projects > everglades ET measurement and modeling > 1998 proposal
Project Proposal for 1998
Program: FRAGILE ENVIRONMENTS
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 Justification: 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 project (FY1999) understanding of the ET processes should be advanced to the point that accurate quantities can be predicted by hydrologic models.
Project Objectives: 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 Shuttleworth-Wallace), which accounts for the effects of environmental factors (i.e., solar radiation, air temperature, humidity, soil moisture availability, 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.
Overall Strategy, Study Design, and Planned Major Products: Nine (9) field sites have been installed for determination of evapotranspiration and the meteorological and other quantities needed for physics-based ET models. The sites are distributed throughout the Everglades drainage unit in a variety of environmental settings representative of the natural Everglades environment (not including agricultural areas, brackish areas, or forested areas). Energy budget and meteorological data are monitored at 30-second intervals over a 2- year period. The Bowen-ratio method is used to determine daily ET from the energy budget data. ET data for the individual sites will be extrapolated over the entire natural Everglades area using models driven by solar energy, water level, and vegetative cover. The planned major product is a report describing ET in the natural Everglades system, a detailed discussion of the importance of meteorological and other factors (plant cover and water level, for example) in determining ET rates, and a model or method for estimating ET over the entire area. It is also likely that the raw data resulting from this project will be a valuable resource for future research.
Overall: Select data-collection sites that are representative of the natural Everglades system. At each site, install meteorological and energy-budget sensors for continuous monitoring of data necessary for ET measurement. Determine the ET at each site using the Bowen-ratio method. Develop site- specific physics-based models of ET, such as the Priestley Taylor or Shuttleworth-Wallace models. From these site models, develop a regional model of ET for the entire natural Everglades system. The regional models will predict ET as a function of solar-energy and other meteorological data input, vegetative cover, and water level. The raw data, and the computed ET, will be stored in the USGS ADAPS system, and will be accessible through the Internet.
Planned Outreach Activities: This project and Task 1.10 (Greg Desmond) will be working together to develop methods of regionalizing the ET data from specific sites to the entire natural Everglades area. Plans are to use remote sensing data, specifically Landsat thermatic mapper images, to characterize ET at a resolution of 30 m2. Use of other types of remote sensing will also be investigated.
New Directions, Expansion of Continuing Project (if applicable): An additional site was added to the original network of 8 sites in January 1997. The new site is located in the Canal C111 area, and was added to increase coverage of relatively dry parts of the Everglades system. This was done because findings indicate that water level is one of the most important factors in determining ET rates.
Accomplishments and Outcomes, Including Outreach: The Priestley-Taylor model was fitted to daily ET for the 1996 calendar year for five sites where preliminary estimates of daily ET have been made. The model explains at least 93 percent of the variation in daily ET at each site. The Priestley-Taylor coefficient ranged from 0.97 to 1.02 at the three wettest (sawgrass) sites; was 0.86 at a partially dry site, and was 1.18 at an open-water site. The difference in the coefficients at the vegetated sites and the open-water site indicates that the presence of aquatic vegetation tends to decrease ET as compared with rates that occur in unvegatated water at a selected level of net radiation, However, the net radiation at a given solar intensity appears to be lower at the open-water site than at the vegetated sites, possibly because of a greater amount of reflection and long-wave radiation from the water surface. The net effect of the higher Priestley-Taylor coefficient and lower net radiation at the open water site may be to make ET at the open-water site about the same as that which occurs at some perennially wet vegetated sites.
The preliminary estimates of ET at the five stations for 1996 ranged from 50.9 in. at the open-water site to 42.2 in. at the partially dry site. Among the perennially wet sites, the ET ranged from 47.2 to 50.9 in. during 1996. The low ET at the relatively dry site indicates that water level is probably more important than vegetative type in determining ET. Locations where the water level is always above land surface will probably have ET rates that are largely a function of the solar energy input and not greatly affected by the type or density of vegetative cover.
Net solar radiation generally is the most significant term in the energy budget, and the most significant in determining ET. At times, however, the heat stored in water can contribute significantly to ET for a few hours or days. This generally occurs during passage of cold fronts, when the heat stored in the surface water is released partially as latent heat.
Also during 1997, meteorological data and other support were furnished to other projects, including Task 1.5 (John Lee), Task 1.8 (Ray Schaffranek), and projects of the Mercury Studies group (Dave Krabbenhoft).