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Procedure for Working with Gauge Station Data
- Put the data from the chip in the gage station into Excel.
- Using the 15 minute averages, use Excel to calculate
a Daily Mean Flow value. This will just be the average
of all the heights measured for each day. The number will
be in meters.
- Create these columns: "Date", "Height measured
in meters", "Height converted to feet",
"Correction for the sensor height", "Q
linear", and "Q curved".
- Input the date, and the Daily Mean Flow in the "Height
measured in meters’ column.
- Convert the Height in meters to Height in feet. (The rating
curve is in cubic feet per second (cfs) so our measurements
need to be in feet!) Put this in the "Height converted
to feet" column.
- Add 1.673 feet to correct for the distance from the bottom
of the bridge (what the rating curve was based upon) to
the tip of the sensor. Put this in the "Correction
for the sensor height" column.
- Using the equations from Ryan Linthicum’s Rating
Curve, calculate the discharge (Q) that corresponds to
each height that we measured (now in the "Correction
for the sensor height" column). The exponential curve
is similar to most other rating curves, and represents
an "ideal" situation. Its equation is Stage
= 6.28 Q –0.0375 where stage is the measurement taken
(depth to water surface), and Q is discharge.
- The linear rating curve was added because the gage station
is directly under a bridge. When the flow is increasing,
the bridge acts as a "bottleneck" for the stream;
that is, as Q is increasing, the height under the bridge
will increase abnormally fast, when compared to a "natural"
channel. Because the Rating Curve is not well constrained
yet, both the linear and exponential rating curves should
be considered. The linear rating curve’s equation
is: Stage = -0.0204Q + 5.9787.
- Now you have daily average discharges, based upon two
different rating curve interpretations. From these you
can calculate the Mean, Maximum, and Minimum for the time
of record. This should be done on the daily averages,
but it can also be done on weekly averages.
- Exceedance values also need to be calculated for 20%,
50% and 80% exceedance. 20% exceedance should be reported
as a single discharge measure (in cfs). This means that
20% of the daily average discharges are above the discharge
that you report. (For example, summer 2001 data had discharges
ranging from 0 to 37.4 cfs, and the 20% exceedance value
was reported as 7.5 cfs)
- Note:
The formula entered into excel for the linear data
is Q= (stage/-0.0204) – 5.9787
The formula for the exponential curve is (LN(stage/6.282)/-0.0375.
Now you need to take the answer and put it to the base
of e. In Excel, type EXP(the number), and it will give
you the discharge. On the TI-85 calculators, its 2nd LN
the number.
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