Spatial and temporal variability in spectral-based surface energy evapotranspiration measured from Landsat 5TM across two mangrove ecotones

Abstract

Coastal mangroves lose large amounts of water through evapotranspiration (ET) that can be equivalent to the amount of annual rainfall in certain years. Satellite remote sensing can play a crucial role in identifying regional ET trends and surface energy changes after disturbances in isolated and inaccessible areas of coastal mangrove wetlands, like those found in Everglades National Park in southern Florida. Using a combination of long-term datasets acquired from the NASA Landsat 5TM satellite database and the Florida Coastal Everglades Long-Term Ecological Research project, the present study investigates how ET as well as radiation and other energy balance parameters in the Everglades mangrove ecotone have responded to multiple hurricane events and restoration projects over the past two decades. An energy balance model using satellite data was used to estimate latent heat (λE) in tall and scrub mangrove environments. An eddy-covariance tower and weather tower supplied long-term data of multiple environmental and meteorological parameters that were used in calibrating and testing the modeled results from the Landsat images. Results identified significant differences in λE and soil heat flux measurements between the tall and scrub, and fringe and basin mangrove environments. The scrub mangrove site had the lowest λE rates, highest soil heat flux and lowest biophysical index (i.e., Fractional Vegetation Cover (FVC), Normalized Difference Vegetation Index (NDVI), and Soil-Adjusted Vegetation Index (SAVI) values. Mangrove damage and mortality associated with two strong hurricanes decreased FVC, NDVI, and SAVI, and increased soil heat flux at the tall mangrove site located in a basin-type environment. Recovery of the spectral characteristics, energy balance parameters and λE following hurricane disturbance was quicker in fringe mangroves than in basin mangroves. Latent heat fluxes (λE) were also relatively high after each storm and may have increased as a result of increasing vapor pressure deficits. Remote sensing of the surface energy balance and λE of mangrove forests can help our understanding of how these environments respond to disturbances to the landscape and the effect that these changes can have on the energy and water budget. Moreover, relationships between energy and water balance components developed for the coastal mangroves of southern Florida could be extrapolated to other mangrove systems in the Caribbean to measure changes caused by natural events and human modifications.