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Instrumentation

Evapotranspiration Measurement in Controlled Environment Chambers

A Comparison between Time Domain Reflectometry and Accumulation of Condensate from Cooling Coils

^a USDA-ARS Crop Systems and Global Change Lab, Bldg. 001, Room 342 BARC-W, 10300 Baltimore Ave., Beltsville, MD 20705-2350
^b Univ. of Washington, Seattle, WA 98195
^c USDA-ARS Cropping Systems Research Lab., P.O. Box 909, Big Spring, TX 79720

^* Corresponding author (Dennis.Timlin{at}ars.usda.gov)

Received for publication December 16, 2005. The measurement of water fluxes from canopy and soil surfaces is performed in sunlit controlled environment chambers by measuring condensate draining from cooling coils in a constant humidity environment. This provides a direct measure of evapotranspiration (ET). However, in growth chambers with soilbins, this does not give information on soil water status or root activity. The objective of this study was to compare ET measurements from the condensate system with ET calculated from measurements of water content by TDR. Data from an irrigation x carbon dioxide (CO₂) study on potato (Solanum tuberosum L.) were used for this study. The soil water contents in the growth chamber soilbins were monitored once an hour at five vertical depths with three measurement locations per depth using an automated TDR system. The correspondence between daily ET rates for the two systems was good. Maximum daily ET rates were near 6.1 to 7.1 mm cm^–2 d^–1 (7–8 L d^–1 on a chamber basis) and differences were on the order to 0.89 to 1.8 mm cm^–2 d^–1 (1–2 L d^–1). At the higher daily ET rates, the daily values from the two methods were closer. The correspondence between hourly measurements of ET measured from the condensate system and calculated from TDR water contents was poor due to instrument and soil variability. A significant source of error was vertical variation in water content in the soil between horizontally placed TDR probes, especially during irrigation events. Evapotranspiration estimates from TDR measurements were much more robust for calculation of water use over a period of time. Data from the condensate system were most useful for quantification of diurnal transpiration rates and were better correlated with radiation.

Abbreviations: DAE, days after emergence • ET, evapotranspiration • PAR, photosynthetically active radiation • RMSE, root mean square error for the regressions • SPAR, soil plant atmosphere research • TDR, time domain reflectometry

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