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Environmental Modeling Community of Practice

Evapotranspiration Calculator (ETC)

ET Calculator Pre-Release
Version Release Date
Release Notes
Beta version June 20, 2015
Desktop version January, 2016


The Evapotranspiration Calculator estimates evapotranspiration time series data for hydrological and water quality models. The tool was developed specifically for the Hydrologic Simulation Program - Fortran (HSPF) and the Stormwater Management Model (SWMM), but can be used with other models if they use time series ET data as input.

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The ET Calculator was developed primarily for hydrological and water quality modelers.

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A web-based Evapotranspiration calculator developed for hydrologic and water quality models such as the Hydrological Simulation Program - Fortran (HSPF), the core watershed model in BASINS (Better Assessment Science Integrating Point and Nonpoint Sources), is presented. HSPF is widely used to set total maximum daily loads (TMDLs) for water quality-impaired water bodies in the U.S. The model’s dominant hydrological processes include infiltration, interception, surface depression storage, evapotranspiration, runoff, groundwater, and soil moisture accounting.  HSPF also has extensive water quality process representations and modeling capabilities at user-specified spatial and temporal scales.

Using different ET methods, the tool’s objective is to calculate evapotranspiration time series data from meteorological data.  This process is important in climate change assessment studies, watershed modeling, drought and flood management, and modeling effectiveness of low impact development (LID) controls.

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Applications and Possible Uses

The most common use of the ET Calculator is calculating evapotranspiration time series input data for hydrological and water quality models such as HSPF and SWMM.  Applications include modeling green infrastructure performance assessment studies, climate change assessment studies, and water balance studies at different temporal and spatial scales.

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Software History

The web-based Evapotranspiration Calculator was developed under the green infrastructure task of the Safe and Sustainable Water Resources National Research Program. This is a Beta Version of the ET calculator and it has not been tested adequately with a range of modeling applications. Based on future test results and user requests, the calculator will have additional enhancements and new versions may be available.

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Technical Support and Training

Questions regarding the Evapotranspiration Calculator can be directed to CEAM Help Desk at the EPA’s Research Laboratory in Athens, GA.  A tutorial is now available for download at ET Tool Download. A document describing different ET methods will be available in 2015.

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This tutorial has been reviewed by the National Exposure Research Laboratory (NERL), U.S. Environmental Protection Agency (USEPA), Athens, Georgia and approved for publication. The ET calculator has not yet been tested extensively with different meteorological data sources and parameter sets. Additional quality assurance testing is needed and we will appreciate tool users reporting any inaccuracies. The authors and the U.S. Environmental Protection Agency are not responsible and assume no liability whatsoever for any results or any use made of the results obtained from this program, nor for any damages or litigation that result from the use of this tool for any purpose.

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