Assessment of Changes in Storm and Seasonal Runoff Response of Watersheds Impacted by Mt. St. Helens Ash Deposition

摘要

The most significant effects of the ash deposition following the major Mt. St. Helens eruptions during May to July 1980 were twofold. The immediate consequence of the ashfall was either to increase or decrease the runoff from snow-covered areas, depending upon the thickness of the ash cover. The second consequence of the ashfall was to change the hydrologic response of ash covered catchments to a given amount of precipitation. To study the changes in the hydrologic response of a catchment, it was necessary to model both the snowmelt process to determine effective precipitation and the transformation of effective precipitation to runoff. As an alternative, it was decided to modify the Constrained Linear System (CLS) model in such a way that the physical characteristics of the catchments could be explicitly incorporated. These results may be attributable to a post-eruption increase in ponding or depression storage capacity due to debris and blast deposits in the catchment.