Lecture 9 and 10- Hydrograph
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Transcript of Lecture 9 and 10- Hydrograph
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Hydrograph – Week 5
Hydrograph analysisSources of streamflowSeparation techniques
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StreamflowPreviously,• Hydrologic cycle• Precipitation, (gains = input)• infiltration, evaporation, transpiration
(losses=output)• Streamflow occurs throughout the
year• Magnitude (flow) varies• Procedures to estimate flow depending
on data availability
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Hydrographs
• What are they• Types• Components • Effects of physiography and
meteorology• Groundwater• Recession curves• Separation techniques
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Hydrograph[hydrograph] - A graphical representation of stage,
flow, velocity, or other characteristics of water at a given point as a function of time .
McGraw Hill Professional – Sci Tech Dictionary
Source: Oxford Press Geography Dictionary
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Hydrograph• Streamflow occurs throughout the year=>Magnitude (flow)
varies• Procedures to estimate flow depending on data availability• The runoff estimation methods from previous lectures• Consider assumptions/ limitations at each step
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HydrographA hydrograph is the result of the physiographic
and hydro–meteorologic effects of the watershed
Physiographic –size, shape, length of main channel, slope, soils ….
Hydro-meteorologic- rainfall intensity, duration, direction of storm, soil moisture……
• Hydro meteorologic and Physiographic => just think ‘precipitation and watershed features’
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HydrographPrecipitation and watershed features• Watershed topography and geology (i.e. bedrock permeability) • The area of a basin receiving rainfall • Land-use (e.g. agriculture, urban development, forestry
operations) • Drainage density• Duration of rainfall and precipitation intensity and type • Evapotranspiration rates • River network • The season • Previous weather • Vegetation type and cover • River conditions (e.g. dams) • Initial conditions (e.g. the degree of saturation of the soil and
aquifers) • Soil permeability and thickness Source: Wikipedia
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HydrographRelevant properties of the hydrograph
Rising limbPeak flowRecession curveTime baseBasin lagPeak runoffTime to peak runoffBase flow
Important to know these- how do they relate to inputs and outputs
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Hydrograph
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Hydrograph
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Types of Hydrographs
Consider different ways to look at information
• Surface water hydrograph (stage and discharge)
• Subsurface water hydrograph• Unit hydrograph• Instantaneous unit hydrograph• Geomorphologic instantaneous unit
hydrograph• Synthetic unit hydrograph• Hourly, Daily, Monthly, Yearly hydrograph
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Types of Hydrographs
Surface water hydrograph - Time record of the water level or flow for the stream (point of interest)
Stage hydrograph- A specific type representing water level
Discharge (streamflow) hydrograph – A specific type representing water flow
Main input is precipitationMain ‘output’ is streamflow
Representation of how watershed responds to rainfall- these are ‘natural’
Discharge hydrograph is what is usually referred to as a ‘hydrograph’
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Types of Hydrographs
Subsurface water hydrograph-– In subsurface hydrology a hydrograph is a record
of the water level below the ground surface– Typically, recorded for monitoring of heads in
aquifers during non-test conditions (e.g., to observe the seasonal fluctuations in an aquifer).
– ‘natural’ hydrographSource: Wikipedia
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Types of Hydrographs
Unit hydrograph- hypothetical unit response of the watershed to a unit input of rainfall
In other words, it is a typical hydrograph for that particular area. It correlates rainfall with runoff and indicates the runoff (and timing) associated with one unit of rain falling evenly over watershed
Important tool - much more later (next week)
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Types of Hydrographs
Instantaneous Unit Hydrograph- similar to the unit hydrograph, BUT it is the flow sequence that corresponds to an instantaneous application of the unit rain
Consideration of ‘instantaneous’ leads to mathematical tools => Calculus
Important tool - more later (next week)
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Types of Hydrographs
Geomorphologic Instantaneous Unit Hydrographsimilar to the instantaneous unit hydrograph, BUT it
considers only the topologic information
Only the number of streams of a given order, the mean length of streams of a given order, and the mean land area draining directly to streams of a given order are absolutely required (and can be estimated rather than explicitly calculated if necessary).
For interest only -No more later
Source: Wikipedia
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Types of Hydrographs
Synthetic Unit Hydrograph- similar to the unit hydrograph, BUT it is an estimate for a location for which streamflow data is not available
In other words, when no hydrograph is available for your point of interest, (or time of interest), you must synthesize (build) one.
Can be based on • hydrograph/ watershed characteristics• dimensionless unit hydrograph• model of watershed storage
Important tool - more later (2 weeks)
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Types of Hydrographs
Hourly, daily, monthly, yearly Hydrograph-only difference is the time scale as applied to types of hydrographs
Simple, but balance data needs with availability and computing. Consider watershed size and reaction time
Appropriate time scale.
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Components of HydrographsFour components,1. Direct surface runoff2. Interflow3. Groundwater (base) flow4. Channel precipitation
12
3
41
32
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Components of Hydrographs
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Components of Hydrographs
Majority of cases, two components are of interest,
• Direct runoff• Groundwater
• Interflow usually minor –why?• Channel precipitation usually minor –why?• When could these be important?
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Precipitation and Watershed Characteristics
Rainfall intensityConsider how hard it has to rain to overcome
infiltration (phi index), depression storage, interflow movement
Infiltration/ depression=> no runoff, Only direct channel precip
Runoff apparentInterflow and groundwaterapparent
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Precipitation and Watershed Characteristics
Rainfall on different areasLonger distance to travel means longer time to peak
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Precipitation and Watershed Characteristics
Watershed shape
The longer it takes for water to travel, the longer the time of concentration, etc
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Precipitation and Watershed Characteristics
Storm direction
Consider that as storm moves across a watershed, the rain is not falling uniformly over entire watershed (as many models assume)
If rain system moves from far reaches towards outlet, the rising limb may be steeper than compared to uniform
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GroundwaterGroundwater hydrology is not within scope of
this course, BUT, some understanding of how groundwater and surface water interact is useful
Consider subsurface as variable, nonhomogeneousUnderstanding of detailed flow is difficult –often
unnecessary for surface water investigation
Surface and GW interaction occurs when and how?
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Groundwater
Surface and groundwater are connected.
Consider gradient required to have flow(water can move uphill!)
Groundwater gains from rain, snowmelt , river ‘losing’ reaches,
Groundwater losses from movement out of watershed, river ‘gaining’ reaches, seepage faces, artesian wells
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GroundwaterInteraction between GW and surface water, and
relationship to hydrograph,
Rain/snowmelt soaking into ground lost as runoff may become GW –where else could it go?
Rivers/lakes may gain or lose or likely both through the watershed
Surface water moves quickly and changes from precipitation easy to identify
GW moves slowly, and precipitation has little impact in short term
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Groundwater RecessionTo relate rainfall to runoff, we need to separate
out the other 3 components (groundwater, interflow, channel precipitation)
But, if we assume that channel precipitation and interflow are minor, then only need to separate out groundwater component
Consider cases where channel precipitation and interflow may be important?
When you assume they are minor, where are they (in the GW or runoff component)?
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Groundwater RecessionSeparate out groundwater (or baseflow)
time
stre
amflo
w
Streamflow Hydrograph Direct Runoff HydrographGroundwater Hydrograph
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Groundwater RecessionTechniques available to separate surface runoff
and groundwater flows in hydrograph
Based on groundwater depletion curve,Qt=QoKr
t Qt=QoKr-t
Qo=flow at some time initial time Qt=flow measured t days laterKr=recession constant (includes surface, interflow
and groundwater)What happened to channel precipitation?
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Groundwater Recession
Based on groundwater depletion curve,Qt=QoKr
t Qt=QoKr-t
Let t=1 for both cases, (how convenient):. Kr=(Qt/Qo) :. Kr=(Qo/Qt)
Since Qo>Q1 Since Qo>Q1
:. Kr<1 :. Kr>1
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Groundwater RecessionSlope of semilog plot represents value of K
K is function of baseflow (groundwater), interflow, surface runoff
Kr=Krb + Kri + Krs
Typical values Surface runoff……….0.05 to 0.2Interflow…………..….0.5 to 0.8Baseflow (GW)……..0.85 to 0.98
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Groundwater StorageOver time, with groundwater flow into the river, the groundwater
becomes depletedThis change in storage for groundwater, dS, may be of interest
If qtdt= volume of water coming from groundwater
But -dS is equal to qtdt (water out of storage =water coming into river from storage)
Let qt=qoKt, then integrate,
qtdt= qoKtdt = - dS
S= (qt-qo )/(logeK)
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Groundwater StorageConsider red line – as time increases, flow decreases –approaching some limit
Green line is quantity released from storage-increases quickly – then approaches a limit – at that point S =~constant and consider what that means for groundwater flow and hydrograph
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break
• Review on Wednesday previous day• Flow vs time => hydrograph• Components of hydrograph flow• How watershed shape etc effect the hydrograph• Importance of groundwater to hydrograph• Basic shpae of groundwater flow• Seperation of groundwater or baseflow• Now some techniques to seperate
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Baseflow separation techniquesFor the streamflow hydrograph – interest often to
separate the surface runoff from the groundwater flow component
Many subjective techniques• Straight line • Bent line• Extension of recession curves• Master curve• Important to know basis for making subjective
decisions
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Baseflow seperation techniquesStraight separation lineBasically drawing a straight line from start of surface
runoff to end of surface runoffMost basic is horizontal line
Trick is –where to Start and where to Stop
Once start/stop knownstraightforward
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Baseflow separation techniquesBent lineAccount for change in base flow >how is base flow
changing?
Trick is –where to Start and where to Stop and where to bend
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Baseflow separation techniquesIdentify start (start of surface runoff)• Usually sharp rise (from the ‘depletion curve’
existing before start of surface runoff)• Rainfall shows sharp increase at or before rise
in hydrograph (why ? Consider rain needed to overcome abstractions – infiltration, ‘phi index’, ET.
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Baseflow separation techniquesIdentify stop (end of surface runoff) method 1• Rule of thumb!, N = A 0.2
• N= # of days from peak flow to point where surface runoff ends
• A = watershed area in square miles• Crude –what happened to all the otherconsiderations?, ie slope, length of channel etc
N
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Baseflow separation techniquesIdentify stop (end of surface runoff) method 2• Plot recession portion of streamflow hydrograph
on semi log paper• Where lines intersect –indicates where surface
runoff ends (actually where slope of Q vs T graph described by function of e)
timetime
Log
Q
Q
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Baseflow separation techniquesIdentify stop (end of surface runoff) method 3• Plot (Q/Q t) vs t (after peak)• Slope change indicates end of runoffWhere does runoff end?At Q or Q t ?
Neither => it occurs between
6 hour intervals
time
Q
Q/Q
6
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Baseflow separation techniquesIdentify stop (end of surface runoff) method 4• Develop a ‘master’ depletion curve • Q to log Q
QLo
g Q
time
time
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Baseflow separation techniquesIdentify stop (end of surface runoff) method 4• Log plot back to Q – then use on original plot
time
time
QLo
g Q