Norman Maclean Snowmelt Flow rate Storm flows fs (c flow m a tre S

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1 Eventually, all things merge into one, and a river runs through it. Norman Maclean Understanding Streamflow ADEQ SW Short Course June 13, 213 Phoenix, AZ Hydrographs Discharge (Q) USGS flow data & plots Flood frequency Factors affecting hydrographs Unit hydrograph Outline 1 2 Bottom line: one page summary Hydrographs show streamflow changes with time. Discharge is flow volume/time (cfs) USGS is source of real time& historical flow data A 1 yr flood has probability of occurrence of 1%/yr Watershed changes over time affect hydrographs A unit hydrograph captures a normalized basin response useful for predicting future storm events Hydrographs / Hyetograph 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. Hydrographs tell us a story about the WS 3 4 Shape of the streamflow hydrograph from an individual storm Water Year Hydrograph Snowmelt Flow rate 25 Streamflow (cfs) Storm flows 8/12 1/1 11/2 1/9 2/28 4/19 6/8 7/28 9/16 Date Source: pratt/images/hydrograph.gif Time Baseflow 5 6

2 Remember Def. of Discharge? How do we calculate discharge or flow (aka Q)? Synonyms: flow rate, stream flow Discharge: The volume of water that flows past a certain point in a stream over a specific period of time a.k.a FLOW = volume/time To solve flow you need: area (ft 2 ) & velocity (ft/s) or time (s) & volume (ft 3 ) What we have: area (ft 2 ) & velocity (ft/s) time (s) & volume (ft 3 ) How do we use these variables to get Q? ft Q s 3 Area ( ft 2 ft ) Velocity s OR 3 3 ft Volume ( ft ) Q s Time ( s) 7 8 Stream Rating Curve Important Flow Units for water cfs (ft 3 /s) cubic feet per second Often used for streamflow gpcd gallons per capita per day Residential use of water per person, per day gal/min gallons per minute (45 gpm = 1 cfs) Flow rates for household fixtures (showers, faucets, etc.) af/year acre feet per year (724 af/yr = 1 cfs) Amounts of water for irrigation, water utilities & many quantified water rights cms (m 3 /s) cubic meters per second (1 cms = 35.3 cfs) Also used for streamflow ADEQ SW Short Course The University of Arizona 9 1 Types of Data Available Time series: current/historical waterdata.usgs.gov/az/nwis/nwisman/?site_no=9484 Flow rate Storm flows Baseflow Time 11 12

3 Time series: Daily Data Sabino Creek gauge height (historical) Flow rate Gage height San Pedro River(historical) What is going on here? Flow rate log scale Hydrograph Shape Depends on: basin size soil moisture precipitation rate % coverage Time series: Annual statistics Sabino Creek 17 18

4 Time series: Annual statistics Seasonal Patterns Green River 19 2 Salt River, Arizona Colorado River 15, cfs 3,5 cfs Mississippi River at Baton Rouge River Comparisons pics2.city-data.com/picthumb/thumbv1142.jpg 619, cfs Area Area Length length discharge discharge river mi 2 1 km 2 mi km cfs cms Amazon 2,27, 5,879 3,92 6,37 6,354, 18, Mississippi 1,247, 3,23 3,87 6, ,339 17,545 Colorado 246, ,45 2,333 15, 425 Salt 5, , 86 Verde 6,

5 Flood Freq: What is a 1 year Flood? 1 year and 5 year floods? The maximum level of flood water or flow that occurs, on average, once every 1 years. It seems simple, but What is the probability that a 1 year flood will occur in any given year? 1% chance of a 1 year flood every year 1% 1years 1 % yr 1% 5years %.2 yr 1% % 1 1years yr What is the problem with this statistical method? What if we can t predict the future based on the past? Changes in normal Stationarity: the idea that natural systems fluctuate within 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 Stationarity is Dead, Milly et al. Science, Feb 28 IPCC model predicted runoff change by 25 from Flood Freq. from Recurrence Interval Flood Probabilities don t incorporate Watershed Changes! What changes in a watershed can affect the frequency and/or intensity of floods? Land Use Changes Human Activities Land Cover Changes Natural Watershed Alterations Fires, Ag Practices Changes in Infrastructure Building, Diversions 29 3

6 Vegetation removal Land Use Changes Increases runoff & erosion Land cover change Shrub land to grassland Forest to grassland Forest fires Increase runoff & erosion SWH 3(5), Neary 31 Human Influence and Impacts 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 impairs water quality 32 Land use changes affect hydrographs 33 Rate of Runoff or Rainfall Runoff Magnitude and Timing Hyetograph Stalagtite--Stalagmite WYN2K: This change is typical for a disturbed WS slide Rainfall peak Time lag Runoff Peaks for Unburned Hydrograph and from Burned Hydrograph from burned watershed unburned watershed flashy Time 34 Slide: Youberg/Deb Martin, USGS Non Uniform rainfall affects hydrography Flow Decrease = Channel Loses ADEQ SW Short Course Comet, DHM The University of Arizona

7 How would you make a representative hydrograph? Theoretical Hydrographs The Unit Hydrograph Most slides from: 37 A unit hydrograph captures a normalized basin response useful for predicting future storm events 38 Unit Hydrographs The Basic Process Excess Precip. Model Excess Precip. Necessary for a single basin Uses & Graphical Representation Uses of UH Translate rainfall to runoff for a basin Estimating floods Ungaged basins Duration of excess precip. Excess Precip. Runoff Hydrograph Basin Routing UHG Methods Stream and/or Reservoir Routing Runoff Hydrograph Downstream Hydrograph 39 Storm Characteristics duration distribution amount intensity Lag time Base flow Time of concentration 4 Unit Hydrograph The hydrograph that results from 1 inch of excess precipitation (or runoff) spread uniformly in space and time over a watershed for a given duration. The key points : 1 inch of EXCESS precipitation Uniform over area spread evenly over the watershed Uniform in time precip. excess rate is constant over the time interval Duration known 41 Deriving a UHG from a Storm sample watershed = 45 mi 2 Flow (cfs) hourly time step Precipitation (inches)

8 Separation of Baseflow Find Direct Runoff Volume by summing flows under excess flow curve (cfs af) Excess curve inflection Continuous process represented with discrete time steps Σ = 37, af Find Direct Runoff Depth by dividing Vol. by WS area (af/ac ft) Obtain UHG Ordinates A = 288, ac 37, af depth =.138 ft 1.65 in, 1.65 Unit hydrograph ADEQ SW Short Course The University of Arizona Estimating Excess Precip. Precipitation Excess: The uniform infiltration necessary to cause 1.65 inches of excess precipitation was determined.8to be approximately.2 inches per hour. Precipitation (inches) Uniform loss rate of.2 inches per hour Σexcess = 1.65 in Excess Prec. (inches) Excess Precipitation Small amounts of excess precipitation at beginning and end may be omitted. 6 hr UHG Derived unit hydrograph is the result of approximately 6 hours of excess precipitation

9 6 Change Duration: Develop S Curve to match duration of new storm or for comparison 6. Convert to 1 hour Duration The 1-hour unit hydrograph should have a higher peak which occurs earlier than the 6-hour unit hydrograph Flow (cfs) Continuous 6-hour bursts Unit Hydrograph Flow (cfs/inch) S-curves are lagged by 1 hour and the difference is found. 1-hour unit hydrograph resulting from lagging S-curves and multiplying the difference by Flow (cfs) 49 5