Mt. St. Helens volcanic ash: Effect of incorporated and unincorporated ash of two particle sizes on runoff and erosion

Abstract

The effect of ash from the 1980 Mt. St. Helens eruption on runoff and rill erosion of cultivated land was studied in 1981 and 1982. Ash from the Pullman (fine particles) and Yakima, WA (coarse particles), areas was applied to a silt loam soil, and water was applied. Treatments were: bare soil (fine-silty, mixed, Mesic Pachic Ultic Haploxerolls); ash incorporated with soil; and unincorporated ash on the soil surface. Sediment concentration in runoff first increased with time to a maximum, then decreased to an approximate steady state. Sediment concentrations were higher for unincorporated ash as compared to ash incorporated with the underlying soil, and were even lower for bare soil. Sediment concentration was higher with Yakima ash as compared to Pullman ash or bare soil because of the less cohesive nature of Yakima ash. Rills became wide and shallow with Yakima ash, as compared to deeper and narrower rills for Pullman ash, because of the relatively more cohesive nature and finer particle size of Pullman ash. The finer Pullman ash caused formation of a surface seal, thus restricting the infiltration rate and producing a higher runoff rate than from rills with Yakima ash or bare soil.