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Getting to Know ET (Evapotranspiration)—USGS Shares Expertise About this Important Component of the Hydrologic Cycle in Florida

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Typical ET tower
Above: Typical ET tower; the USGS maintains stations like this in several different types of landcover. Here, Amy Swancar services an ET field-data station in an orange grove in Arcadia, Florida. [larger version]

Ann Tihansky uses climbing gear to access instrumentation on an evapotranspiration tower
Above: Hydrologist Ann Tihansky uses climbing gear to access instrumentation on an ET tower. All staff who service these stations have been trained in climbing safety and use standard safety climbing gear. [larger version]

Florida map showing existing and planned evapotranspiration stations in Florida in May 2006
Above: Existing and planned evapotranspiration (ET) stations in Florida in May 2006. Each station is operated collaboratively or solely by the USGS, the Smithsonian Environmental Research Center, Dynamac Corp., the University of Florida, the University of New Hampshire, or the University of Virginia. Abbreviations: NWFWMD, Northwest Florida Water Management District; SRWMD, Suwannee River Water Management District; SJRWMD, St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; SFWMD, South Florida Water Management District. [larger version]
The hydrologic cycle describes the various states of water as it moves through a landscape, transforming from liquid to gas and back to liquid again. Water moves from precipitation as rain, snow, and fog into ponds, lakes, and rivers; some infiltrates into the ground as ground water. Most precipitation in Florida returns to the atmosphere by way of evapotranspiration (ET), a term for the combined processes of evaporation and transpiration. Evaporation occurs directly from free-water surfaces, such as lakes, streams, and temporary rainfall accumulations (puddles on a sidewalk, for example, or droplets on top of a leaf); transpiration occurs as plant roots extract water from the soil and release water vapor into the atmosphere through plant-leaf stomata.

ET rates can vary depending on meteorological conditions, the type of land-surface cover (paved, wetland, wooded, agricultural, and others), the time of day, the time of year, and soil moisture. In spite of the relative importance of ET within the hydrologic cycle—after rainfall, it is the largest component of the water budget—reliable data for ET have historically been scarce. Strategic water management requires quantification of ET for reliable hydrologic analyses. The U.S. Geological Survey (USGS) currently operates 13 ET stations throughout Florida in various environmental settings; the stations measure ET at daily or shorter time scales. Another 6 stations are scheduled for installation in 2007.

The USGS Florida Integrated Science Center (FISC) office in Orlando hosted an ET workshop on November 13, 2006, to share ET information and to discuss plans for future partnerships and collaboration. More than 40 scientists, regulators, and technical managers representing the USGS, the Florida Water Management Districts, the Florida Department of Environmental Protection, Tampa Bay Water, and eight universities attended the meeting. They gathered to discuss the results of an effort to estimate ET rates throughout Florida, using a combination of satellite- and weather-station meteorological data.

FISC director Barry Rosen opened the meeting with an overview of FISC, explaining how the center is organized and how it integrates geology, biology, and water science in studies that cover a range of topics—water quality and availability; the effects of invasive species; natural hazards and associated coastal processes; and interaction among ground water, surface water, and ecosystems, to name a few. After Rosen's welcome, USGS hydrologist David Sumner introduced the focus of the meeting with a comprehensive talk titled "Evapotranspiration Measurement and Estimation in Florida—State of the Art and Future Directions."

Sumner, who organized the meeting, has been managing a project to produce a database of daily ET rates covering the entire State of Florida at 2-km resolution; this database is expected to be used widely by water-resource planners and regulators. University scientists conducted the research used to generate the database, and several of them—Jennifer Jacobs of the University of New Hampshire, Ellen Douglas of the University of Massachusetts, and John Mecikalski and Simon Paech of the University of Alabama, Huntsville—presented results at the meeting. Validation data for the ET database project were provided primarily by USGS ET field stations operated by Sumner, Ed German (retired), Amy Swancar, and Trey Grubbs. Future plans are for the USGS to maintain and update the ET database in a Web-deliverable manner.

The November meeting was funded by all five Florida State Water Management Districts, which also provided funding for the ET database project. Representatives of three of the five districts gave talks at the meeting, including a proposal for State-wide coordination of ET research and data-collection programs.

Originally conceived as a simple project meeting, the November gathering ended up with a larger scope, providing a focal point to further the development of a State-wide, coordinated approach to ET research in Florida.

Related Web Sites
Florida Integrated Science Center
U.S. Geological Survey (USGS)
Florida Water Management Districts
State of Florida

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cover story:
Deadly Tsunami Hits Solomon Islands

Tsunami-Forecasting System Tested by Recent Earthquakes

Sub-Sea-Floor Methane in the Bering Sea

Outreach USGS Donates Equipment to Local Nonprofit Theater

Meetings Getting to Know ET (Evapotranspiration)

International Workshop on High-Seas Biogeography

Awards USGS Researcher Receives Award from the American Society of Civil Engineers

USGS Sirenia Project Receives Manatee Hero Award


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