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

Why is Snow Important? Water Resources Flooding Economics Transportation from http://www.nohrsc.nws.gov/

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.

SNOTEL

Snow Measurement Airborne Snow Survey Program (SWE) from http://www.nohrsc.nws.gov/

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 2000-2009 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 https://eng.ucmerced.edu/people/rbales/CV/Talks/

Snow Pack Characteristics from http://www.nohrsc.nws.gov/

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 http://www.nohrsc.nws.gov/

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 http://www.nohrsc.nws.gov/

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 http://www.nohrsc.nws.gov/

Snow Energy Exchanges (K-K) + (L  - L) + Qe + Qh + Qg + Qp = Q b 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 http://www.nohrsc.nws.gov/

Energy to melt snow Latent heat of fusion L= 79.8 cal/g [333 kJ/kg] Ice heat capacity ci = 0.501 cal g-1 K-1 Phases of Melting Warming Ripening Melting

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

Snow and Climate Change Land surface temperatures 5-yr average departure from 1901-2000 mean From Roger Bales https://eng.ucmerced.edu/people/rbales/CV/Talks/

Observed changes in water cycle less snow/more rain Mote, 2003 TRENDS (1950-97) 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 https://eng.ucmerced.edu/people/rbales/CV/Talks/ 16

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, 1950-1999 Bales et al., 2006 From Roger Bales https://eng.ucmerced.edu/people/rbales/CV/Talks/

USU lysimeter drainage farm Cache Valley