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Worksheet Grade 9 Science

Thornbury Ridge Snowpack: Water Equivalent and Spring Runoff

A grade 9 worksheet where students turn snow survey depths into snow water equivalent, scale them to basin runoff in acre-feet, and compare a warmer modeled year.

Objectives

  1. Students will calculate snow water equivalent from measured snow depth and snow density.
  2. Students will scale a basin average snow water equivalent into a runoff volume in acre-feet.
  3. Students will compare a recent average snowpack with modeled output for a warmer year and explain the effect on water supply.

Vocabulary

Snowpack
The snow that piles up in the mountains over the winter. As it melts in spring and summer, it acts like a giant water tank feeding rivers and reservoirs.
Acre-foot
The amount of water it takes to cover one acre of land, about the size of a football field, one foot deep. Western states use acre-feet to measure large amounts of water.

Also: snow water equivalent, runoff, cryosphere

Teacher overview

Snow that falls in the mountains above the fictional town of Thornbury is stored for months before any of it reaches the river. Snow surveyors measure the depth of the pack and its density, then convert those two numbers into snow water equivalent, which is the depth of water the snow would produce if it melted all at once. This worksheet has students run that conversion for three snow courses in the Vesper basin, scale the basin average into a volume in acre-feet, and compare a recent average year against output from a warmer modeled year. One misconception is worth naming before students begin: many students read snow depth as the amount of water in the pack, so a 96-inch snowpack looks like 96 inches of water to them. Density is what separates the two numbers, and light early-season snow holds far less water than the same depth of dense spring snow.

Thornbury Ridge Snowpack: Water Equivalent and Spring Runoff — Grade 9 Science
Name: ____________________________ Date: ______________

Student Worksheet

Surveyors measured three snow courses in the Vesper basin on April 1. The basin drains into Thornbury Reservoir. Use the table below for items 1 through 9. Snow water equivalent equals snow depth times snow density. Round to one decimal place unless an item says otherwise.

Snow course Elevation (ft) Snow depth (in) Snow density (as a decimal)
Aster Saddle 6,200 48 0.25
Vesper Divide 7,800 72 0.30
Granite Bench 9,100 96 0.35
  1. Find the snow water equivalent at Aster Saddle, in inches.
  2. Find the snow water equivalent at Vesper Divide, in inches.
  3. Find the snow water equivalent at Granite Bench, in inches.
  4. Find the average of your three snow water equivalent values. Use this basin average in the items that follow.
  5. The basin above the reservoir covers 96,000 acres. Multiply your basin average from item 4 by 96,000 acres to find the water held in the snowpack, in acre-inches.
  6. Convert your answer from item 5 into acre-feet. There are 12 acre-inches in 1 acre-foot.
  7. About 70 percent of the snowpack water reaches the reservoir. The rest turns straight from snow to vapor, evaporates after melting, or soaks into the soil. Find the runoff that reaches the reservoir, in acre-feet.
  8. Thornbury Reservoir holds 90,000 acre-feet of usable storage. How many acre-feet of the runoff in item 7 are more than the reservoir can hold, so that the water must be used or released as it arrives?
  9. Thornbury and its neighbors use 55,000 acre-feet of water in a year. The runoff in item 7 is what percent of that yearly demand? Round to the nearest whole percent.

Items 10 through 14 use the table below. It reports output from a basin snowpack model that the fictional Thornbury utility ran for a warmer winter.

Scenario Basin average snow water equivalent (in) Date of peak runoff
Recent 30-year average 22.4 May 28
Modeled warmer year 15.4 May 7
  1. Using the modeled snow water equivalent of 15.4 inches and the same 96,000 acres, find the water held in the modeled snowpack, in acre-feet.
  2. Apply the same 70 percent runoff fraction to your answer from item 10 to find the modeled runoff, in acre-feet.
  3. By what percent does the modeled runoff in item 11 fall below the runoff you found in item 7? Round to the nearest tenth of a percent.
  4. In the modeled year, peak runoff arrives about three weeks earlier. Explain why an earlier peak causes trouble for a reservoir that must also keep space open in spring for flood control.
  5. Three Earth systems appear in this data: the atmosphere, the cryosphere, and the hydrosphere. In two or three sentences, describe the chain that connects a warmer winter to a smaller summer water supply.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 12.0 inches. 48 in times 0.25 equals 12.0 in of snow water equivalent.
  2. Answer: 21.6 inches. 72 in times 0.30 equals 21.6 in.
  3. Answer: 33.6 inches. 96 in times 0.35 equals 33.6 in.
  4. Answer: 22.4 inches. 12.0 plus 21.6 plus 33.6 equals 67.2, and 67.2 divided by 3 equals 22.4 in.
  5. Answer: 2,150,400 acre-inches. 22.4 in times 96,000 acres equals 2,150,400 acre-inches.
  6. Answer: 179,200 acre-feet. 2,150,400 acre-inches divided by 12 equals 179,200 acre-feet.
  7. Answer: 125,440 acre-feet. 179,200 times 0.70 equals 125,440 acre-feet of runoff.
  8. Answer: 35,440 acre-feet. 125,440 minus 90,000 equals 35,440 acre-feet beyond usable storage.
  9. Answer: 228 percent. 125,440 divided by 55,000 equals 2.2807, which is 228 percent of yearly demand.
  10. Answer: 123,200 acre-feet. 15.4 in times 96,000 acres equals 1,478,400 acre-inches, and 1,478,400 divided by 12 equals 123,200 acre-feet.
  11. Answer: 86,240 acre-feet. 123,200 times 0.70 equals 86,240 acre-feet.
  12. Answer: 31.3 percent. 125,440 minus 86,240 equals 39,200, and 39,200 divided by 125,440 equals 0.3125, which is 31.3 percent lower.
  13. Answer: The reservoir cannot hold the whole spring flood at once, so an earlier peak forces operators to release water while they still need empty space for flood control. Water released in early May is not available in August, when demand is highest and the snowpack is already gone. Accept any answer that connects earlier arrival to releases that cannot be stored.
  14. Answer: A warmer atmosphere delivers more winter precipitation as rain and melts the pack sooner, so the cryosphere stores less water for less time. The hydrosphere receives that water as an early, sharp pulse in the river instead of a slow release through summer. Reservoir storage has to replace the storage the snowpack used to provide, and it is not large enough to do so.

Differentiation

Support

Give students the first table row worked out, showing that 48 times 0.25 equals 12.0 inches, and provide a conversion strip stating the acre-inch to acre-foot relationship. Allow items 5 through 7 to be completed with a partner before students return to independent work at item 8.

Extension

Ask students to find the snow density the modeled warmer year would need at Granite Bench, holding depth constant at 96 inches, for the basin average to return to 22.4 inches. Assume Aster Saddle and Vesper Divide fall by the same 31.25 percent as the basin average in the modeled year (to 8.25 and 14.85 inches). Have them explain whether that density is physically reasonable for spring snow.

ELL supports

Pre-teach depth, density, equivalent, and runoff with a labeled drawing showing a column of snow beside the shorter column of water it melts into. Provide a sentence frame for items 13 and 14: "A warmer winter means the snow melts ___, so the river rises ___ in the year."

Extensions

Have students plot the two peak-runoff dates on a calendar strip alongside the months of highest water demand in their own region, then write a short paragraph naming which month becomes hardest to supply if the modeled shift holds.