Lecture 9 and 10- Hydrograph

Hydrograph – Week 5

Hydrograph analysisSources of streamflowSeparation techniques

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

Hydrographs

• What are they• Types• Components • Effects of physiography and

meteorology• Groundwater• Recession curves• Separation techniques

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

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

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’

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

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

Hydrograph

Hydrograph

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

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’

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

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)

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)

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

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)

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.

Components of HydrographsFour components,1. Direct surface runoff2. Interflow3. Groundwater (base) flow4. Channel precipitation

12

3

41

32

Components of Hydrographs

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?

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

Precipitation and Watershed Characteristics

Rainfall on different areasLonger distance to travel means longer time to peak

Precipitation and Watershed Characteristics

Watershed shape

The longer it takes for water to travel, the longer the time of concentration, etc

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

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?

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

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

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)?

Groundwater RecessionSeparate out groundwater (or baseflow)

time

stre

amflo

w

Streamflow Hydrograph Direct Runoff HydrographGroundwater Hydrograph

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?

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

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

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)

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

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

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

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

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

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.

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

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

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

Baseflow separation techniquesIdentify stop (end of surface runoff) method 4• Develop a ‘master’ depletion curve • Q to log Q

QLo

g Q

time

time

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