Lecture ERS 482/682 (Fall 2002) Snow hydrology ERS 482/682 Small Watershed Hydrology

Lecture ERS 482/682 (Fall 2002) ~13% of average precipitation in US is snow Significance of snow Figure 4.16 (Manning 1987)

Lecture ERS 482/682 (Fall 2002) ~13% of average precipitation in US is snow Significance of snow Figure 5-11 (Dingman 2002)

Lecture ERS 482/682 (Fall 2002) ~13% of average precipitation in US is snow Significance of snow In some areas, snowmelt is main source of water supply Can affect water quality

Lecture ERS 482/682 (Fall 2002) Snow properties Granular ice pore spaces If T<0°C: ice, air If T=0°C: ice, water, air

Lecture ERS 482/682 (Fall 2002) Snow properties Snow density porosity water content i = ice w = water

Lecture ERS 482/682 (Fall 2002) Definitions precipitation –depth of rainfall plus water equivalent of snow, sleet, and hail snowfall –incremental depth of snow and solid precipitation snowpack –accumulated snow on the ground snowmelt –amount of liquid water produced by melting leaving the snowpack ablation –total loss of water substance from snowpack (includes evaporation/sublimation) water output –total of liquid water leaving snowpack All typically expressed in units of depth [L]

Lecture ERS 482/682 (Fall 2002) Snow measurement Precipitation gages Figure 3-5 (Linsley et al. 1982)

Lecture ERS 482/682 (Fall 2002) Snow measurement Precipitation gages Snow board/snow stake Figure 4.18 (Manning 1987)

Lecture ERS 482/682 (Fall 2002) Snow measurement Precipitation gages Snow board/snow stake Universal gage (Fig. 5-6) Snow survey Federal sampler Depth probe

Lecture ERS 482/682 (Fall 2002) Snow measurement Precipitation gages Snow board/snow stake Universal gage (Fig. 5-6) Snow survey Snow pillow (Fig. 5-8)

Lecture ERS 482/682 (Fall 2002) Snow measurement Precipitation gages Snow board/snow stake Universal gage (Fig. 5-6) Snow survey Snow pillow (Fig. 5-8) Lysimeter

Lecture ERS 482/682 (Fall 2002) Snow measurement Acoustics Radar Satellite Table 5-1 summarizes methods

Lecture ERS 482/682 (Fall 2002) Snowmelt processes Snowpack metamorphism –Involves changes in Snow structure Density Temperature Albedo Liquid water content

Lecture ERS 482/682 (Fall 2002) Snowmelt processes Accumulation period –h m increases –net energy input is negative –average snowpack temperature decreases Melt period net energy input is positive snowpack T s increases to 0°C melting occurs, but no water output; T s = 0°C –Warming phase –Ripening phase –Output phase energy input  water output

Lecture ERS 482/682 (Fall 2002) Warming phase where Q cc = cold content c i = heat capacity of ice  w = density of water h m = snow water equivalence T s = temperature of the snowpack T m = melting point temperature 2102 J kg -1 °C kg m -3 0 °C [°C] [m] [J m -2 ] Snowpack temperature increases to T s = 0°C Note: Q m1 = Q cc at beginning of melt period

Lecture ERS 482/682 (Fall 2002) Ripening phase where Q m2 = net energy to complete ripening phase  ret = maximum volumetric water content f = latent heat of fusion Figure MJ kg -1 [J m -2 ] Before water output when the snowpack is isothermal at T s = 0°C

Lecture ERS 482/682 (Fall 2002) Output phase where Q m3 = net energy to melt the rest of the snow h wret = liquid water-retaining capacity of snowpack [m] [J m -2 ] Water output after snowpack is ripe Equations 5-14 and 5-15 where  w = incremental water output from snowpack S = net rate of energy exchanges into snowpack  t = time period [J m -2 day -1 ] [day] [m]

Lecture ERS 482/682 (Fall 2002) Energy exchange processes S = K + L + H + LE + R + G Equation 5-26 where K= shortwave (solar) radiation input L= longwave radiation H= turbulent exchange of sensible heat with atmosphere LE= turbulent exchange of latent heat with atmosphere R= heat input by rain G= conductive exchange of sensible heat with ground All expressed in units of [E L -2 T -1 ]

Lecture ERS 482/682 (Fall 2002) Shortwave radiation input, K Energy input due to sun’s energy –Depends upon slope, aspect, cloud cover, vegetation cover, albedo Figure 14.1 (Brooks et al. 1991)Figure (Dunne & Leopold 1978)

Lecture ERS 482/682 (Fall 2002) Shortwave radiation input, K Energy input due to sun’s energy –Depends upon slope, aspect, cloud cover, vegetation cover, albedo Figure 14.2 (Brooks et al. 1991) Figure 5-23 (Dingman 2002)

Lecture ERS 482/682 (Fall 2002) Shortwave radiation input, K Energy input due to sun’s energy –Depends upon slope, aspect, cloud cover, vegetation cover, albedo where K in = amount of shortwave radiation reaching snow  = albedo (fraction reflected back to atmosphere) Table D-6

Lecture ERS 482/682 (Fall 2002) Longwave radiation, L Terrestrial radiation, reflected solar radiation –Depends on temperature of earth’s surface, air temperature, canopy cover, cloud cover where  at = effective emissivity of the atmosphere  = Stefan-Boltzmann constant T a = 2-m air temperature 4.90  MJ m -2 day -1 K -4 °C Equation 5-38, 5-39 or 5-40 Adjusts for snow surface T ss = 0°C Equation 5-42

Lecture ERS 482/682 (Fall 2002) Turbulent sensible heat exchange, H Occurs whenever there is temperature difference between surface and air –Assumes neutral atmospheric conditions, wind speed and air temperature measured at 2 m above snow surface in clear non-forested area where v a = wind speed at 2 m above snow surface T a = 2-m air temperature T ss = temperature at snow surface m s -1 °C Equation 5-43 °C If there is a temperature inversion, should apply stability correction factors

Lecture ERS 482/682 (Fall 2002) Turbulent latent heat exchange, LE Occurs whenever there is vapor pressure difference between surface and air –Assumes neutral atmospheric conditions, wind speed and air temperature measured at 2 m above snow surface in clear non-forested area Equations 5-45 or 5-46 cold snow (T ss < 0°C) melting snow (T ss = 0°C)

Lecture ERS 482/682 (Fall 2002) Heat input by rain, R Rainwater gives up heat due to cooling to freezing point and/or freezing Equation 5-47 usually assumed = T a where c w = heat capacity of water r = rainfall rate T r = temperature of rain T m = melting temperature 4.19  MJ kg -1 K -1 [L T -1 ] °C 0 °C

Lecture ERS 482/682 (Fall 2002) Sensible heat exchange with ground, G Occurs whenever there is a temperature difference between snow and ground Usually negligible, but can be significant during accumulation period

Lecture ERS 482/682 (Fall 2002) Modeling snowmelt Energy balance approach (Figure 5-29) Temperature-index approach where  w = snowmelt for a certain time period M = melt coefficient T a = air temperature T m = melting temperature

Lecture ERS 482/682 (Fall 2002) Temperature-index approach Figure 14.4 and Table 14.2 (Brooks et al. 1991) wwww M