Laboratory 3 – CE 321, Fall 2012
Water Budget of the Monocacy Creek
You may work in pairs at the computers.
Objectives
 do a water balance analysis for a nearby watershed similar to the Bushkill using publicly
available data from the www
 determine and plot annual values of P, Q, and ET and determine the Q/P and ET/P ratios and
their variation
Background
Maintaining adequate clean water supplies for humanity and ecosystems is an increasing
challenge in the 21st century. In areas like the southwest, there are no additional sources of water
to be tapped. One of the goals in sustainable water use or water resource engineering is to
maintain the “natural water balance”. Urbanization can impact the water balance negatively by
increasing runoff and decreasing transpiration from plants. Overpumping of groundwater for
irrigation or water supply is equivalent to mining a nonrenewable resource. For development to
be sustainable from a water resource perspective, components of the water cycle like the amount
of streamflow and evapotranspiration should be maintained indefinitely.
The water balance equation for a watershed is an example of the continuity equation or
mass balance:
dS
 PQ ET
dt
(1)
where S = water storage in the watershed (lakes, groundwater, soil moisture, etc.), P =
precipitation, Q = streamflow, and ET = evapotranspiration.
Of the four terms of the equation, P and Q are routinely measured using rain gages and
stream flow gages respectively, but dS/dt and ET are more difficult to measure. For example,
changes in storage (dS/dt) require measuring water table elevations from wells, and also soil
moisture at many locations in the watershed. ET measurements require sophisticated
instrumentation mounted on towers above the vegetation – again at many locations.
However, when summed over a year, the changes in storage volume are generally much
smaller than the volume of inputs and outputs, and we can neglect this term. So equation (1)
simplifies to:
Pann  Qann  ETann
(2)
This equation says that over the span of a year, the incoming precipitation is approximately
balanced by the sum of streamflow and evapotranspiration (where both have been converted to
an equivalent height of water, e.g., inches). Note that we are assuming here that there are no
major additional anthropogenic sources and sinks of water within the watershed.
Equation (2) is often used to calculate annual water budgets, that is, the division of the input P
between Q and ET. For such calculations, the water year (WY) does not follow the normal
calendar year, but rather runs from Oct 1 to Sept 30 of the following year.
Procedure
Data files to use in this lab will be taken directly from the web.
For daily streamflow from Monocacy Creek, just west of the Bushkill watershed, we will use:
http://waterdata.usgs.gov/pa/nwis/dv/?site_no=01452500
For output format, select “Tab-separated” and enter the dates, and hit Go
Now copy the data into Excel and separate into columns
Be sure to record the watershed area from the website – we will need this to convert the
streamflow data (in cfs) to inches.
For daily precipitation from the nearby Lehigh Valley airport, we will use:
http://climate.met.psu.edu/www_prod/ida/index.php?t=3&x=faa_daily&id=KABE
You can navigate there from http://climate.met.psu.edu/www_prod/data/
Select data archive, and then FAA Daily in the pull-down menu, then the Allentown airport
Now select the dates and the data desired (24 hour precip)
If you change the output filetype to CSV file, you will get a data file that is easy to read
with Excel
NOTE: there are some Missing Dates in the precip data (e.g. June and Aug 2000, possibly others) – you may
want to insert lines here so that your data for P and Q match up line-for-line!
Using the last 40 years of data (Oct 1, 1970 - Sept 30, 2010), do the following in Excel – note,
everyone should work through the first year together, and then repeat the process:
1. Sum up the daily P and Q data for each water year (Oct 1 - Sept 30), using inches for both.
You should wind up with one value for P (in) and one value for Q (in) for each year.
Note: to convert the flow Q for each day from cfs to inches, you must divide the volume of
flow by the watershed area [pay attention to units!]:
Q (in ) 
Q ( cfs) x 24hrs x 3600 sec/ hr 12in
x
Area ( ft 2 )
ft
2. Using equation (2), determine the annual ET for each year
3. Make a column plot of P, Q, and ET vs. year
4. Make a column plot of the ratios Q/P and ET/P vs. year
Discussion questions:
1. What is the average percentage of P that becomes Q? What is the average percentage of P
that becomes ET? What is the range of variation of each?
2. Why might the Q/P and ET/P ratios be varying from year to year (note: it is not because P
varies, the question is why does the ratio vary)?
3. List the assumptions we are making in the calculations.
4. Do you think these are reasonable assumptions? Explain your answer.
5. List 5 academic tools used throughout this lab session that were useful in addressing the
problem presented. (example…working with large amounts of data)
For your lab write-up, provide a short summary with your plots and answers to the five questions
above.
Due Date – Next Lab Session – Must be dropped of to my office no later than 1 hours after
laboratory session.