Runoff from a Burn
Scar—Glen Eyrie Castle and the Waldo Canyon Fire
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]
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:
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.
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?
