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 p roductivity in the tundra and in boreal forests, for estimating potent ial damage to living plants due to frost drought, and for evaluating m ajor 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 rad ar scattering properties. In this article, we investigate the possibil ity 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 Ag ency's Earth Remote Sensing satellite (ERS-1). Repeat-pass SAR images are mosaicked together along a north-south transect across Alaska, cor egistered, and analyzed using a change detection algorithm that determ ines when the landscape freezes based on a decrease in radar backscatt er greater than 3 dB relative to a known thawed, wet state of the land scape. Air-temperature recordings from seven airport weather stations and in situ observations from three monitored forest stands in interio r Alaska concur to indicate SAR accurately maps frozen areas across th e entire state. The technique does not apply to open water areas becau se calm water and frozen water are confused. Elsewhere, ERS-1 SAR coul d monitor thaw/freeze transitions of terrestrial ecosystems at the reg ional scale, at a spatial resolution of several tens of meters and ind ependent of cloud cover and vegetation type.