WSM 462/562: Watershed Management
Spring, 2012: Exam
Name:____________________
Open Book
Important: SHOW ALL WORK AND UNITS!
Conversions: 1 sq. mile = 640 acres, 1 acre = 43,560 sq. ft., 1 inch = 2.54 cm, 1 hectare = 10,000 sq.
meters, 365 days = 1 year, 1 foot = 12 inches, 1 meter = 100 centimeters = 1000 millimeters, 1 sq.
meter = 10,000 sq. centimeters, 1 kilogram = 1000 grams, 1 U.S. short ton = 2000 lbs, 1 kilogram =
2.205 lbs.
1. What is a watershed? Identify and briefly discuss three physical/hydrologic characteristics that
make arid and semiarid watersheds in the southwestern United States unique (10 points).
2. You are studying interception in a forest ecosystem on a 0.5 hectare plot. For a 1.25
centimeter rainfall event you collected a total of 1,000 grams of water from 15 rain gauges with
an inside diameters of 10 centimeters located under the forest canopy. For stemflow 800,000
grams of water was collected from the plot. What was the interception capacity of the forest canopy
for this event (10 points)?
3. Estimate the actual annual evapotranspiration between Oct. 1, 1987 and Sept. 30, 1988 from a
research plot given (10 points):
Annual Precipitation (Water Year between Oct. 1st – Sept 30th ) = 50 cm/yr
Maximum Snowpack Accumulation (SWE) = 15 cm
Maximum Snowpack Depth = 75 cm
Average Annual Temperature = 15 C
Annual Runoff from Plot = 0.7 cubic meters/yr
Estimated Interception on Plot = 10 cm/yr
Soil Depth of soil samples = 3 meters
Average Bulk Density for 3 meters = 1.35 g/cm^3
Average Soil Moisture by Weight on Oct 1 = 0.2
Average Soil Moisture by Weight on Sept 30 =0.22
Average Soil Moisture by Volume on Oct. 1 = 0.27
Average Soil Moisture by Volume on Sept. 30 = 0.30
Deep Seepage Loss from Plot = 5 cm/year
Forest Basal Area = 20 cubic meters/ha
Herbaceous Ground Cover = 80%
Number of Rattlesnakes Found on Plot = 2 snakes
Plot Size = 10 square meters
4. Indicate in the following cases if the channel would undergo aggradation, degradation, or be
relatively stable, and explain why (10 points)?
a. Increase in the number and magnitude of runoff events into a stream from a developed area.
b. Increase livestock damage on the channel banks causing the banks to start slumping.
c. The local flood control district has cleaned all the large woody debris from a stream channel.
d. Increase in unimproved logging roads near and crossing a stream channel.
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e. An undisturbed wildland stream is incorporated into a protected wilderness area.
5. You have been requested to review the effects of a high density residential development (1/4 acre
lots) on peak discharges (Qp) for the 1 hour, 100 year rainfall event near Tucson, Arizona (P = 3.5
inches). The watershed is 1.2 square miles in size. A 0.4 square mile tributary within the watershed
will be developed. The outlet of the developed tributary will have a retention pond. The time-ofconcentration for the watershed is 0.7 hours before development and is 0.6 hours after development.
Initially, the watershed consists of hydrologic soil group C, with a natural desert shrub vegetation
cover (good condition). See attached tables. (20 points).
a. What is the volume (inches) and peak discharge (cfs) from the whole watershed before
development?
b. What is the volume (inches) and peak discharge (cfs) from the whole watershed after
development, without the retention pond?
c. What does the capacity (in cubic feet) of the retention pond for the tributary have to be in order to
retain the additional runoff (compared to the undeveloped case) from the tributary after
development?
d. The developer has proposed installing stormwater wells and infiltration strips with the tributary
watershed. The plan would be to route 15% of the developed subwatershed to these structures (and
these areas will not yield runoff). How would that affect the required capacity of your retention
pond?
6. Thirty years ago a stock pond was built in the channel of a small 400 acre watershed. The pond
has since collected the sediment from upstream watershed. The original design water storage
capacity of the pond was 10,000 ac-ft. Thirty years later the water storage capacity is 9,750 ac-ft
(15 points).
a. Estimate the annual rate of erosion in tons/ac-yr from this watershed. Assume a sediment
density of 16 lbs/cubic feet and that the sediment delivery ratio for the watershed is 1.
b. Given R = 80, average slope gradient of 10% and average slope length of 165 feet, what is the
cover factor (C) for the USLE for this watershed? Assume the P factor is equal to 1.0. A soil
survey has indicated that soil texture is: 50% silt and fine sand, 35% sand, and 15% clay, 1%
organic matter, medium granular structure and moderate permeability.
c. Given this C factor, would you say the watershed had sparse cover (20% total cover) or good
cover (80% total cover)?
7. You have been asked to evaluate the impact of a road restoration project on the Boise National
Forest, ID. The road segment is 5 m wide, has a gradient of 6%, and has a length of 500 m. The
buffer slope is about 35 percent and the current buffer length is 25 m. The fillslope length is 5 m
with a 45 percent slope. The current ditch spacing is 100 m. The nearest climate station is
Deadwood Dam. The site is on the Idaho Batholith, a coarse-grained soil most closely described
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as sandy loam (20% rock). The road is well established with a vegetated ditch (insloped). Road
has a Native surface and has High traffic. Use the WEPP: Road tool at
http://forest.moscowfsl.wsu.edu/fswepp/ http://forest.moscowfsl.wsu.edu/fswepp/ (15 points)
a. What would be the impact on sediment yield if the road was moved to there was a buffer length
of 80 m? The road would still be insloped, but with a rocked ditch. This is the most costly option
($$$$). Is this a viable alternative?
b. What would be the impact on sediment yield (sediment leaving buffer) if the road was
converted to outslope, rutted? This is the second most costly option ($$). Is this viable
alternative?
c. What would be the impact on sediment yield is the fill slope was modified to 35 percent slope
with a length of 8 m. The buffer length would be 18 m. T his is the least costly option ($). Is this a
viable altnernative?
d. What would you do? Why?
8. You have a check dam on a 0.5 square mile watershed that has a time-of-concentration of 0.5
hours and curve number of 75. The broad rectangular spillway of the check dam is 1.5 meters wide
(L) and 0.2 meters in height (H), including freeboard. Would the spillway be able to handle the
peak discharge from a 2.0 inch rainfall event (10 points)?
Take Home Questions (Due May 1, 2012)
10. You have been asked to develop a presentation (60 minutes) to a stakeholder group on the
relationship upland watershed condition and riparian/channel condition. Outline the important
points you want to illustrate in the presentation (20 points).
11. What is the role of watershed management in society (30 points)?
There is no right or wrong answer to this question, but if I receive a "throw away answer" and I
will give a "throw away grade." The response does not need to be long, something between 4 to
6 pages, but give it some thought and reflection. What are today's problems and what role do
you see for yourself in solving them. Students in the past have changed their educational
programs based on their answers to this question. Hopefully answering this question will help
you define why you are here and why you took (or shouldn’t have taken) this class.
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