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Rain-on-snow events are common on mountain slopcs within the transient-snow zone ofthc Pacilic Northwest. These events make more water available :for runoff than does precipitation alone by melting the snowpack and hy adding a small amount of condensate to the snowpack. In forest openings (such as those resulting from clearcut logging), the amount of snow thal ac~~m~lnt~s and the turhulent-energy input to the snowpack are greater than helow forest stands. Both factors are believed to contribute to a greeter amount of water available for runoff during rain-on-snow wents in forest openings thnn forest stands. Because increased water available for runoff may lead to increased downstream flooding and erosion, knowledge of Ihe amount of snowmelt that can occur during rain on snw and the processes that control snowmclt in forest openings is useful when making land-use decisions. Snow accumulat:on and melt were simulated for clcarcut conditions only, using an energy-balance approach that accounts for the most important energy and mass cxchangcs between a snowpack and its environment. Mctcorological mcasurcments provided the input for the simulations. Snow accumulation and melt were not simulated in forest stands because interception of precipitation proccsscs are too complex to simulate with a numerical model without making simplifying assumptions. Such a model, however, would need to be extensively tested against rcprcscntative observations, which were not availahlc for this study. Snowmelt simulated during three mill-on-snow events (measured in a previous study in a clcarcui in the transient-snow zone of the H.J. Andrew Experimental Forest in Oregon) demonstrated that Imelt generation is most sensitive to turbulent-energy exchanges bctwccn the air and the snowpack surface. As a result, the most imporlant climate variable [hat conlrols snowmelt is wind speed. Air temperature, however, is a significant variable also. ‘The wind speeds were light, with a maximuln cof3.3 meters per second during one event and average wind speeds for all three events ranging from I .I to 2.1 mcte:ls per second. For observed and estimated conditions, the average simulated snowmelt mngcd from 0.2 to 0.8 millimeter liquid water per hour, and turbulent-energy exchange prwided 51 percent of the energy that led to snowmelt during the largest of the three rain-on-snow events. When wind speeds were multiplied by a factor of 4, the simulated snowmelt ranged from 1.0 lo 2.5 millimeters per hour. Similarly, when wind speeds were multiplied by a factor of 6, the simulated snowmelt ranged from I .6 to 3.7 millimeters per hour. Turbulent-energy exchange providcd a dominant 88 and 92 percent of the energy input to the snowpack during the largest rain-on-snow cvcnt when average wind speeds were multiplied by factors of 4 and 6, rcspcctively. During the same event, the contribution to melt by the sum of net solar and net thermal radiation (net all-wave radiation) was roughly equal to the contribution of sensible energy carried by the precipimtion ilsclf (advectiw heat). Estimates of snowmelt resulting from rain on snom, f(,r climate conditions other than those obscrvcd and cslimated in the simulated plot-scale data were expanded by simulaling snowmc1t for 24.hour presumed rain-on-snow cvcnts extracted from the reconstructed, long-term historical climate records for Cedar Lake and Snoqualmic Pass National Weather Service stations in Washingron State. The selcctcd events exceeded 75 millimeters of precipiration in 24 hours. When clcarcut <:onditions were assumed to he identical Io those at the H.J. Andrew Experimental Forest site and a ripe snowpack that never completely melted was assumed to be available, simulated 24.how snownell ranged from 4.2 10 47.0 millimeters (0.2 to 2.0 millimeters per hour) for low wind speeds (1.5 meters per second) and from 10.3 to 178.8 millimeters (0.4 to 7.5 millimeters per hour) for high wind speeds (8.2 meters per second). The ranges in melt for a given wind speed rcsultcd from the different combinations of air tcmperawe, dewpoint temperature, and precipitation depth that were characteristic of the synthetic CYCIILS. The avcrage of the median 24.hour snowmelt 81 Cedar Lake and Snoqualmie Pass was 15. I millimeters (0.6 millimeters pa hour) at low wind speeds and 49.6 millimeters (2.1 millimeters per hour) at high wind speeds. Condensa-

Simulation of water available for runoff in clearcut forest openings during rain-on-snow events in the western Cascade Range of Oregon and Washington