Some characteristics and consequences of snowmelt during rainfall in western Oregon

Abstract

Hydrometeorological data for two watersheds in western Oregon indicate snowmelt during rainfall has been a dominant hydrologic process which is responsible for erosion within headwater areas and for downstream flooding. The majority of the larger peak flows in both watersheds result from snowmelt during rainfall. In a stream draining a 60-ha watershed in the zone of transient shallow snowpacks, a major peak flow of 10 l/s per ha is five times more likely to result from rain-on-snow than from rain alone. In a 62.4-km 2 watershed, largely within the transient snowpack zone, 85% of all landslides which could be accurately dated were associated with snowmelt during rainfall. By increasing melt caused by condensation and convective heat transfer, clearcut logging, especially on southwestern-facing slopes, may be increasing water input to soil up to 25% during infrequent combinations of shallow snowpacks, heavy rains, relatively warm air and wind. Under more frequent combinations, increases in water input still could be 10%. Limitations in present knowledge are discussed.