Evapotranspiration (ET) is the term applied to the removal of water from the land surface through a combination of direct evaporation and vegetation transpiration. As it is in many ecosystems, ET is a primary component of the Everglades water budget. ET varies widely through both space and time along with the biological and meteorological factors that drive it. Spatial and temporal heterogeneity of vegetation type and function, as well as differences in available energy and water, all influence the rate at which ET occurs. Determining appropriate ways to account for the variation in these factors, and therefore in ET itself, is one of the pressing research issues facing those who need to describe, understand, and predict the behavior of hydrologic and climate systems - especially at regional scales for which sparse in-situ meteorological data are inadequate. Satellite remote sensing systems offer the promise of providing repetitive, synoptic views of variables related to ET.

Methods currently being developed that use satellite systems for ET modeling and mapping span a broad range of complexity. Empirically derived statistical relationships, physically based analytical approaches, numerical models, and the exploitation of observed relationships between radiant surface temperatures (Tr) and spectral vegetation indexes like the Normalized Difference Vegetation Index (NDVI) are all being developed (Kustas and Norman, 1996). However, many factors complicate the use of satellite data for ET estimation. Satellite-measured fluxes of energy are distorted by the mixture of land cover type and vegetation amount within the instantaneous field of view. Variability in the condition as well as the thermal and reflective properties of the materials being imaged complicates the derivation of important information. In addition to variability in the surface targets of interest, materials in the atmosphere block, absorb, and reflect energy differently through space and time, introducing additional noise in the measurements recorded by the sensor.

[This poster] presents preliminary attempts to use satellite remote sensing technology for extrapolating point measurements of ET over broader spatial scales. For this analysis, atmospheric corrections were applied to Landsat thematic mapper (TM) multispectral data spanning the visible, near-infrared, and thermal regions of the electromagnetic spectrum. These data were processed to yield reflectance, NDVI, and Tr values. Detailed meteorological and hydrologic data collected at nine locations as part of the South Florida Ecosystem Program (German, 1996) were used to model ET. Assuming the proportion of available energy going to latent heat of vaporization is constant (that is, a constant ET fraction) throughout the day (Hall and others, 1992), the values for reflectance, NDVI, and Tr for the areas of sufficient distances surrounding ground meteorological sites were regressed against the daily total ET for the date of the satellite overpass. The observed relation was used to extrapolate ET to all locations within the scene.

For longer term monitoring and modeling, the ability to estimate ET by means of satellite remote sensing without the need for extensive ground-based data collection is desirable. The methods and data sets developed through the analysis reported here will serve as one measure of spatially distributed ET against which subsequently developed techniques can be compared.

REFERENCES

German, Edward, 1996, Regional evaluation of evapotranspiration in the Everglades: U.S. Geological Survey Fact Sheet FS-168-96, 4 p.

Hall, F.G., 1992, Satellite remote sensing of surface energy balance: success, failures and unresolved issues in FIFE: Journal of Geophysical Research, v. 97, p. 19,061-19,090.

Kustas, W.P., and Norman, J.M., 1996, Use of remote sensing for evapotranspiration monitoring over land surfaces: Hydrological Sciences, v. 41, no. 4, p. 495-516.

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