Contrasting Methods for Measuring Evapotranspiration in Soybean.

Crop scientists are often interested in canopy rather than leaf water fluxes. Canopy measurements are difficult to obtain because instrumentation is expensive, investigations require a high level of comprehension in micrometeorological methods, and treatment comparisons are usually limited. The objective of this research was to contrast measurement of evapotranspiration (ET) using eddy covariance methods compared to summing the evaporation (E) and transpiration (T) components of ET in a soybean canopy during reproductive development. Soil water evaporation was estimated using weighable microlysimeters by mass balance and T was estimated using leaf gas exchange on sunlit and shaded leaves. Summing E+T always produced higher estimates than eddy covariance. Differences ranged from 0.71 to 1.50 mm or from 21 to 46%. Soil water evaporation ranged from 4 to 11% of the total E+T on the five measurements dates. The largest difference between methods (46%) was measured on a day with the lowest net radiation and vapor pressure deficit, lower wind speed, and moderate turbulence intensity.