently, due in part to the choice of power input. The open-loop power control algorithm maintained upstream-downstream surface temperature differences between 1.5 and 2pC during midday, but the algorithm was not responsive enough to the very rapid rate of change in Tr for about two hours during the morning, resulting in anomalous overestimates of Tr.