Contrasting methods for estimating evapotranspiration in soybean

Crop scientists are often interested in canopy rather than leaf water estimates Comparing canopy fluxes for multiple treatments using micrometeorological approaches presents limitations because of the large fetch required The goal of this study was to compare leaf-scale to field-scale data by summing soil water evaporation (E) and leaf transpiration (T) versus ET using tower eddy covariance (EC) and scaling leaf transpiration to the canopy level using a two-step scaling approach in soybean [Glycme max (L) Merr I Soybean transpiration represented 89-96% of E+T when combining the soil water evaporation with leaf transpiration on the five measurement days during reproductive growth Comparing E+ Tversus ET from the EC system the E+T method overestimated ET from 068 to 1 58 mm In terms of percent difference the best agreement between the two methods was 15% on DOY 235 and the worst agreement occurred on DOY 234(41%) A two-step scaling method predicted average ET within 001 mm of the EC ET between 10 00 and 14 15 on an hourly time-step on DOY 227 under uniform sky conditions and average Er within 0 03 mm of the EC ET on DOY 235 under intermittent sky conditions between 10 00 and 15 15 Pooling the scaled-leaf data and comparing them with the measured EC ET data exhibited a strong linear relationship (r = 0 835) after accounting for bias (6%) Findings from this study indicate satisfactory results comparing absolute differences are likely not obtainable by summing leaf transpiration with soil water evaporation to calculate canopy water fluxes However scaling leaf transpiration provided a robust measure of canopy transpiration during reproductive growth in soybean under these conditions and merits additional study under different climatic and crop conditions Published by Elsevier B V