Evapotranspiration: Pitfalls to Avoid and Why It’s Easier Than You Think

Measuring evapotranspiration (ET) to understand water loss from a native or a managed ecosystem is easier than it looks, but you have to know what you’re doing.

Mistakes that kill your estimates

Measuring evapotranspiration (ET) to understand water loss from a native or a managed ecosystem is easier than it looks, but you have to know what you’re doing. If you can’t spend the time or money on a full eddy-covariance system, you'll have to be satisfied with making some assumptions using equations such as Penman-Monteith.

Like any model, the accuracy of the output depends on the quality of the inputs, but do you know what measurements are critical for success? Plus, as your instrumentation gets more inaccurate, the errors get larger. If you’re not careful, you can end up with no idea what’s happening to the water in your system.

      Get the right number every time

      You don’t have to be a meteorologist or need incredibly expensive equipment to measure ET effectively. In this 30-minute webinar, Campbell Scientific application scientist Dr. Dirk Baker and METER research scientist Dr. Colin Campbell team up to explain:

      • The fundamentals of energy balance modeling to get ET
      • Assumptions that can simplify sensor requirements
      • What you must measure to get adequate ET estimates
      • Assumptions and common pitfalls
      • How accurate your equipment should be for good estimates
      • Causes and implications of uncertainty

      Next steps


      Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum. 


      • Blonquist Jr, J. M., R. G. Allen, and Bruce Bugbee. "An evaluation of the net radiation sub-model in the ASCE standardized reference evapotranspiration equation: Implications for evapotranspiration prediction."Agricultural water management 97, no. 7 (2010): 1026-1038. (Article link)
      • Fuchs, Marcel, Ehud Dayan, David Shmuel, and Isaac Zipori. "Effects of ventilation on the energy balance of a greenhouse with bare soil." Agricultural and Forest Meteorology 86, no. 3-4 (1997): 273-282. (Article link)
      • Ham, J. M. "Uncertainty analysis of the water balance technique for measuring seepage from animal waste lagoons." Journal of environmental quality 31, no. 4 (2002): 1370-1379. (Article link)
      • Kimball, Bruce A., Kenneth J. Boote, Jerry L. Hatfield, Laj R. Ahuja, Claudio Stockle, Sotirios Archontoulis, Christian Baron et al. "Simulation of maize evapotranspiration: an inter-comparison among 29 maize models." Agricultural and Forest Meteorology 271 (2019): 264-284. (Article link)


      Dr. Dirk V. Baker has been with Campbell Scientific since 2011 and is an Application Research Scientist in the Environmental Group. Areas of interest include ecology, agriculture, and meteorology—among others. He has a bachelor's degree in wildlife biology and a doctorate in weed science, both from Colorado State University. Dirk’s graduate and postdoctoral research centered around measuring and modeling wind-driven plant dispersal.

      Dr. Colin Campbell has been a research scientist at METER for 20 years following his Ph.D. at Texas A&M University in Soil Physics. He is currently serving as Vice President of METER Environment. He is also adjunct faculty with the Dept. of Crop and Soil Sciences at Washington State University where he co-teaches Environmental Biophysics, a class he took over from his father, Gaylon, nearly 20 years ago. Dr. Campbell’s early research focused on field-scale measurements of CO2 and water vapor flux but has shifted toward moisture and heat flow instrumentation for the soil-plant-atmosphere continuum.


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