Advances in Streamgaging | Reach-Scale Monitoring Experiment (2 of 2)

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Detailed Description

In this second installment of a two part series, The Arizona Water Science Center, in partnership with the USDA Agricultural Research Service, reviews the results of an experiment to verify remote methods that measure flood flows at the Walnut Gulch Experimental Watershed. What makes this experiment unique are the super-critical runoff flumes that precisely measure discharge, which will act as a control, allowing USGS scientists to verify their methods and gain insight into the potential for Reach-Scale Monitoring at other stream gages. 

For more information, visit Part I of this video series to learn more about the background of this experiment! 


Date Taken:

Length: 00:05:54

Location Taken: Tucson, AZ, US


(soft music)

- [Claire] The USGS has been
collecting stream flow data

in Arizona since the 1890's.

And over 200 stream
gages are in operation

in the state today.

- Traditionally,
collecting stream flow data

requires a hydrographer
to travel to the guage

to verify discharge during the flow.

But given the remote nature
of many of our gages,

we are working to develop new techniques

that will help us collect
more reliable stream flow data

at our hard to reach gages.

In order to ensure that these methods

will meet USGS standards,

they need to be tested in
a controlled environment.

The walnut gulch experimental watershed

in Tombstone, Airzona, provided our team

with the perfect setting
to develop and experiment

to test these methods.

- My name is Brandon Forbes,

I'm a hydrologist with the
USGS out of Tucson, Arizona.

And today we are back at Walnut Gulch

experimental watershed.

We're at flume six.

We had a great monsoon flow.

We were able to capture the entire flood

on all of our sensors.

And today we're going to
GPS survey the sensors.

The cross sections at each sensor,

and all the high water marks here

so we can make sure the
stage sensors worked well.

And once we have that,

we'll be able to calibrate
both one and 2D flow models

to the record collected the flume

to see how well we're
actually able to recreate

the flow that occurred here.

- These watersheds in Walnut Gulch

are meant to measure very intensively.

So in this 60 square mile watershed,

we have over 100 reporting rain gages.

And we have almost 30 nested watersheds

that are gaged with
instrumentation like these flumes

and several other designs.

Walnut Gulch experimental
watershed has been used

for a variety of collaborations.

In this particular instance,

we're working with the
local USGS science center

and they have to set up gaging stations

all over the state of Arizona

where they don't have these big structures

and these flumes like this.

So this allows them to basically
validate their methodology.

And because you have fixed geometry

and you know the slope,

you can compute the discharge
if you know the depth

of flow within this.

- A recent advance in
surveying within the USGS

is the use of unmanned
aerial vehicles, or UAVs.

UAVs provide an efficient
way to collect data

at our sites by documenting the conditions

of the channel on the day of the survey.

But what we are most
interested in collecting

from these aerial surveys
is topographic data.

In this experiment,

we flew a UAV above the channel

to take high resolution photographs.

Within the data collected
from these flights,

we can build a digital elevation model

of our channel.

Furthermore, by analyzing the images

on a pixel by pixel basis,

we can better document
current channel conditions,

including channel roughness,

and create what is known
as a 2D flow model.

This model can simulate
different magnitudes of flow

through the system,

which ultimately allows
us to gain knowledge

of all the potential flow
scenarios of that watershed.

- This reach scale
experiment at Walnut Gulch

has provided us with very valuable data.

We tested the continuous slope area method

during this experiment,

and we were able to verify
the accuracy of the CSA method

by comparing the discharge
of computed using our sensors

with the true discharge
measured by flume six.

After reviewing the data gathered

from our indirect
measurement, the CSA sensors,

and the flume, we were
able to begin our analysis.

We used the surveyed high water marks

to do a slope area computation
to determine stream flow

at the peak of the event.

Without the CSA sensors in the channel,

this peak discharge value
would be the only measurement

available after a flow event.

Our CSA sensors measure water
depth every five minutes,

allowing us to determine stream flow

in five minute intervals.

With that data,

we can then compare the
entire event recorded

by our sensors to the discharge
measured by the flume.

After reviewing the data,

we determined that the CSA
sensors can compute flow

above approximately 800
cubic feet per second.

We did experience some
discrepancy in the beginning

of the hydrograph,

which may be to due to
challenges measuring water depth

as it was rapidly changing.

However, when we compared the
rest of the stream flow data

computed by the sensors

to what was recorded by the flume,

we found that our methods were accurate

within 5% of the true discharge.

Measuring accurate discharge
during an entire event

without requiring a
hydrographer to be present

during the flood,

is a significant step forward
for our stream gages.

With this one event at Walnut Gulch,

we were able to create
an accurate rating curve

for our continuous slope area reach,

showing that the CSA method
can be used to improve

the quality of data collected at gages

where we can't easily
measure during floods.

We are excited by the
results this experiment

and are planning additional
installations using

the continuous slope area
method across the state.

We hope to be able to
remotely collect reliable

stream flow data to provide
accurate information

to our partners and
citizens throughout Arizona.

We will be continuing the
Advances in Streamgaging series

with follow up videos on the progress

of our field applications

and will soon be highlighting
a step by step process

on how we produce our results.

If you have questions or comments,

feel free to contact us at the
Arizona Water Science Center.

Thanks for tuning in.

(piano music)