Activity 3 Data Diving
During monsoon season when the rains are strong the streets of Tucson flood, the
arroyos fill with water and the Santa Cruz River flows. The Santa Cruz River passes by
Nogales, Tucson and Maranna – which way is it going and where is the flow strongest?
And is the monsoon season the only time of year you find water flowing in the Santa
Cruz River?
In this activity you’ll use rainfall and stream flow data from data collection stations
(gaging stations) along the Santa Cruz River to answer these questions. Before you
start with the actual data answer the following questions. Take a guess if you’re not
sure:
1. In what direction does the water in the Santa Cruz River flow?
2. Where is the flow of the Santa Cruz River the strongest?
3. In what months of the year does the Santa Cruz River flow the most?
Materials
Santa Cruz River data (EXCEL).
Tucson and Nogales USGS topographic 1:250,000 maps
Calculator
Ruler
Part I: Getting acquainted with the Santa Cruz River gaging stations
1. Mark all four gaging stations on a map using coordinates provided on your
datasheet. Label gaging stations and mark their elevations on the map.
2. Draw the approximate boundaries for the watersheds for each gaging station on
the map.
3. Determine the distance of each gaging station from the highest gaging station
(the station that is farthest upstream). Record the distances in the empty column
marked distance in Table I – remember to note the units you are using.
Table I.
Station
Name
1) highest
Elevation
Distance from
Station (1)
0
2)
3)
4) lowest
4. Create a channel profile. Make a line graph and plot distance on the x-axis and
elevation on the y-axis for each gaging station. Label your axes (include units)
and mark your points with the appropriate name of each gaging station. See
Channel Profile example PDF.
5. Calculate the change in elevation, the change in distance and the slope
(gradient) between each gaging station. Record your data in Table II. See
Gradient Calculation example PDF.
Table II.
Station
Names
#1
Change in
elevation (ft)
(elev1 –
elev2)
#2
Change in
distance
#3
Gradient
(= #1/ #2)
Stations 1-2
Stations 2-3
Stations 3-4
Stations 1-4
(total)
6. Using your channel profile, mark the downstream direction on your map with an
arrow.
a) Which interval has the steepest (biggest) gradient?
b) Between which stations do you expect water to travel the fastest? Explain
your reasoning.
c) During periods of heavy rains, which station would record the highest
streamflow? Explain your reasoning.
Part II: Pick your own gaging station and get to know the data
Each group member chooses a dataset from one gaging station to analyze. Make sure
that each member of your group chooses a different dataset. You want to be able to
compare data across the entire channel profile.
1. For your individual data set make the following plots – refer to the directions (MS
WORD) for help plotting the data in Excel:
a) Streamflow vs. Time (plot Discharge on y-axis, Time in months
on x-axis).
b) Precipitation vs. Time (plotPrecipitation on y-axis, Time in
months on x-axis).
*** Remember to give your plots titles and label the axes (include units)***
2. The precipitation and discharge data cover the same time interval. Line up the
graphs one above the other and compare how precipitation and discharge vary
with time.
i. Which months have high precipitation?
ii. Which months have high discharge?
iii. Are precipitation and discharge related? Would you expect them
to be related? Why or why not?
3. Compile your data with the data from the other gaging stations with your group,
use the following chart:
Table IV.
Tucson
Nogales
Marana
Lochiel
Elevation
Months of high
streamflow
Months of high
precipitation
Maximum
streamflow
Maximum
precipitation
Watershed size
(rank the stations
from highest to
lowest)
4. Compare the results between the gaging stations
a) In 1999, did the other gaging stations experience high streamflow and high
precipitation when yours did? Would you expect this result?
b) In 1999, did the maximum streamflow vary between gaging stations? Is this
what you would expect? Can you explain the data?
c) Explain how streamflow and precipitation might be related.
d) Explain how streamflow and the watershed size might be related.
Part III: Hypothesis check
Having analyzed the precipitation and discharge data from the Santa Cruz River gaging
stations address the following questions.
1. In what direction does water in the Santa Cruz River flow?
Support your response with data.
2. Where is the flow of the Santa Cruz River the strongest?
Support your response with data.
3. In what months of the year does the Santa Cruz River flow the most?
Support your response with data.
How do these responses compare to your responses to the same questions at the
beginning of this activity?