Lesson Plan from DW

Runoff from a Burn Scar—Glen Eyrie Castle and the Waldo Canyon Fire

 

Standards Addressed:

CDE Earth Science Content Standard #6: “The interaction of Earth's surface with water, air, gravity, and biological activity causes physical and chemical changes”

CDE Earth Science Content Standard #7: “Natural hazards have local, national and global impacts such as volcanoes, earthquakes, tsunamis, hurricanes, and thunderstorms”

Background:

[The following topics will already have been discussed in class prior to attempting this activity:

Watersheds

Wildfires and wildfire policy—characteristics, history, and effects]

 As a result of fire suppression over the last century, wildfires burn hotter at the ground now than they did prior to suppression activity.  These hotter fires result in “moonscapes”:  surfaces with no living plants remaining.  Since living plants hold soil in place, runoff from “moonscapes” tends to wash away large volumes of soil from the burn area.  In addition, torching of the upper few inches of soil tends to make the soil more hydrophobic, so less rainfall is able to infiltrate, increasing runoff volume and the potential for flash flooding.  Consequently, debris- and soil-laden flash floods are common in burn areas for years after the fire.

 

Most of the watershed above Glen Eyrie Castle near Colorado Springs was torched during the 2012 Waldo Canyon Fire.  In this activity, you will estimate how much water will run off from the watershed during a given rain storm and compare it to how much would have runoff during a storm of the same intensity prior to the fire.

 

Procedure:

1.      Go to www.arcgis.com and sign in using the student login information provided in class.

2.      Once logged in, click on “Groups”

3.      Inside “Groups”, click on “Chaparral HEES”; a map called “GE Watershed Map” should appear.

4.      Click on the word “Open” below the small map; the full-size map should open up.

5.      You should see a red flag northeast of Colorado Springs.  This flag marks the location of Glen Eyrie Castle, one of the oldest and most historic buildings in the area (see http://www.visitcos.com/glen-eyrie for the castle’s history).  Click on the flag to confirm you have found the right one—the words “Glen Eyrie Castle” should appear.

6.      The orange line on the map represents the extent of the area burned by the Waldo Canyon Fire; the green line outlines the watershed above Glen Eyrie.

 

7.      Measuring the area of the watershed:

a.    click on “Measure” above the map.  A box will appear that says “Find Area, Length, or Location.”

b.    Click on the first icon—the one that says “Area” when you hover over it.  You now are ready to measure.

c.     Click on the watershed boundary and release—you can start anywhere.  Now move the pointer to a new place on the water boundary and click again.  The program will draw a blue line between your clicked points.  It will kind of look like you’re stretching a rubber band around pins.  Do this all the way around the watershed, following its shape so your area is accurate.

d.    When you’ve made it all the way around, double-click on the last point.  A number of square miles will appear below “Measurement Result.”

e.     Record the area measurement.

8.      Calculating the total rainfall:  If a storm drops 1.5 inches of water over every point in the watershed area, how much total water volume is that?  Calculate your answer in cubic meters.

9.      Estimating the total runoff under “normal” (pre-fire) conditions:  Actual infiltration rates depend upon many factors—many more than we can consider here.  To keep it simple, let’s estimate that under pre-fire conditions, 80% of the rainfall volume would infiltrate the ground and not run off.  Calculate how much water (in m³) would run downstream under these conditions.

10. Estimating the total runoff under post-fire (“moonscape”) conditions:  Because fire-torched soil is significantly more hydrophobic, runoff from a “moonscape” burn scar is approximately seven times greater than under pre-fire conditions.

a.    Calculate how much water (in m³) would run downstream in this watershed under these hydrophobic soil conditions.

b.    What percentage of the rainfall infiltrates under these conditions?

Questions for discussion:

1.      The rate of rainfall influences how much water will infiltrate the ground.  For instance, more water will infiltrate if an inch of water falls over three hours than if it falls over only one hour.  Why does this occur?

2.      Three to four times more soil is swept away by runoff from a burn scar than would have been swept away under pre-fire conditions.  Why does this occur?

3.      Describe (list and explain) at least three characteristics of runoff from a burn scar that make it more destructive than runoff from pre-fire forest land.

4.      What do you think the caretakers of Glen Eyrie should do to prevent damage to the historic castle from flash floods while preserving the castle’s historic setting and conditions?