Monitoring freeze-thaw along north-south Alaskan transects using ERS-1 SAR

Monitoring freeze-thaw transitions of the soil and vegetation in high latitude terrestrial ecosystems is useful for determining the length of the growing season, and for monitoring potential damage to living plants due to freezing and frost drought. The authors present a technique for monitoring freeze-thaw cycles using repeat-pass SAR data from the European Remote Sensing Satellite, ERS-1, 100 km in swath width, and mosaicked together along a north-south Alaskan transect 1400 km in length. Freezing of the soil and vegetation is detected based on a 3 dB decrease in radar cross-section σ⁰ relative to a known thawed state of the landscape. The decrease in σ⁰ is explained by radar backscatter models as resulting from a large decrease of the dielectric constant of the soil and vegetation with freezing. The technique is validated using air-temperature recordings from three forest stands along the Tanana River near Manley Hot Springs, and from local weather stations along the transect. The technique does not apply to areas of standing water, but is independent of the type of vegetation cover, and permits the spatial and temporal monitoring of freezing of the natural landscape at the regional scale