Historical Perspective on the Science and Estimation of Evapotranspiration for Operational Water Management, Systems Design, Research, and Monitoring—Successful Evolutions (Centennial)

The presentation will provide historical and modern perspectives on ET estimation and measurement. It will include a history on the traditional estimation of ET from ground-based point observations and evolution of parameterizations and data requirements. This description will include the current FAO56 dual crop coefficient – reference ET approach and its history and modern-day use in operational ET estimation. A review will be made of the FAO and ASCE standardizations for reference ET and why they have been widely accepted internationally. In addition, the dependence of reference ET method performance on the measurement of weather data in well-watered settings is important, along with means to mitigate when weather data site conditions are not ideal.

The dual crop coefficient approach provides an empirical, but consistent and rational method for partitioning total ET between evaporation and transpiration fluxes that is applicable to operational estimation and that requires little or no calibration. A brief mention will be made of a few example operational models that employ the dual crop coefficient method and that are used for water rights litigation, mitigation and management, namely the University of Idaho ETIdaho and U.S. Bureau of Reclamation ETDemands models as well as the SIMS remote sensing model.

The presentation will reflect on historical measurement of ET as it pertains to development and calibration of ET estimation methods. Both measurement and estimation descriptions will emphasize modern desires for accuracy, data integrity and adherence to physics, as well as supporting ease of use and repeatability. Common mistakes made during operation and management of lysimeters and soil moisture measurement will be described.

A section of the presentation will make brief mention of applying the single-layer Penman-Monteith ET method and multi-layer or multi-source models directly to specific vegetation types, including challenges in consistent parameterization and availability of meteorological data that reflects the equilibrium boundary layer above the surface being studied.

Advances in remote sensing of water flux will be briefly described, especially where they utilize some traditional methods or principles for calibration or interpolation over time. The case for thermal remote sensing and thermal satellite platforms to support applications at the field-scale will be made.