Mountain Watersheds Lab - student handout
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Colorado State University, Fort Collins *
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Course
121
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Geology
Date
Dec 6, 2023
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Uploaded by MegaSummerElephant42
GEOL 121
Name __Logan Ondrak__
Lab Section ___L21_____
Mountain Watersheds Lab
Measuring water in remote mountain watersheds can be challenging! But over three billion
people on Earth depend on water that flows down from high mountain areas, so it is also really
really important. In this exercise you will investigate in more detail:
o
how we measure surface water (snow and water)
o
what different methods are better at measuring
o
how seasons, drought cycles, climate change, and human use affect surface water
The case study site will be a Western Mountain Watershed in Colorado USA, but the same issues
are present in many mountains and the areas downstream of them around the world.
Answer the numbered question. The lettered sub-headings indicate the different sub-topics.
In the provided spreadsheet, you will find hydrologic data for a mountain watershed in southwest
Colorado, USA as measured by three difference methods: SNOTEL, reflection GPS, and stream
gaging.
You should see several tabs at the base of the document – each takes you to a difference
spreadsheet in this Excel Workbook.
SNOTEL stations – snowpack measurements
1.
Check out the link to the SNOTEL website, showing the map of stations
(
https://www.wcc.nrcs.usda.gov/snow/snow_map.html
- you will need to click on one of
This lab was developed by Jon Harvey (Fort Lewis College) and Becca Walker (Mt. San Antonio College)as a part of
the GETSI Project (
https://serc.carleton.edu/getsi/index.html
).
Page 1
Figure 1. Screenshot showing tabs at the
base of the workbook.
Figure 2. Map of USDA SNOTEL stations from
https://www.nrcs.usda.gov/wps/portal/wcc/home/quicklinks
Mountain Watersheds Lab
GEOL 121
the links to bring up the map…try
Snow Water Equivalent > Percent of 1981-2010
Median > Stations Only
). How would you describe the coverage of that network?
Where does it seem most stations are located?
(if you cannot access the website, look at Figure 2 instead)
IT seems like a lot of the stations are located witin or around the rocky mountains and in
higher elevated areas of the United states.
We will go to southwest Colorado, to the watershed of Los Piños River. This is a high-altitude,
mountainous setting in the southwestern US. Water users include residents of the towns of
Bayfield, Ignacio, the Southern Ute tribe, recreational users of Vallecito Reservoir, and farmers
who irrigate the lowlands using water from the Los Piños River (Figure 3).
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Figure 3. Map
showing location
of the Los Piños
River and Vallecito
Creek watershed
divides (black
outlines), gaging
stations (green
circles), and
SNOTEL station
(red circle) used in
the assignment.
Also shown are
the two nearest
towns (white
circles).
Mountain Watersheds Lab
GEOL 121
Precipitation is enhanced by the mountainous terrain, and the higher elevations receive up to 5
times as much precipitation as the lowlands (~50 in/yr in the high country and ~10 in/yr in the
low). Most of the high-elevation precipitation falls as snow, which accumulates throughout the
winter and melts away in the spring and summer. Rainfall occurs in the warm months too, but
only larger storms contribute significantly to river flow. Water from smaller showers is quickly
re-evaporated, taken up by plant roots, or otherwise absorbed by parched soils.
We will take a look at the most important water delivery mechanism to this area -
snowfall
. How
do we keep track of that resource? You may recall from the pre-lab videos that
SNOTEL
stations
track the depth and water content (snow-water equivalent, or SWE) of snowpack at that station
throughout the winter accumulation season.
A. How does the snowpack change over the course of a year?
On the ‘SNOTEL’ tab of your spreadsheet, you will find SNOTEL data from the “Stump Lakes”
station (location shown in Figure 1) from 2000 to 2023. Open it up and inspect the plot. Note that
you can mouse-over the plot to get the date and value at that point on the curve.
2.
Let’s start by describing the basic patterns present in the data:
a.
During which month(s) does snow typically
start
accumulating in a given year?
Snow typically starts occurring during December => march and even to April om
the data.
b.
During which month(s) does it typically reach its annual max?
It typically reaches its annual max within the month of December.
c.
By which month is it typically done melting?
It is typically done melting within around may, June or July.
d.
Which takes longer to occur - the accumulation or the melting?
It takes longer for the accumulation of snow, melting happens very fast.
B. How does snowpack change from year to year?
3.
Zoom out and, looking at the whole plot, make a prediction as to which two years in the
record had the most problems with water scarcity. What were the highest snow water
equivalent (SWE) values measured those years?
2002, and 2018 had the most water scarcity and most problems with it as 2002 had roughly 6
inches of snow equivalent and 2018 had around 8 inches of snow water equivalent which is
rough considering that every other year is above 10.
Reflection GPS – snowpack measurements
SNOTEL stations are not the only way to measure accumulating snowpack. As you learned in
the pre-lab videos GPS stations can give us estimates of snow depth at the station by measuring
the GPS satellite radio signals reflected off the snow surface. Note that these data do not provide
snow-water equivalent
values, so we get depth only. Here is a link to a website that shows the
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network of GPS stations that participate in monitoring snowpack:
https://cires1.colorado.edu/portal/
C. How do SNOTEL and GPS snow data compare?
In your spreadsheet, under the ref_GPS tab you will see data from the nearest Reflection GPS
station (120 km away in Montrose, CO).
Remember that most SNOTEL stations measure both snow
depth
and snow
water equivalent
(SWE; the actual amount of water stored in the snowpack) and Reflection GPS measures only
snow depth.
4.
Compare the Montrose, CO reflection GPS data to the SNOTEL data. How well do the
two curves resemble each other (keeping in mind that the station is ~75 mi away)?
The curves resemble each other surprisingly well but with the SWE there is less then
snow depth as snow depth and less water can be a result of different locations. But they
are relatively similar.
(REWRITE THIS A LITTLE BIT)
5.
List a few reasons that could explain the differences you observed.
A few reasons for the differnces are the location, height of where the snowfall is, as well
as temperature that is keeping the snow in snow form instead of melting it down.
D. What happens as the snow melts?
All of that snow melts as temperatures rise in the spring and summer. Some of it enters the
groundwater system, some evaporates or is used by plants, but much of it ends up flowing
downhill in rivulets and gullies that enter streams, which join to form larger rivers. This “surface
water” transport pathway is important to monitor as it delivers most of the meltwater to its
ultimate users. To help keep track of this surface water resource, the USGS maintains a network
of river gages across the country. Check out the coverage map here:
https://waterdata.usgs.gov/nwis/rt
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Mountain Watersheds Lab
GEOL 121
6.
List any parts of the country (e.g. state names or regions) that appear to be experiencing
high water at the moment (look for stations with dark blue dots). Make a separate list for
areas that are dealing with unusually low water (orange and red dots).
More of the mountain west region is facing high water at the moment on this chart. And places
that are dealing with lower water are in the east central area, as well as a little
bit in the western
great lakes area.
Back to our study watershed in southwest Colorado: The first river gage that melting snow in the
Los Piños watershed will encounter is the Vallecito Creek gage (See Figure 3 for location). Go to
the SurfaceWater tab of your Excel sheet to see data from that gage.
Inspect the plot. The Y-axis is showing the volume of water flowing past the gage in a given
time. In this case, it is the number of cubic feet per second (cfs) flowing past the gage. For
reference, there are around 7.5 gallons in one cubic foot. So, a flow of 100 cfs = 750 gallons
flowing past the gage every second.
7.
Describe the behavior of the flow in a given calendar year.
a.
During what month(s) does the flow start to increase in a given year?
The flow starts to increase in march and really picks up in April, may and June.
b.
By what month does it usually trail back down to background levels?
It usually trails back down in around the end of July and beginning of august.
8.
Based on that annual cycle, during what months should farmers have access to surface
water for irrigation?
The farmer should have access to surface water in march => June.
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Mountain Watersheds Lab
GEOL 121
E. Do we see correlations between drier snow years and decreased river discharge?
9.
Thinking back to question #3 about years with low snowpack - How accurate were your
predictions about years of water scarcity? In which two years do we see the lowest
discharges coming down Vallecito Creek? Write down the years and corresponding peak
snowmelt discharges of those years. Are those the same years that you identified in your
answer to #3? Explain.
My prediction was correct as the water scarcity was in 2002 as well as 2018,
in 2002 the creek
snowmelt discharge was 400 CFS and in 2018 it was a little above 400 CFS. And these are both
the years that I identified as the ones with the lowest snowmelt.
F. What about reservoirs? Do they show similar changes to rivers?
Like so many river systems in the western US, the waters of Vallecito Creek are eventually
impounded behind a dam (forming Vallecito Lake) that provides flood control, water storage
capacity, and recreation through boating and fishing. The water storage capacity provided by the
reservoir serves an important role - it allows farmers to stretch the normal runoff season longer
into the growing season. Reservoirs like Vallecito also serve as important benchmarks for water
users downstream - in general, they tell us something about how much water is left to send
downstream.
On the same sheet in your Excel file, take a look at the plot of lake elevation for Vallecito
Reservoir. The following questions are based on that plot (remember that you can get the date
and Y-axis value off the chart by mousing over it in you are using Excel):
10. During what month(s) does the lake level typically reach its annual peak?
It reaches its annual peak in the middle of the year which could be around may, June and
July.
11. Based on the chart, what is the maximum elevation of the reservoir? (hint: the water level
cannot rise any higher than the maximum)
The maximum elevation of the reservoir is going to be around 7665 ft.
12. During what month(s) do the dam operators tend to release water? (hint: releases send
water to downstream users, and are marked by sudden downward trends in the lake
elevation).
Dam operators typically release the water from July to October is when they will be
releasing the water. This causes a huge down spike in water elevation .
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13. During the low-water years you identified in #3 and #9, what happened with the lake
elevation?
The lake elevation did not meet the maximum peak, rather the highest it got would be
roughly around how much water was in the river after the dam release.
G. How do dams and diversions impact river flow below them?
Most of the users of the Los Piños River are
downstream
of Vallecito Reservoir. See Figure 2 for
location of the Los Piños river gage. This instrument can tell us how much water is being
delivered to downstream users and is important to monitor because
hydrographs below dams
often look quite different from those upstream of the dam.
14. How do flows below the dam (bottom plot) compare to natural flows in Vallecito Creek
(top plot)? Do you see any major differences in timing or magnitude of flows? Explain.
They compare as the below the dam plot has less water while above the dam has more water. The
timing of the water flow is also roughly the same between both of the rivers. The below river has
slightly higher peaks in discharge compared to the higher plot though.
15. How did the low-water years you identified in the snowpack data (#3) show up
downstream of the dam? Speculate on the impact that year would have had on farmers
and other users downstream. What options might they have had?
Downstream of the dam or below the dam the years with low water did not even show up,
they barely had any water, this can impact farmers downstream as less water makes it harder
for crops to grow and thus a less productive year for the farmers.
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Mountain Watersheds Lab
GEOL 121
16. Based on the above info, what hydrosphere measurement technique(s) would be most
appropriate for a farmer in Bayfield, CO (Figure 3) to use to keep track of water
resources as they prepare their upcoming farming/irrigation season?
The best hydrosphere measurement technique would be to have flow meters for
determine what would be the intake of water to the crops, Soil moisture sensors and water
quality tests.
H. Comparing snow and water the data sets and what they imply for society
Most of the charts used in this exercise are compiled in the ‘Charts’ tab. Open that sheet, and
take ~5-10 minutes to inspect how water resources move through this mountain watershed.
Questions to ponder:
17. How does timing of ‘peak water’ change from snowpack to river flow to reservoir level
to below-dam river flow?
peak snowpack: 36 inches
peak streamflow: 1800 CFS
peak reservoir levels: 7665
peak below-dam streamflow: 2,600 CFS
18. Where in this system are drought years likely to be felt most strongly?
The drought years are to be most likely felt in moderate amount of snow water equivalent
years, as seen in a medium influx in 2001 and then little to none in 2002.
19. Consider 2015. Note from the SNOTEL chart that it was an unimpressive snow year.
Speculate on how 2015 could also have produced one of the largest flows on Vallecito
Creek and the Los Piños River? (it is OK if you are unsure – there are multiple plausible
answers here – just give your ideas).
It didn’t have lots of snowfall, but when it did snow, it all happened at the same time,
creating a large pack of snow that takes longer to melt over time, which in turn makes the
graph look more stretched out.
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Mountain Watersheds Lab
GEOL 121
20. Check out the flood on Vallecito Creek in Oct 2006. What was its peak discharge? What
does that flood wave look like downstream on the Los Piños gage? Explain.
The peak of the discharge occurred on 10/23/06 and was valued at around 765, this flood
wave had a large impact downstream which resulted in it affecting the los Pines gage
21. Under global climate change, many western mountain watersheds are expected to see
more of their winter precipitation fall as rain not snow; and snow that does come is
expected to melt earlier.
Sketch what a typical year of SNOTEL data looks like (solid line). (Hint: you can just
look at the graph of the SNOTEL station to get the idea for this, but only sketch one
“typical” year here
.)
Add in a dashed line of what we would expect it to look like with a warmer climate.
Sketch what a typical year of the Vallecito Creek gage looks like (solid line). Add in a
dashed line of what we would expect it to look like with a warmer climate.
Page 9
JanMarMayJulSepNov
Snow
amount
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How might that affect the local cycle of water storage and water availability for users in
the area?
Warmer climates will have less water storage and can affect water storage and avalibility for its
user
22. Based on the example of the Los Piños river system, describe one benefit and one
drawback to putting dams on rivers in western mountain watersheds.
The benefits are that they have water storage that they can then use for later use and the
drawback would be if they don’t have it lifted it can result in droughts.
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JanMarMayJulSepNov
River
discharge