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 southern 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 severely 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 sensitive for the discrimination of affected areas than the raw AVHRR channels. Moreover, optimal timing for such an assessment is discussed considering the NDVI time profiles derived over the pre-eruption period. Visual analysis of the two post-eruption images available partly confirmed the ash removal date predicted by EPIC simulation model. However, both methods cannot be strictly compared since the diagnosis on whether ash remains or not, is sometimes hampered by the confusion between pre-eruption natural cover and ash. Furthermore, a time lag is necessary for vegetation to recover its initial vigour after the date of ash removal.