Although the temperature during a storm may contribute to detention of snow, 

 tree-branching characteristics very likely play an important role. In observing 

 sapling trees at a lower elevation (2,500 feet], we have found that the bulk of the 

 snow readily slides off the narrow, limber branches in a matter of hours after each 

 storm (8) . The stiffer branches and wider foliage of mature trees at a higher elevation 

 (4,300 feet) provide a more sturdy "platform" for holding snow loads. 



Comparison of precipitation records in the forest and small openings as well as 

 close observations on weekly trips to the study site indicate that snow tends to persist 

 on the mature trees--sometimes for several weeks. Unlike the climatic conditions in 

 Colorado (5), very little snow blows off the canopy and redeposits in the small openings. 

 When snow does dislodge, usually it falls to the forest floor as massive wet clumps. 



This lengthy persistence of snow in the canopy enhances the opportunity that the 

 snow will be acted upon either by subsequent storms or by clear weather conducive to 

 evaporation or melting. It is not uncommon for a new storm following in the wake of 

 an old storm to be accompanied by above- freezing temperatures and thus produce rain. 

 The rainy, thawing condition rapidly changes the physical properties of canopy-held 

 snow and drip begins to fall within a short time. Snow clumps heavy with water slide 

 out of the trees, but much of the intercepted snow holds fast to branches and foliage 

 until dissipated by melting. (Our field crews often experience considerable discomfort 

 when working under the canopy on a "heavy-drip" day.) 



Noteworthy information derived from the study has been the high intensity and 

 volume of outflow (primarily throughfall-drip percolate) recorded in the forest usually 

 on the beginning day of a rainy, thawing condition. This volume of outflow often 

 exceeds the outflow (primarily rain percolate) in the small openings by a wide margin. 

 By inference, the excess outflow is attributed to drip from snow held over in the 

 canopy from a previous snowstorm. 



This poses the question as to whether the entire contribution of excess outflow 

 has its origin as drip from snow in the canopy. There are two other possible sources-- 

 differential input by rainfall and differential melt in the snowpack. There is no 

 scientific evidence to show that rainfall incident on the canopy is substantially 

 greater than that which falls in the small openings. Most investigators claim that the 

 input to the forest canopy may in fact be less than in the openings (7). Indeed, if 

 this is the case, then by our analysis technique the outflow assigned to drip is a 

 conservative estimate. 



There is the theoretical chance that more heat energy from the warm air masses is 

 absorbed by the crowns and transmitted to the snowpack by convection and radiation. We 

 discount this possibility because, during periods when canopies are bare of snow and 

 warm air masses are passing in absence of rain (i.e., warm winds or Chinook conditions), 

 the lysimeter charts in the forest exhibit no sudden surge of snowmelt percolate. In 

 actuality, when these conditions prevail the sky is usually heavily overcast and the 

 melt rate is comparable or slightly faster in the small openings. 



The first-day outflow of throughfall-drip in the forest often percolates down 

 through the snowpack at a rapid rate. Possibly there are extended periods during 

 winter when the snowpack in the forest maintains an isothermal condition concomitant 

 with complete or near-complete recharge of liquid water. As a consequence, the snow- 

 pack responds rapidly to the smallest input of throughfall-drip or heat energy. This 

 is substantiated by the brief lag time before outflow appears in the lysimeter. 



In the deeper snowpack in the small openings, conditions are different. The initial 

 lag time is usually greater, especially after an extended period of clear, cold weather 

 and, therefore, responses to rain and heat energy are correspondingly slower--probably 

 because of the freezing of rain or refreezing of surface melt deeper in the pack. 



15 



