USGS Washington Water Science Center
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WA400 - Estimating Evapotranspiration at Klamath Marsh, Oregon, and Lower Klamath National Wildlife Refuge, California - Completed FY1999
Problem - Estimates of evapotranspiration are required for the purpose of evaluating ecosystem water balances. For many ecosystems, more water is lost by evapotranspiration than is lost by any other process. Because of this, reliability of water-balance assessments for areas such as Klamath Marsh and Lower Klamath National Wildlife Refuge hinges upon the reliability of evapotranspiration estimates.
Objectives - The objectives of the proposed U.S. Geological Survey (USGS) investigation are: (1) to estimate evapotranspiration during a portion of the growing season for one or more habitat types each at Klamath Marsh and Lower Klamath National Wildlife Refuge (figure 1); and (2) to develop one or more models for predicting evapotranspiration from those habitat types.
At this time, the investigation is envisaged to begin in April 1996. Field measurements will begin in May 1996 and conclude by July 31, 1996. The final report, which will be a USGS Administrative Report, will be delivered to the U.S. Fish and Wildlife Service (USFWS) by January 31, 1997.
Approach - Micrometeorological techniques, such as eddy correlation and energy-balance Bowen ratio, are the most effective techniques currently available for estimating evapotranspiration independently from other components of the site water balance. The eddy correlation technique is particularly useful because reliability of results can be confirmed by independent examination of the surface energy balance. For this reason, eddy correlation is the technique of choice for this proposed investigation.
Eddy correlation and other micrometeorological techniques are not suited for use in all terrestrial environments. Because of the hydrodynamic principles on which these techniques are based, extensive areas of sufficiently homogeneous surface cover are required to insure reliable results. For this reason, the reliability of eddy correlation measurements in many wildland environments, which typically have heterogeneous surface cover, can only be established by making measurements and evaluating the results. The proposed investigation will consist of two phases. During phase 1, eddy correlation measurements will be made on a limited trial basis and reliability of the results will be evaluated. Phase 2 will involve more eddy correlation measurements if the eddy correlation technique yields reliable results in phase 1. If eddy correlation results are not reliable, evapotranspiration measurements will be made in phase 2 using a simpler, substitute approach.