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Streamflow

Outline:Outline:• Hydrographs

• Discharge (Q)• Discharge (Q)

• Flood frequency

Fl l l ti• Flow calculations

Arizona Water Issues © 2011 The University of Arizona – HWR203 1

Measuring Flow – gauging stations


www.usgs.gov

Rating Curves: stage to discharge


www.nws.noaa.gov/om/hod/SHManual

Sonic transponders

www.sontek.com


Hydrographs

Hydrographs illustrate how streamflow changes with timechanges with time.

Th d t t t i it ti• They can demonstrate response to precipitation as well as the effects of urbanization.

• Daily and longer hydrographs provide a picture• Daily and longer hydrographs provide a picture of baseflow, storm flow, and snowmelt.

• Average annual streamflow graphs provide a• Average annual streamflow graphs provide a picture of water resource variability.

Arizona Water Issues 5© 2011 The University of Arizona – HWR203

Shape of the streamflow hydrograph from an individual stormfrom an individual storm


Source: www.oaa.pdx.edu/CAE/Programs/sti/ pratt/images/hydrograph.gi

Water Year Hydrograph

Snowmelt

2500

Flow rate

1500

2000

mflo

w (c

fs) Storm flows

0

500

1000

Stre

am

08/12 10/1 11/20 1/9 2/28 4/19 6/8 7/28 9/16

Date

TimeBaseflow

Time


What is discharge?

• Discharge: The volume of water that flows past a g pcertain point in a stream over a specific period of time

• a k a FLOW = volume/time• a.k.a FLOW = volume/time• To solve flow you need:

– area (ft2) & velocity (ft/s)area (ft ) & velocity (ft/s)or– time (s) & volume (ft3)time (s) & volume (ft )


How do we calculate discharge or flow (aka Q)?

• What we have:What we have:–area (ft2) & velocity (ft/s)ti ( ) & l (ft3)–time (s) & volume (ft3)

• How do we use these variables to get Q?

33 ⎞⎛⎞⎛⎞⎛ ff 3

)( Time)(Volume 33

sft

sftQ =⎟⎟

⎠

⎞⎜⎜⎝

⎛OR⎟⎠⎞

⎜⎝⎛×=⎟⎟

⎠

⎞⎜⎜⎝

⎛sftft

sftQ Velocity )( Area 2

3


Streamflow Cross‐Section

Cross-Section Area (ft2):

Width of stream (ft) x depth of stream (ft)

Streamflow velocity (f/s)

⎞⎛⎞⎛ ff 3

⎟⎠⎞

⎜⎝⎛×=⎟⎟

⎠

⎞⎜⎜⎝

⎛sftft

sftQ Velocity )( Area 2

3


Flow from a pump

Common Elements:Common Elements:

• Flow rate: Volume/time, ex: cfs )( Time

)( Volume 33

sft

sftQ =⎟⎟

⎠

⎞⎜⎜⎝

⎛

• At what rate …

• How much …

• How long …to fill 30 gal tank if flow is 5 gal/min

• To convert gpm to cfs:1 cf/7.5 gal * 1 min/60 sec


Important Flow Units for water

• cfs (ft3/s) – cubic feet per second– Often used for streamflow

• gpcd – gallons per capita per day– Residential use of water per person per dayResidential use of water per person, per day

• gal/min – gallons per minute (450 gpm = 1 cfs)– Flow rates for household fixtures (showers, faucets, etc.)

f/ f (724 f/ 1 f )• af/year – acre‐feet per year (724 af/yr = 1 cfs)– Amounts of water for irrigation, water utilities & many quantified

water rights

• cms (m3/s) – cubic meters per second (1 cms = 35.3 cfs)– Also used for streamflow


Daily Hydrograph

Flow rate

Storm flows

Flow rate

BaseflowBaseflow

Time


Rillito Streamflow


Sabino Creek – gauge height


San Pedro River


Sabino Creek – annual streamflow


Green River


Salt River, Arizona

www.inaraft.com/.../images/photo-salt-14.jpg

3,500 cfs


Colorado River

15,000 cfs


Mississippi River at Baton Rouge

619,000 cfspics2.city-data.com/picthumb/thumbv10142.jpg


Flood Freq: What is a 100‐year Flood?

• The maximum level of flood water or flow thatThe maximum level of flood water or flow that occurs, on average, once every 100 years.

• It seems simple, but…p ,

• What is the probability that a 100‐year flood will occur in any given year?y g y

• 1% chance of a 100‐year flood every year

%%100yryears%1

100%100

=


10‐year and 500‐year floods?

%%100 %%100yryears%10

10%100

=yryears%2.0

500%100

=

What is the problem with this statistical method?


Stationarity is Dead, Milly et al.

• Stationarity: the idea that natural systems fluctuate y ywithin an unchanging envelope of variability.

• What else besides climate might affect this assumption?

• Probability density functions, stochastic models, GCM’s, LC/LU change, infrastructure adequacy, time series analysis

IPCC model predicted runoff change by 2050 from 1900‐1970


Science, Feb 2008

Changes in “normal”


Flood Freq. from Recurrence Interval


Flood Probabilities don’t incorporate Watershed Changes!Watershed Changes!

What changes in a watershed can affectWhat changes in a watershed can affect the frequency and/or intensity of floods?

– Land Use Changes• Human Activities

– Land Cover Changes• Natural Watershed Alterations

Fi i• Fires, cropping

– Changes in InfrastructureB ildi Di i• Building, Diversions


Land Use Changes

• Vegetation removalVegetation removal– Increases runoff & erosion

• Land cover change• Land cover change– Shrub land to grassland

– Forest to grassland

• Forest fires– Increase runoff & erosion

SWH 3(5), Nearywww.siskiyou.org


Human Influence and Impacts

• Urbanization creates impervious surfaces, leading to increasedrunoff and frequency of floods.

– Less water infiltrates

– More water enters streams

– Velocity increases

• Diversion regulation dammingDiversion, regulation, damming, channelization, etc. change streamflow

• Erosion increases impairsErosion increases impairs water quality


Land use changes affect hydrographs


Calculating Flow (1)

• These problems are similar to “rate‐type”These problems are similar to rate type problems where velocity is the rate.

• How long would it take to fill an Olympic• How long would it take to fill an Olympic swimming pool (~400,000 gal) if all you had was a garden hose (~5 gal/min)?was a garden hose (~5 gal/min)?

• How do we solve this problem?


Flow variables

• What are our variables?What are our variables?– Know: Volume

? Minutes– ? Minutes

• Which equation?

ftVelocityftAreaftQ ×=⎟⎟⎞

⎜⎜⎛

)( 23

OR )( 33 ftVolumeftQ =⎟⎟⎞

⎜⎜⎛

sVelocityftArea

sQ ×⎟⎟

⎠⎜⎜⎝

)()(sTimes

Q ⎟⎟⎠

⎜⎜⎝


Flow Example (1)How long would it take to fill an Olympic swimming pool (~400,000 gal) if all you had was a garden hose

)(galVolumegalQ =⎟⎠⎞

⎜⎝⎛

(~5 gal/min)?

(min)min TimeQ ⎟

⎠⎜⎝

)(4000005 galgalQ =⎟⎞

⎜⎛

(min)min timeQ =⎟

⎠⎜⎝

Time(s) = 80,000 min = 55.5 days


Calculating Flow (2)

• How much water can a ditch carry per monthHow much water can a ditch carry per month if its cross‐sectional area is 10 m2 and the flow rate is 1 m/sec?rate is 1 m/sec?

H d l hi bl ?• How do we solve this problem?


Flow variables

• What are our variables?What are our variables?– Know: Area, rate

? flow– ? flow

• Which equation?

)( 33 ftVolumeftQ =⎟⎟⎞

⎜⎜⎛ORftVelocityftAreaftQ ×=⎟⎟

⎞⎜⎜⎛

)( 23

)(sTimesQ ⎟⎟

⎠⎜⎜⎝s

VelocityftAreas

Q ×⎟⎟⎠

⎜⎜⎝

)(


Flow Example (2)How much water can a ditch carry per month if its cross-sectional area is 10 m2 and the flow rate is 1 m/sec?

mVelocitymAreamQ ×=⎟⎟⎞

⎜⎜⎛

)( 23

sVelocitymArea

sQ ×=⎟⎟

⎠⎜⎜⎝

)(

mmQ 1)(10 23

⎟⎟⎞

⎜⎜⎛

sm

sQ 1)(10 2 ×=⎟⎟

⎠⎜⎜⎝

Flow(cms) = 10 m3/s


River Comparisons

Area Area Length length discharge discharge

river mi2 1000 km2 mi km cfs cms

Amazon 2,270,000 5,879 3,920 6,307 6,354,000 180,000

Mississippi 1,247,000 3,230 3,870 6,227 619,339 17,545

Colorado 246,700 639 1,450 2,333 15,000 425

Salt 5 824 15 3 000 86Salt 5,824 15 3,000 86

Verde 6,200 16 140 225 500 14


Download - Outline - Sahraweb.sahra.arizona.edu/.../lesson8/Hwr203streamflow.pdf · as well as the effects of urbanization. • Daily andand longerlonger hydrographshydrographs provideprovide

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