1. Snow is an important part of water supply in much of the world and the Western US.
Objectives
Describe how snow is quantified in terms of depth, density and water equivalent
Calculate the energy involved in phases of snowmelt
Describe how snow over a wateshed is measured
Describe the role played by snow in the changing hydrologic cycle

3. Why is Snow Important? Water Resources Flooding Economics
Transportation
from

4. S. John Wilkin/Herald Journal Utah hyrologists Ray Wilson, left, and Randy Julander weigh a sample of snow for "unit volume."
Snow pillow at SNOTEL site
S. John Wilkin/Herald Journal Ray Wilson takes a measurement of how compact the snow is at a site in Logan Canyon near Right Hand Fork.

5. SNOTEL

6. Snow Measurement Airborne Snow Survey Program (SWE)
from

7. Sierra Nevada fractional snow covered area (SCA) from MODIS
MODSCAG algorithm – Painter et al., 2008
SCA at left is binned into 4 classes for ease of viewing
Pixel size: 500 m
Data available for
Despite differences in elevation, latitude & accumulation, there is a remarkable consistency to rate & extent of snowmelt with elevation & latitude – Eastern Sierra Nevada example
SCA for March 10, 2008
from Roger Bales

8. Snow Pack Characteristics
from

9. Snow Pack Characteristics
Primary physical characteristics of deposited snow
Hardness
Strength
Water Equivalent
Depth
Grain Shape
Temperature
Density
Grain Size
Impurities
Liquid Water Content
Albedo
from

10. Snow Pack Characteristics
Snow Water Equivalent (SWE)
The height of water if a snow cover is completely melted, on a corresponding horizontal surface area.
Snow Depth x (Snow Density/Water Density)
from

11. Density of Snow Cover Snow Depth for One Inch Water Snow Type
Density (kg/m3)
Wild Snow
10 to 30
98” to 33”
Ordinary new snow immediately
after falling in still air
50 to 65
20” to 15”
Settling Snow
70 to 90
14” to 11”
Average wind-toughened snow
280
3.5”
Hard wind slab
350
2.8”
New firn snow
400 to 550
2.5” to 1.8”
Advanced firn snow
550 to 650
1.8” to 1.5”
Thawing firn snow
600 to 700
1.6” to 1.4”
from

12. Snow Energy Exchanges (K-K) + (L  - L) + Qe + Qh + Qg + Qp = Qb e d o w C a n o p y S h r t v e R d u c i L g E m s W T l A b x M E L T I N G R F Z S n o w a i V p r l e f c t d / m g v W C u K H u m i d t y E N R G Y M A S L L Q p K  Q Q e h Q Q g
from

13. Energy to melt snow Latent heat of fusion L= 79.8 cal/g [333 kJ/kg]
Ice heat capacity ci = cal g-1 K-1
Phases of Melting
Warming
Ripening
Melting

14. Example
Given 60 cm of snow at density 150 kg/m3 and Ts=-2 C, what is the SWE and energy required to melt it.
If air temperature is 4 C in the Sierra Nevada (Table 2-2 melt equations), how long will it take for this snow to melt

15. Snow and Climate Change
Land surface temperatures
5-yr average departure from mean
From Roger Bales

16. Observed changes in water cycle
less snow/more rain
Mote, 2003
TRENDS ( ) in
April 1 snow-water content at
western snow courses
Knowles et al.,
2006
-2.2 std devs
LESS as snowfall
+1 std dev
MORE as snowfall
less spring snowpack
earlier greenup
earlier snowmelt
Cayan et al., 2001
Stewart et al., 2005
From Roger Bales 16

17. Influence of +3ºC on SNOW vs RAIN
More rain, less snow
Earlier snowmelt
More winter floods
Historical, 0 to -3oC
Derived from UW’s VIC model daily inputs,
Bales et al., 2006
From Roger Bales

18. USU lysimeter drainage farm Cache Valley