Comparative snow accumulation and melt during rainfall in forest and clearcut plots in western Oregon

A study was conducted to compare snow accumulation and meltduring rainfall in adjacent forest and clearcut plots in thetransient snow zone of the western Cascade Range in Oregon. Snowaccumulation differences were determined by comparing the waterequivalents of forest and clearcut snowpacks. During rain-on-snowperiods, energy balances were analyzed to evaluate the differences inenergy transfers acting to melt snow in the two plots. In this way,snowmelt differences can be linked to the microclimatic alterationsrelated to clearcut logging. Snowmelt estimated by energy balaceanalyses was compared with snowmelt determined by snowmelt lysimeterdata and snow survey information.
The forest canopy played a strong role in controlling snowaccumulation. Snow trapped in the forest canopy melted faster thansnow that accumulated on the ground in the clearcut plot. Snowsurvey information indicated that the water equivalents-of clearcutsnowpacks averaged 29 mm, but were up to 74 mm greater than forestsnowpacks.
For five rain-on-snow events during the winter of 1983-1984, fourof which rainfall amounts were smaller than that called for in thestudy design, a comparison of snowmelt estimated by the three methods
had variable resulti. Only during the last two rain-on-snow events,of which one was the annual rainfall event, did results from thethree methods all show greater snowmelt in the clearcut plot.Snowmelt determined by lysimeter information was more reliable duringthese events than the previous three events because of improvedrainfall sampling.
Snowmelt estimated by energy balance analyses was consistentlygreater in the clearcut plot. Longwave radiation was the greatestsource of snowmelt for all events, contributing 38-88 percent of thetotal computed snowmelt of each event. Snowmelt attributed to netlongwave radiation was 22-56 percent greater in the clearcut plot.However, the combined fluxes of latent and sensible heats accountedfor a large portion of the snowmelt differential between the plots.Although the combined fluxes ranked second in importance as a sourceof heat for snowmelt (6-36 percent of the total snowmelt-for eachevent), in the clearcut plot they were 226-300 percent of thecombined fluxes in the forest plot.
During the largest rain-on-snow event (February 11-13, 1984),total snowmelt was 55-111 percent greater in the clearcut plotdepending on the method of measurement. The increased snowmelt ofthe clearcut plot is attributed to (1) greater snow accumulationprior to the event and (2) greater energy inputs during the event.