Snow

snowfall

• more snow falls on Canada than on any other country
• as a percentage of annual precipitation, snow accounts for 5% globally, but about 36% in Canada
• on the prairies, snowmelt accounts for about 80% of the stream flow and water stored in sloughs; in west-central Saskatchewan its 80-85% whereas snowfall accounts for 18-48% of the total annual precipitation

snow crystals

• the hexagonal structure of snow flakes reflects the molecular structure or water, solid water forms when the molecules bond in a hexagonal lattice
• small and simple snow crystals (grains) grow slowly, with cold temperatures and low humidity
• large and more complex snow crystals (flakes) form quickly with warmer temperatures and higher humidity

measurement

• snow gauges
  
  
  • accuracy of about +/- 20% because snowflakes tend to be blown past the opening of gauges
  • thus all snow gauges are shielded
  • snow gauges have to mounted on brackets such that the gauge can be raised as the snow accumulates
  
  
• direct measurement
  
  
  • the most accurate method of determining snowfall amounts, but requires frequent measurement of changes in the depth of snow on ground, because snow tends to quickly settle and undergo metamorphosis
  
  
• units
  
  
  • depth of snow
  • water equivalent: depth of water produced when the snow melts, assuming a density of 0.1 g cc^-1 for newly fallen snow, i.e., a ratio of 10 to one between the density of liquid water and snow
  
  

hydrological significance of snow

• total annual snowfall is only one factor determining the contribution of snow to a water budget
• snow is first stored for days to months before participating in the hydrological cycle and therefore, the important considerations are

1. the spatial distribution of snow fall, especially in terms of altitude
   
   
   • in the Himalayan Karakoram (Pakistan), the altitudinal zone of maximum snowfall is 13,000 to 17,000 feet asl; catastrophic flooding and most of the annual erosion occurs during 20% of the year when thawing temperatures occur at these elevations
   • in contrast, snowmelt in regions of low relief (plains) is simultaneous over large areas and thus of relatively short duration
   
   
2. depth of the snowpack during the melt season
   
   
   • this depends on snowfall over the winter and on the duration of the snowcover (number of days that the ground is snow covered)
   • the periodic ablation of the snowpack over the winter seriously limits the snow cover during the period of conssitently higher temperatures, i.e., when ice is leaving the ground, rivers and lakes and thus snowmelt runoff can be stored
   • in the continental interior, the onset and waning of the winter snow cover occurs with advance and retreat of colder air, and thus a regional snowline, with latitude and elevation (annual analogue of the advance and retreat of a continental glacier)
   
   
3. the rate of melt
   
   
   • the rate and degree of temperature increase above 0^o, determines the rate of snowmelt relative to the capacity of the watershed to accommodate the snow meltwater
   
   

• the snowpack consists of ice crystals, water vapour and, with temperatures near 0^o, liquid water
• unlike ice, snow is permeable and thus air and water circulate through the snow pack causing changes in structure and texture with variations in temperature and mechanical stresses
• ultimately snow is transformed into glacier ice or meltwater
• the most rapid changes usually occur immediately after snow is deposited, because conditions on the ground (soil heat, drifting snow, gravitational stress) are much different than in the atmosphere where the snow crystals form

gravtitational metamorphosis

equi-temperature (ET or destructive) metamorphosis

temperature gradient (constructive) metamorphism

melt metamorphosis (firnification)

• with the large hydrological significance of snow in higher latitudes and altitudes, snow surveys and melt water forecasting is conducted for the purposes of
  
  
  • irrigation planning
  • domestic and industrial water supply, including planning for water shortages
  • hydro-electric power production: reservoir levels, spillway discharges, possible shortfalls in power production
  • forecasting floods and managing flood control structures; Manitoba established a snow survey network in response to the 1950 Red River flood

methods of snow survey

• in addition to the regular measurements of snow fall at meteorological stations, snow courses have been established to monitor the depth and density of snow packs, especially in the mountains
• for example, there are about 1500 snow courses in the 11 western US states; in Oregon, 43 agencies cooperate to maintain 180 snow courses
• repeated measurements (once or twice per month and more frequently as the melt season approaches) of depth and density are converted to water equivalent, where WE = snow depth * snow density / water density
  
  
  • depth is measured in a drift-free area by inserting a graduated probe or reading from a graduated pole fixed in the ground
  • density is determined by from the average weight of snow sampled in aluminum tubes of know volume, or from depth-integrated measurements:
  • the density of a snow core taken through the complete depth of the snow pack
  • pressure applied by the snow pack on a rubber of sheet metal snow pillow (metres in diameter and centimetres thick) filled with antifreeze that is expelled to a calibrated pressure gauge by the weight of the snow
  
  
• a snow melt runoff is forecast by establishing the statistical relationships (correlation and regression) among water equivalents, soil moisture, antecedent precipitation and stream flow

sources:


• Gray, D.H. and Male, D.H. 1981. Handbook of Snow.
• extensive research on prairie snow melt by Division of Hydrology, College of Engineering, U of S in the Bad Lake watershed, southwest of Rosetown


• melt of the prairie snowpack depends on weather and the depth and density of the snow, especially as affected by the interaction of wind, topography and land cover
• stubble retains about 37% of the water equivalent of the winter snowpack
• fallow soil retains about 9%
• the stubble captures snow blown off the fallow soil, so strip cropping controls both soil erosion and soil moisture
• regarding density, newly fallen snow (0.04 - 0.25 g m^-3) quickly reaches the average density of the over-winter snowpack (0.25 - 0.30 g m^-3) as the result of wind abrasion of the snow crystals and packing (gravitational metamorphosis)

energy budget of the melting snowpack

• source of energy for snow melt
  
  
  • solar radiation (insolation)
  • sensible heat of the air
  • sensible of heat of rain
  • latent heat
  • soil heat
  
  
• the relative importance of these energy sources is depends on
  
  
  • continuity of the snowpack
  • cloud cover
  • humidity
  • time of day and season
  
  
• melt occurs fairly rapidly on the prairies compared to other environments, especially mountains, given the small range of elevations, whereby the snow cover over large areas is simultaneously (rather than sequentially) exposed to warm temperatures
• also the prairie snowpack is characteristically thin and discontinuous
• once serious melt begins in March or early April, much of the natural snow cover disappears in less than week
• only the drifted or protected snow persists, e.g., in ditches, coulees, shelterbelts
• initially much of the melt water is stored in the snowpack or the soil and thus, significant runoff does not occur until about 20-30% of the watershed is bare

1. incoming radiation
   
   
   • most important source of energy for melt, such that cloud cover and melt are strongly negatively correlated
   • becomes increasingly important during the melt season, with increasing sun angle and day length, e.g., at 51^o N (latitude of Winnipeg), there is a 50% increase in incoming solar radiation between March 10 and April 15
   • but there also is strong aspect variation; during a six day 1972 study in the Bad Lake watershed:
     
     
     • net radaition was five times greater on south- versus north-facing slopes
     • south-facing snow on bedrock produced essentially no runoff, most of the snow sublimated and evaporated, while the north-facing snowpack yielded 82% of the water equivalent
     • on treed slopes, both aspects generated 70% of the water equivalent as runoff, but more rapidly on the south-facing slopes
     
     
   • much incoming radiation is reflected from winter snowpack, given its high albedo (70 - 85%)
   • but with melting, the albedo of the dirtier coarser snow decreasing exponentially
   • this accelerated change in albedo contrasts with the deeper, more continuous mountain snowpack, where melt and more prolonged and the underlying ground much less influence on the surface albedo
   
   
2. outgoing radiation
   
   
   • the emission of longwave radiation tends to be high on the prairies because of the generally clear skies and low humidity, although these conditions may be prevented during the melt season with the evaporation of snow melt water
   • after noctural outputs of longwave radiation and thus cold nights, the melting of snowpack must be restarted, rather than just maintained, by the incoming radiation
   • cloud cover can sustain, and significantly enhance, the rate of melt and thus appreciable snow melt does not usually occur until mean daily temperatures exceed about 5^o C, i.e., there are warm temperatures through the day and night, and not just warm afternoons
   
   
3. sensible heat flux
   
   
   • heat transferred from the air is especially important with patches of bare ground and at night, when the sensible heat flux can offset longwave radiation loss and thus prevent refreezing of the snowpack
   • during the six day study in southwestern Saskatchewan (Gray and O'Neil, 1973), net radiation represented 93% of the energy budget with continuous snowcover but, on an isolated snow patch, only 56% of the energy was net radiation, while about 44% was sensible heat
   • this result suggests that advection of warmer air from above snow-free ground is a significant source of energy
   • spring rain also is a source of sensible heat and thus rapid spring runoff often results from rain onto snow
   
   
4. latent heat flux
   
   
   • with higher vapour pressure, in the evening of when moist air overlies snow, condensation and sublimation can supply small amounts of energy to the snowpack
   • energy available for melt is lost during evaporation of snow melt water, although the low temperature and thus high relative humidity of the air over snow tends to limit energy loss by evaporation and sublimation
   • chinooks are the exception, when warm dry air is constantly advected over the snow surface, supplying sensible heat and maintaining a lower relative humidity
   
   
5. soil heat flux
   
   
   • about 1-5% of the energy for snow melt
   • the flux of soil heat is inhibited by a low temperature gradient between the frozen or cold ground and the bottom of the snow pack, especially compared to the temperature gradient between the top of the snow pack and the air
   • also, soil tends to have low thermal conductivity
   • soil heat serves mostly to offset nocturnal heat loss and thus can prevent snow from refreezing when minimum temperatures are around 0^o
   
   
6. internal energy (changes in storage)
   
   
   • the energy stored in the snowpack is small compared to the magnitude of the energy fluxes
   • it also is highly variable between night and day and with variation in depth, density and free water content, causing spatial and temporal variation in the production of runoff
   • the thin prairie snowpack has especially small energy storage and thus peak melt and runoff nearly always occur when the stored energy, and heat fluxes prevent the snow from refreezing at night