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Monitoring freeze-thaw cycles along North-South Alaskan transects using ERS-1 SAR


Monitoring freeze-thaw cycles of high latitude terrestrial ecosystems is useful for estimating the length of the growing season and annual productivity in the tundra and in boreal forests, for estimating potential damage to living plants due to frost drought, and for evaluating major changes in heat fluxes between land and atmosphere. At microwave frequencies, freezing results in a dramatic decrease of the dielectric constant of soil and vegetation, which significantly alters their radar scattering properties. In this article, we investigate the possibility of monitoring freeze-thaw cycles of terrestrial ecosystems using C-band frequency (5.3 GHz), vertical transmit and receive polarization, synthetic-aperture radar (SAR) data gathered by the European Space Agency's Earth Remote Sensing satellite (ERS-1). Repeat-pass SAR images are mosaicked together along a north-south transect across Alaska, coregistered, and analyzed using a change detection algorithm that determines when the landscape freezes based on a decrease in radar backscatter greater than 3 dB relative to a known thawed, wet state of the landscape. Air-temperature recordings from seven airport weather stations and in situ observations from three monitored forest stands in interior Alaska concur to indicate SAR accurately maps frozen areas across the entire state. The technique does not apply to open water areas because calm water and frozen water are confused. Elsewhere, ERS-1 SAR could monitor thaw / freeze transitions of terrestrial ecosystems at the regional scale, at a spatial resolution of several tens of meters and independent of cloud cover and vegetation type. © 1994.

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