How Much Water is in a Mountain Snowpack? Data Analysis Activity to Accompany “Water: A Zero Sum Game” Video Grade Levels: 6-­‐12 Background In Colorado, the majority of the water supply comes from snowmelt. Water managers, whose job is to plan for water supply for Colorado residents, closely monitor snowpack because it tells them how much water will be available the following spring. SnoTel is a network of snow depth sensors that water managers rely on for data on water availability in the snowpack. Researchers at Niwot Ridge Long Term Ecological Research site monitor snowpack properties by maintaining a SnoTel site, and also take weekly measurements by hand. CU Boulder students dig two snow pits at Niwot each week at different elevations and take measurements on snowpack depth, density, and stratigraphy (layers of different densities in the snowpack). In this activity, you will assume the role of a snow hydrologist and a water manager, and use data from Niwot Ridge to determine how much water is held in the snowpack of Green Lakes Valley. Green Lakes Valley is in the Boulder Creek watershed, and is located on the continental divide about 40 miles away from Boulder. Calculating Snow Water Equivalence (SWE) Snow water equivalence (SWE) is the amount of water contained in a volume of snow. Using data collected by researchers at Niwot Ridge in Green Lakes Valley, you will calculate maximum SWE for years 2013 and 2014. The SWE results represent the “height” of liquid water contained in the snowpack at a single point – for example, if a 1 meter deep snowpack with a SWE of 10 centimeters somehow melted instantaneously, there would be 10 centimeters of water on the ground. Data from a snow pit is recorded on a “pit sheet.” An example pit sheet is on the right. The pit sheet information used for calculating the snow water equivalence (SWE) are found in the columns second and third from the left: • “Net weight (g)” is the mass of the snow collected in a 1 liter snow sample. Thus, the value found in this column is g/L and is the density of the snow. • “Height above ground (cm)” is the measured height for each interval where density was measured. Thus this is the height of the top and bottom of the snow layer. Notice the top of one layer is the bottom of the layer above it. To find the depth of each layer, simply subtract the two heights. Most of the layers are 10cm high. 1 Snow density changes over the depth of a snowpack. In order to capture the different densities (and thus the different water contents) of varying layers throughout the snowpack, snow density measurements are made every 10 centimeters from the snow surface to the ground. The density at each 10 cm interval is recorded and used to calculate the SWE for that layer. By understanding density changes within the snow pack, we can more accurately calculate SWE. 𝑠𝑛𝑜𝑤 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
𝑆𝑊𝐸 = 𝑠𝑛𝑜𝑤 𝑑𝑒𝑝𝑡ℎ ∗ 𝑙𝑖𝑞𝑢𝑖𝑑 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
Liquid water density is a constant equal to 1000 kg/m3. The total estimated SWE for the snowpack is the sum of all the layers. Example calculation: Calculate the SWE for a layer that has a density of 450 g/L. Remember that g/L = kg/m3. 𝑆𝑊𝐸 = 𝑠𝑛𝑜𝑤 𝑑𝑒𝑝𝑡ℎ ∗ !"#$ !"#$%&'
!"#$"% !"#$% !"#$%&'
= 10𝑐𝑚 ∗ !"
!!
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!""" !!
!"# = 4.5 𝑐𝑚 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑤𝑎𝑡𝑒𝑟 Attention! Carefully examine the pit sheet data. You’ll notice that not every layer is 10cm high. For instance, the surface layer is 83cm-­‐80cm = 3cm. By using the above equation for each layer, you can accommodate the different layer heights in the snow pit. 2 Using the snow pit data below, calculate SWE for 2013 and 2014. 2013 2014 Layer Layer Snow Snow Position in Position in Depth Density SWE (cm) Snow pit Snow p
it (cm) (g/L) (cm) (cm) 180-­‐191 170-­‐180 160-­‐170 150-­‐160 140-­‐150 130-­‐140 120-­‐130 110-­‐120 100-­‐110 90-­‐100 80-­‐90 70-­‐80 60-­‐70 50-­‐60 40-­‐50 30-­‐40 20-­‐30 10-­‐20 0-­‐10 11 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 348 348 328 415 512 507 560 543 584 540 512 580 536 494 450 425 425 367 367 Total SWE: Remember: 𝑆𝑊𝐸 = 𝑠𝑛𝑜𝑤 𝑑𝑒𝑝𝑡ℎ ∗ 210-­‐214 200-­‐210 190-­‐200 180-­‐190 170-­‐180 160-­‐170 150-­‐160 140-­‐150 130-­‐140 120-­‐130 110-­‐120 100-­‐110 90-­‐100 80-­‐90 70-­‐80 60-­‐70 50-­‐60 40-­‐50 30-­‐40 20-­‐30 10-­‐20 0-­‐10 Snow Depth (cm) 4 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 Snow Density (g/L) 267 224 230 287 350 447 392 374 407 407 448 408 423 445 442 430 411 401 413 439 413 413 Total SWE: SWE (cm) 𝑠𝑛𝑜𝑤 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
𝑙𝑖𝑞𝑢𝑖𝑑 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
Liquid water density is a constant equal to 1000 kg/m3. The total estimated SWE for the snowpack is the sum of all the layers. Note: Data courtesy of CU-­‐Boulder Professor Mark Williams, Niwot Ridge LTER, http://niwot.colorado.edu 3 Estimating basin-­‐scale snow water resources We can apply the SWE value over an area of snow to get the volume of water stored in the snow pack in a given area. Example calculation (*remember to convert your units): Water Storage (gallons) = SWE (m) * Basin Area (m3) Note: Water managers typically use acre feet as the unit for large volumes of water, not gallons. In gallons, estimate how much water was stored in the Green Lakes Valley basin in 2013. Also estimate water storage for 2014. • The area of the Green Lakes Valley basin is 4 hectares. Let’s assume this entire area is covered with the same depth of snow (in reality, some areas are rocky buttresses, others are deep drifts). • Take the SWE calculated from the pit sheet to be the average SWE in the basin. (Disclaimer: this is large assumption, remember that SWE is highly variable over space) • 1 hectare = 10,000m2 • 1 m3 = 264.2 gallons The average Denver citizen uses 82 gallons of water per day (source: Denver Water). With the amount of water stored in the Green Lake Valley at maximum snow accumulation, how many citizens could be supplied with water for a year by the water stored in the snowpack in Green Lake Valley basin in 2013? How about in 2014? 4