MAPPING OF THE VOLCANIC ASHES FROM THE 1991 HUDSON ERUPTION USING NOAA-AVHRR DATA

The August 1991 eruption of Mount Hudson, a volcano located in the sou thern Andes, spread 8 km(3) of ash into the atmosphere. As a result, a great part of the province of Santa Cruz, Argentina, was affected by a 0.1 to 10 cm ash layer. Plants and consequently animal life were sev erely touched and the region declared disaster-stricken. Comparison of pre- and post-eruption AVHRR images of the region show that at least a 70 000 km(2) strip ranging from the Andes (Mount Hudson volcano) to the Atlantic coast has been affected. The NDVI was found to be more se nsitive for the discrimination of affected areas than the raw AVHRR ch annels. Moreover, optimal timing for such an assessment is discussed c onsidering the NDVI time profiles derived over the pre-eruption period . Visual analysis of the two post-eruption images available partly con firmed the ash removal date predicted by EPIC simulation model. Howeve r, both methods cannot be strictly compared since the diagnosis on whe ther ash remains or not, is sometimes hampered by the confusion betwee n pre-eruption natural cover and ash. Furthermore, a time lag is neces sary for vegetation to recover its initial vigour after the date of as h removal.