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Everglades ET measurement and modeling

Project Proposal for 1999

Project title: Everglades ET measurement and modeling
Geographic area: South Florida - natural Everglades system
Project start date: October 1994
Project end date: September 2000

Project chief: Edward R. German
Region/Division/Team/Section: WRD
Email: egerman@usgs.gov
Phone: 407-865-7575
Fax: 407-865-6733
Mail address: 224 West Central Pk., Suite 1006, Altamonte Springs, FL. 32714

Program(s) South Florida Ecosystem Program

Program element(s)/task(s) Hydrologic modeling and support element, Task 4.1

Project summary: Solution of water problems within the Everglades requires an understanding of the flow systems. Evapotranspiration (ET) is a major component of the Everglades water budget (generally over 40 inches/year) that has not previously been measured. This project will provide the knowledge of ET necessary for use of hydrologic models.

Project objectives and strategy: The overall objective is to develop a process-oriented appraisal of evapotranspiration within the Everglades drainage unit, excluding agricultural and brackish environments. Specific objectives include: 1) field measurement of evapotranspiration at a variety of sites encompassing a regionally representative range of environmental factors; 2) integration of evapotranspiration estimates into a process-oriented model (e.g., Priestley-Taylor or Penman-Monthith), which accounts for the effects of environmental factors (i.e., solar radiation, vegetative cover, etc.) in a physics-based manner; and 3) extrapolation of process-oriented understanding from measured sites/time periods to unmeasured areas/time periods to estimate spatial and temporal trends in Everglades evapotranspiration.

Potential impacts and major products: It is anticipated that information gained in this project during the first two years of data collection will help those engaged in developing hydrologic models of the Everglades system to make some preliminary simulations. By the completion of the report in FY 1999, understanding of the ET processes should be advanced to the point that accurate quantities of ET can be predicted for use in hydrologic models used to understand and manage the Everglades.

Collaborators, clients: The primary users of these data and ET models will be the state and federal entities involved with preservation and management of the Everglades. These are the Big Cypress National Preserve, Everglades National Park, Loxahatchee National Wildlife Refuge, and South Florida Water Management District (SFWMD). These entities are currently involved in development of water-management models that need accurate estimations of ET for successful implementation. Additionally, it is likely that the database resulting from this project will be useful in a wide range of research done by universities as well as government entities.

Time line (FY 1999 to project end): The person responsible for all items in the following list is
E.R. German.

1. FY1998: Select 4 sites for continuing data collection, and maintain at reduced maintenance schedule.
2. FY 1998: Use the newly developed Eddy-correlation equipment to investigate effect of method on measured ET.
3. FY1998: Continue data analysis, especially development of regional models of ET
4. FY 1998: Finish delivery of all ET and meteorological data to WWW site.
5. FY1999: Begin full-schedule operation of four ET stations.
6. FY1999:  Install a fifth ET station in western part of Everglades to support modeling studies.
7. FY1999: Publish final interpretative report describing study data and findings.
8. FY2000:  Operate the network of five ET stations until end of FY2000.

FY 1999 activities:
The four ET stations selected for continuing operation will be operated at a full maintenance schedule (monthly site visits). A final interpretative report describing ET data and regionalization of ET data will be published.

FY 1999 deliverables/products:
An interpretative report describing the ET data and methods for estimating ET within the natural Everglades system will be published.

FY 1999 outreach:.
Publication of the final project report by the end of FY 1999 will address client requirements for a means of estimating ET. The report will describe and discuss regional models that may be used in the models that are the principle tools of clients involved in management and restoration of the Everglades.

New directions or major changes for FY 1999 (if applicable):
Under the old plan, data collection was to terminate on December 31, 1997. However, because of data requirements of on-going projects in the Southern Inland and Coastal Systems (SICS) area, and other parts of the Everglades, there is a need to continue operation of selected Bowen-ratio sites through FY 2000. This revision of the original plan calls for continuation of 4 of the original 9 Bowen-ratio sites. Sites selected for continuation are: Site P33, Everglades National Park (ENP); Old Inghram HW site, ENP; Site F4, WCA 2a; and Site 7, Loxahatchee National Wildlife Refuge. The sites in ENP will provide support for the SICS modeling studies, and the site in WCA 2a will provide support for surface water/ground water studies. The site in Loxahatchee National Wildlife Refuge is an open-water site that probably represents maximum ET rates in the natural Everglades system. Because the Loxahatchee site uses less instrumentation than that required at the vegetated sites, it can be included in the continuing network with little additional expense. Together, these four sites selected for continuation of data collection probably represent the vegetative conditions most important in determining ET: open water (no emergent vegetation), sparse sawgrass, dense sawgrass, and an area of sparse vegetation with no standing water for several months each year.

Some report preparation work scheduled for FY1998 was deferred until 1999, because of time required to maintain the four continuing sites and the Bowen ratio - Eddy correlation comparison studies. This change results in more salary for report work in FY1999 than originally planned, but will not affect report completion data (September 1999).

FY 1998 accomplishments and outcomes, including outreach:
As of May 22, 1998, about half of the data from the ET sites has been processed into ADAPS and furnished to the Ecosystem Web site. The rest of the data will be stored in these databases by September 1998. Work is progressing on development of a regional model of ET based on solar radiation and water depth. Hourly ET data for selected dates have been furnished to John Jones and Greg Desmond (Task 1.10) for use in developing a regional model of ET based on thematic mapper images from Landsat.
A relatively simple 2-parameter model of ET was developed for estimating ET at any location in the southern part of the Everglades from incoming solar radiation data (pyranometer) and depth of water above (or below) land surface. The model is based oh a relation between incoming solar energy and total available energy, and a relation between the Priestley-Taylor coefficient (a measure of evaporative efficiency) and water depth. At 5 sites in the southern half of the study area, the incoming solar radiation accounted for about 86 percent of the variation in energy available for evapotranspiration. This relation indicates that differences in plant community and water depth among the different sites are relatively unimportant in determining how much of the incoming solar energy is captured by the system and is thus available for release as latent (evapotranspiration) and sensible heat. The relation between Priestley-Taylor coefficient and water depth indicates that higher water levels produce a greater ET rate than lower water levels for a selected amount of available energy. Putting the two relations together provides a model that predicted annual ET totals that were within 10 percent or less of measured ET for 1996 at the 5 sites where the model was tested.

Newly developed equipment for measuring ET by the Eddy-correlation technique was used to study effects of method (Bowen ratio or Eddy correlation) on measured ET values. This is important because the two techniques are commonly used in ET determination, but imply different assumptions about energy transport. Thus, it is possible that results from ET studies might be somewhat method dependent, and it is important to know the magnitude of this methods dependency. Work accomplished to date at three sites where the Eddy-correlation equipment was operated simultaneously with the existing Bowen ratio equipment indicates that the Eddy-correlation technique tends to indicate values of ET that are 10-20 percent lower than the ET values measured by the Bowen-ratio technique. This comparison was made in winter and early spring when energy available for ET is relatively low. The comparison will be repeated this summer, when ET rates are relatively high.

FY 1998 deliverables, products completed: A complete set of hourly ET and meteorological data will be available in ADAPS and the Ecosystem Web site. While these data are not considered ready for release to the public until after approval of the final report in FY l999, the data are available to other Ecosystem researches who may find the information useful. Wind, rainfall, and ET data are being used in a flow model of the southern inland and coastal system area being constructed by Eric Swain.

Names and expertise Dave Standard, USGS NRP, Denver, Colorado, has provided project operation and data analysis during each year of the study. We will continue to call upon him for advice on ET modeling and review of study conclusions.

Other required expertise for which no individual has been identified None.

Major equipment/facility needs: None.

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