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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #248493
Title: Contrasting methods for estimating evapotranspiration in soybean

Author
item Singer, Jeremy
item HEITMAN, J - North Carolina State University
item Hernandez Ramirez, Guillermo
item Sauer, Thomas
item Prueger, John
item Hatfield, Jerry

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2010
Publication Date: 10/18/2010
Citation: Singer, J.W., Heitman, J.L., Hernandez Ramirez, G., Sauer, T.J., Prueger, J.H., Hatfield, J.L. 2010. Contrasting methods for estimating evapotranspiration in soybean. Agricultural Water Management. 98(1):157-163.

Interpretive Summary: Crop scientists are often interested in canopy rather than leaf water fluxes. Comparing canopy fluxes for multiple treatments using micrometeorological approaches presents limitations because of the large area (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 evapotranspiration (ET) using tower eddy covariance (EC) and scaling leaf transpiration to the canopy level using a two-step scaling approach in soybean. Soybean transpiration represented 89 to 96% of E+T when combining the soil water evaporation with leaf transpiration on the five measurement days during reproductive growth. Comparing E+T versus ET from the EC system, the E+T method overestimated ET from 0.68 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 EC ET well on four days with a bias of 6%. Findings from this study indicate 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. Developing alternative methods to estimating canopy water fluxes may help crop scientists compare canopy water use efficiency among several treatments.

Technical Abstract: Crop scientists are often interested in canopy rather than leaf water fluxes. 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 evapotranspiration (ET) using tower eddy covariance (EC) and scaling leaf transpiration to the canopy level using a two-step scaling approach in soybean [Glycine max (L.) Merr.]. Soybean transpiration represented 89 to 96% of E+T when combining the soil water evaporation with leaf transpiration on the five measurement days during reproductive growth. Comparing E+T versus ET from the EC system, the E+T method overestimated ET from 0.68 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 0.01 mm of the EC ET between 1000 and 1415 on an hourly time-step on DOY 227 under uniform sky conditions and average ET within 0.03 mm of the EC ET on DOY 235 under intermittent sky conditions between 1000 and 1515. 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.