Streamflow Measuring Flow – Gauging Stations Rating Curves

Streamflow Measuring Flow –gauging stations

Outline: • Hydrographs • Discharge (Q) • Flood frequency • Flow calculations

www.usgs.gov

Rating Curves: stage to discharge Hydrographs

Hydrographs illustrate how streamflow changes with time.

• They can demonstrate response to precipitation as well as the effects of urbanization. • Daily and longer hydrographs provide a picture of baseflow, storm flow, and snowmelt. • Average annual streamflow graphs provide a picture of water resource variability.

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

Shape of the streamflow hydrograph Water Year Hydrograph from an individual storm

Snowmelt

Flow rate

2500

2000 Storm flows fs) c 1500

1000 Streamflow ( Streamflow 500

0 8/12 10/1 11/20 1/9 2/28 4/19 6/8 7/28 9/16 Date

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

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

• Discharge: The volume of water that flows past a • What we have: certain point in a stream over a specific period of time –area (ft2) & velocity (ft/s) • a.k.a FLOW = volume/time –time (s) & volume (ft3) • To solve flow you need: • How do we use these variables to get Q? – area (ft2) & velocity (ft/s) or – time (s) & volume (ft3) ⎛ ft 3 ⎞ ⎛ ft ⎞ ⎛ 3 ⎞ 3 ⎜ ⎟ 2 OR ft Volume ( ft ) Q⎜ ⎟ = Area ( ft )× Velocity ⎜ ⎟ Q⎜ ⎟ = ⎝ s ⎠ ⎝ s ⎠ ⎝ s ⎠ Time (s)

Streamflow Cross‐Section Important Flow Units for water

Cross-Section Area (ft2): • cfs (ft3/s) – cubic feet per second Width of stream (ft) x depth of stream (ft) – Often used for streamflow Streamflow velocity (f/s) • gpcd – gallons per capita per day

3 – Residential use of water per person, per day ⎛ ft ⎞ 2 ⎛ ft ⎞ Q⎜ ⎟ = Area ( ft )× Velocity ⎜ ⎟ ⎝ s ⎠ ⎝ s ⎠ • gal/min – gallons per minute – Flow rates for household fixtures (showers, faucets, etc.) • af/year –acre‐feet per year (72,400 af/yr = 100 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 Rillito Streamflow

Flow rate Storm flows

Baseflow

Time

2 Sabino Creek –gauge height San Pedro River

Sabino Creek – annual streamflow Green River

Salt River, Arizona Colorado River

15,000 cfs

www.inaraft.com/.../images/photo-salt-14.jpg 3,500 cfs

3 Mississippi River at Baton Rouge Flood Freq: What is a 100‐year Flood?

• The maximum level of flood water or flow that occurs, on average, once every 100 years. • It seems simple, but… • What is the probability that a 100‐year flood will occur in any given year? • 1% chance of a 100‐year flood every year 100% % pics2.city-data.com/picthumb/thumbv10142.jpg =1 619,000 cfs 100years yr

10‐year and 500‐year floods? Stationarity is Dead, Milly et al.

• Stationarity: the idea that natural systems fluctuate within an unchanging envelope of variability. • What else besides climate might affect this assumption? 100% % 100% % =10 = 0.2 • Probability density functions, stochastic models, GCM’s, 10years yr 500years yr LC/LU change, infrastructure adequacy, time series analysis

IPCC model predicted What is the problem with this statistical method? runoff change by 2050 from 1900‐1970

Science, Feb 2008

Changes in “normal” Flood Freq. from Recurrence Interval

4 Flood Probabilities don’t incorporate Land Use Changes Watershed Changes!

What changes in a watershed can affect • Vegetation removal the frequency and/or intensity of floods? – Increases runoff & erosion – Land Use Changes • Land cover change • Human Activities – Shru b lldand to grasslldand – Land Cover Changes – Forest to grassland • Natural Watershed Alterations • Forest fires • Fires, cropping – Increase runoff & erosion – Changes in Infrastructure

Human Influence and Impacts Land use changes affect hydrographs

• Urbanization creates impervious surfaces, leading to increased runoff and frequency of floods. – Less water infiltrates – More water enters streams – Velocity increases • Diversion, regulation, damming, channelization, etc. change streamflow • Erosion increases Æ impairs water quality

Calculating Flow (1) Flow variables

• These problems are similar to “rate‐type” • What are our variables? problems where velocity is the rate. – Know: Volume • How long would it take to fill an Olympic – ? Minutes swimming pool (~400, 000 gal) if all you had • Whic h equation ? was a garden hose (~5 gal/min)? ⎛ 3 ⎞ 3 3 ft 2 ft OR ⎛ ft ⎞ Volume( ft ) Q⎜ ⎟ = Area( ft )×Velocity Q⎜ ⎟ = • How do we solve this problem? ⎝ s ⎠ s ⎝ s ⎠ Time(s)

5 Flow Example (1) Calculating Flow (2)

How long would it take to fill an Olympic swimming pool (~400,000 gal) if all you had was a garden hose • How much water can a ditch carry per month (~5 gal/min)? if its cross‐sectional area is 10 m2 and the flow ⎛ gal ⎞ Volume(gal) Q⎜ ⎟ = rate is 1 m/sec? ⎝ min ⎠ Time(min) ⎛ 5gal ⎞ 400000(gal) Q⎜ ⎟ = ⎝ min ⎠ time(min) • How do we solve this problem?

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

Flow variables Flow Example (2)

How much water can a ditch carry per month if its 2 • What are our variables? cross-sectional area is 10 m and the flow rate is 1 m/sec?

– Know: Area, rate 3 ⎛ m ⎞ 2 m – ? flow Q⎜ ⎟ = Area(m )×Velocity ⎝ s ⎠ s

• Whic h equation ? 3 ⎛ m ⎞ 2 m Q⎜ ⎟ =10(m )×1 ⎝ s ⎠ s ⎛ ft 3 ⎞ ft ⎛ ft 3 ⎞ Volume( ft 3 ) Q⎜ ⎟ = Area( ft 2 )×Velocity OR Q⎜ ⎟ = ⎜ ⎟ ⎜ ⎟ 3 ⎝ s ⎠ s ⎝ s ⎠ Time(s) Flow(cms) = 10 m /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 86

Verde 6,200 16 140 225 500 14