Mountain Watersheds Lab - student handout

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Colorado State University, Fort Collins *

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101C

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Geology

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Jan 9, 2024

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docx

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GEOL 121 Name _____________________________ Lab Section ________ 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 the links to bring up the map…try Snow Water Equivalent > Percent of 1981-2010 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 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 ( I would say on mountain ranges We will go to southwest Colorado, to the watershed of Los Pi ñ os 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 Page 2 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 who irrigate the lowlands using water from the Los Pi ñ os River (Figure 3 .( 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 ? September and October b . During which month(s) does it typically reach its annual max ? March and April c . By which month is it typically done melting ? August and September d . Which takes longer to occur - the accumulation or the melting ? Melting 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 have the most water scarce years, they had a SWE of 5.6 and 5.8 Reflection GPS – snowpack measurements SNOTEL stations are not the only way to measure accumulating snowpack. As you learned in Page 3
Mountain Watersheds Lab GEOL 121 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 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 ?( Considering that Montrose is different than most ecosystems the curve is very similar other than the more accumulation of snow in Montrose . 5 . List a few reasons that could explain the differences you observed . - Montrose had more snow accumulation in 2016 - The curves are not totally the same shape 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 Page 4
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