CE 374 K – CE 374 K – HydrologyHydrologyRunoff ProcessesRunoff Processes

Daene C. McKinneyDaene C. McKinney

WatershedWatershed WatershedWatershed

Area draining to a Area draining to a streamstream

Streamflow generated Streamflow generated by water entering by water entering surface channelssurface channels

Affected byAffected by Physical, vegetative, Physical, vegetative,

and climatic featuresand climatic features Geologic considerationsGeologic considerations Stream PatternsStream Patterns

Dry periodsDry periods Flow sustained from Flow sustained from

groundwater (baseflow)groundwater (baseflow)

http://www.epa.gov/owow/watershed/whatis.html

StreamflowStreamflow Atmospheric Atmospheric

WaterWater EvapotranspirEvapotranspir

ationation PrecipitationPrecipitation

Subsurface Subsurface WaterWater InfiltrationInfiltration GroundwaterGroundwater

Surface Surface WaterWater

Atmospheric Moisture

Interception

Snowpack

Surface

Soil Moisture

Groundwater

Streams and Lakes

Runoff

RainSnow

Evaporation

Evapotranspiration

Evaporation

Throughfall and Stem Flow

Snowmelt

Infiltration

Overland Flow

Percolation

Groundwater Flow

Channel Flow

Pervious Impervious

Energy

WatershedBoundary

Streamflow HydrographStreamflow Hydrograph

Peak

Ris

ing L

imb

Recessio

n L

imb

Time

Dis

ch

arg

e,

Q

Beginning of Direct Runoff

BaseflowRecession Baseflow

Recession

Centroid of Precipitation

Basin Lag

Time of Rise

End of Direct Runoff

InflectionPoint

Baseflow SeparationBaseflow Separation

No inflow added to No inflow added to groundwater - groundwater - depletion depletion (recession) curve(recession) curve

Continuity Continuity equationequation

][constant decay

at time flow

at time flow)(

00

Tk

tQ

ttQ

Time

Dis

ch

arg

e,

Q

BaseflowRecession

ktteQtQ /)(0

0)(

)()( tQtIdt

dS

dteQdS ktt /)(0

0

)()( tkQtS

Time

Dis

ch

arg

e,

QA B

Baseflow

Direct Runoff

Baseflow Separation Baseflow Separation TechniquesTechniques

Straight – line Straight – line methodmethod Draw a horizontal Draw a horizontal

line segment (A-B) line segment (A-B) from beginning of from beginning of runoff to runoff to intersection with intersection with recession curverecession curve

Time

Dis

ch

arg

e,

QA B

D

C

2.0AN

Baseflow

Direct Runoff

Baseflow Separation Baseflow Separation TechniquesTechniques

Fixed Base MethodFixed Base Method Draw line segment Draw line segment

(A – C) from (A – C) from baseflow recession baseflow recession to a point directly to a point directly below the below the hydrograph peak hydrograph peak

Draw line segment Draw line segment (C-D) connecting a (C-D) connecting a point N time point N time periods after the periods after the peakpeak

Time

Dis

ch

arg

e,

QA B

C

E

Baseflow Separation Baseflow Separation TechniquesTechniques

Variable Slope Variable Slope MethodMethod Draw line segment (A-Draw line segment (A-

C) C) forwardforward from from baseflow recession to baseflow recession to a point directly below a point directly below the hydrograph peakthe hydrograph peak

Draw line segment (B-Draw line segment (B-E) E) backward backward from from baseflow recession to baseflow recession to a point directly below a point directly below the inflection pointthe inflection point

Draw line segment (C-Draw line segment (C-E)E)

Baseflow

Direct Runoff

Abstraction (Losses) Abstraction (Losses) EstimationEstimation

Phi – Index MethodPhi – Index Method Excess (effective) Excess (effective)

rainfall rainfall Rainfall that is not Rainfall that is not

retained or infiltratedretained or infiltrated Becomes direct runoffBecomes direct runoff Excess rainfall hyetograph Excess rainfall hyetograph

(excess rainfall vs time)(excess rainfall vs time) Abstraction (losses)Abstraction (losses)

Difference between total Difference between total and excess rainfall and excess rainfall hyetographshyetographs

Phi – IndexPhi – Index Constant rate of Constant rate of

abstraction yielding abstraction yielding excess rainfall hyetograph excess rainfall hyetograph with depth equal to depth with depth equal to depth of direct runoffof direct runoff

M

mmd tRr

1

interval time

runoffdriecttongcontributi

rainfallofintervals#

indexPhi

rainfall observed

runoffdirect ofdepth

t

M

R

r

m

d

ExampleExampleTime Observed

Rain Flow

in cfs

8:30 203

9:00 0.15 246

9:30 0.26 283

10:00 1.33 828

10:30 2.2 2323

11:00 0.2 5697

11:30 0.09 9531

12:00 11025

12:30 8234

1:00 4321

1:30 2246

2:00 1802

2:30 1230

3:00 713

3:30 394

4:00 354

4:30 303

0

2000

4000

6000

8000

10000

12000

7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM

Time

Str

eam

flo

w (

cfs)

0

0.5

1

1.5

2

2.5

No direct runoff until after 9:30And little precip after 11:00

Have precipitation and streamflow data, need to estimate losses

Basin area A = 7.03 mi2

Example (Cont.)Example (Cont.)

Estimate baseflow (straight line Estimate baseflow (straight line method)method) Constant = 400 cfsConstant = 400 cfs

0

2000

4000

6000

8000

10000

12000

7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM

Time

Str

eam

flo

w (

cfs)

baseflow

Example (Cont.)Example (Cont.)

Calculate Direct Calculate Direct Runoff HydrographRunoff Hydrograph Subtract 400 cfsSubtract 400 cfs

Total = 43,550 cfs

Example (Cont.)Example (Cont.)

Compute volume of direct runoffCompute volume of direct runoff

37

3

11

1

11

1

ft10*7.839

/sft 550,43*hr5.0*s/hr3600

n

nn

nd QttQV

Compute depth of direct runoffCompute depth of direct runoff

in80.4

ft4.0

ft5280*mi03.7

ft10*7.83922

37

A

Vr dd

Example (Cont.)Example (Cont.)

Neglect all precipitation intervals that Neglect all precipitation intervals that occur before the onset of direct runoff occur before the onset of direct runoff (before 9:30)(before 9:30)

Select Select RRmm as the precipitation values in as the precipitation values in the 1.5 hour period from 10:00 – 11:30the 1.5 hour period from 10:00 – 11:30

)5.0*3*08.220.233.1(80.41

M

mmd tRr

in27.0t

in54.0

in80.4dr

Example (Cont.)Example (Cont.)

0

2000

4000

6000

8000

10000

12000

7:30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM

Time

Stre

amflo

w (c

fs)

0

0.5

1

1.5

2

2.5

t=0.27

SCS Curve Number SCS Curve Number MethodMethod

Soil Conservation Service(SCS) Curve Number (CN) Soil Conservation Service(SCS) Curve Number (CN) model estimates precipitation excess as a function of model estimates precipitation excess as a function of cumulative precipitation, soil cover, land use, and cumulative precipitation, soil cover, land use, and antecedent moistureantecedent moisture

SCS developed the method for small basins (< 400 sq. SCS developed the method for small basins (< 400 sq. mi.) to "before" and "after" hydrologic response from mi.) to "before" and "after" hydrologic response from events. events.

Classify soils (60 or 70 types) into four hydrologic soil Classify soils (60 or 70 types) into four hydrologic soil groupsgroups

Method is simple enough to be used by people that Method is simple enough to be used by people that have little experience with hydrology. have little experience with hydrology.

Converts basin storage into something simpler and Converts basin storage into something simpler and more manageable (a “curve number” CN) more manageable (a “curve number” CN)

Abstractions – SCS Abstractions – SCS MethodMethod

In generalIn general

After runoff beginsAfter runoff begins

Potential runoffPotential runoff

SCS AssumptionSCS Assumption

Solve for Rainfall Solve for Rainfall ExcessExcess

Time

Pre

cip

itati

on

pt

aI aF

eP

aae FIPP

StorageMaximumPotentialS

nAbstractioContinuing

nAbstractioInitial

Excess Rainfall

Rainfall Total

a

a

e

F

I

P

P

PPe

SFa

aIP

a

ea

IP

P

S

F

SIP

IPP

a

ae

2

SCS Method (Cont.)SCS Method (Cont.)

Experiments Experiments showedshowed

SoSo

SIa 2.0

SP

SPPe 8.0

2.0 2

0

1

2

3

4

5

6

7

8

9

10

11

12

0 1 2 3 4 5 6 7 8 9 10 11 12

Cumulative Rainfall, P, in

Cu

mu

lati

ve D

irec

t R

un

off

, P

e, i

n

100

90

80

70

60

40

20

10

SurfaceSurface Impervious: CN = Impervious: CN =

100100 Natural: CN < 100Natural: CN < 100

100)CN0Units;American(

101000

CN

S

100)CN30Units;SI(

25425400

CNCN

S

SCS Method (Cont.)SCS Method (Cont.)

S and CN depend on antecedent S and CN depend on antecedent rainfall conditionsrainfall conditions

Normal conditions, AMC(II)Normal conditions, AMC(II) Dry conditions, AMC(I)Dry conditions, AMC(I)

Wet conditions, AMC(III)Wet conditions, AMC(III)

)(058.010

)(2.4)(

IICN

IICNICN

)(13.010

)(23)(

IICN

IICNIIICN

5-day antecedent rainfall 5-day antecedent rainfall (in)(in)

AMC GroupAMC Group Dormant Dormant seasonseason

Growing Growing seasonseason

II < 0.50< 0.50 < 1.4< 1.4

IIII 0.5 -- 1.10.5 -- 1.1 1.4 – 2.11.4 – 2.1

IIIIII > 1.1> 1.1 > 2.1> 2.1

SCS Method (Cont.)SCS Method (Cont.)

SCS Curve Numbers depend on soil SCS Curve Numbers depend on soil conditionsconditionsGroupGroup Minimum Minimum

Infiltration Rate Infiltration Rate (in/hr)(in/hr)

Soil typeSoil type

AA 0.3 – 0.450.3 – 0.45 High infiltration rates. Deep, High infiltration rates. Deep, well drained sands and gravelswell drained sands and gravels

BB 0.15 – 0.300.15 – 0.30 Moderate infiltration rates. Moderate infiltration rates. Moderately deep, moderately Moderately deep, moderately well drained soils with well drained soils with moderately coarse texturesmoderately coarse textures

CC 0.05 – 0.150.05 – 0.15 Slow infiltration rates. Soils Slow infiltration rates. Soils with layers, or soils with with layers, or soils with moderately fine textures moderately fine textures

DD 0.00 – 0.050.00 – 0.05 Very slow infiltration rates. Very slow infiltration rates. Clayey soils, high water table, Clayey soils, high water table, or shallow impervious layer or shallow impervious layer

Example - SCS Method - Example - SCS Method - 11

Rainfall: 5 in. Rainfall: 5 in. Area: 1000-acArea: 1000-ac Soils: Soils:

Class B: 50%Class B: 50% Class C: 50%Class C: 50%

Antecedent moisture: AMC(II)Antecedent moisture: AMC(II) Land useLand use

Residential Residential 40% with 30% impervious cover40% with 30% impervious cover 12% with 65% impervious cover12% with 65% impervious cover

Paved roads: 18% with curbs and storm Paved roads: 18% with curbs and storm sewerssewers

Open land: 16%Open land: 16% 50% fair grass cover50% fair grass cover 50% good grass cover50% good grass cover

Parking lots, etc.: 14%Parking lots, etc.: 14%

Example (SCS Method – Example (SCS Method – 1, Cont.)1, Cont.)

Hydrologic Soil GroupHydrologic Soil Group

BB CC

Land useLand use %% CNCN ProductProduct %% CNCN ProductProduct

Residential (30% imp Residential (30% imp cover)cover)

2020 7272 14.4014.40 2020 8181 16.2016.20

Residential (65% imp Residential (65% imp cover)cover)

66 8585 5.105.10 66 9090 5.405.40

RoadsRoads 99 9898 8.828.82 99 9898 8.828.82

Open land: good coverOpen land: good cover 44 6161 2.442.44 44 7474 2.962.96

Open land: Fair coverOpen land: Fair cover 44 6969 2.762.76 44 7979 3.163.16

Parking lots, etcParking lots, etc 77 9898 6.866.86 77 9898 6.866.86

TotalTotal 5050 40.3840.38 5050 43.4043.40

8.8340.4338.40 CN

Example (SCS Method – 1 Example (SCS Method – 1 Cont.)Cont.)

Average AMCAverage AMC

Wet AMCWet AMC3.92

8.83*13.010

8.83*23

)(13.010

)(23)(

IICN

IICNIIICN

in25.3

93.1*8.05

93.1*2.05

8.0

2.0 22

SP

SPPe

in93.1108.83

1000 S

8.83CN

in13.4

83.0*8.05

83.0*2.05

8.0

2.0 22

SP

SPPe

in83.0103.92

1000 S

101000 CN

S

Example (SCS Method – Example (SCS Method – 2)2)

Given P, CN = 80, AMC(II)Given P, CN = 80, AMC(II) Find: Cumulative abstractions and excess rainfall Find: Cumulative abstractions and excess rainfall

hyetographhyetographTime Time (hr)(hr)

CumulatCumulativeive

Rainfall Rainfall (in)(in)

Cumulative Cumulative

Abstractions Abstractions (in)(in)

CumulativeCumulative

Excess Rainfall Excess Rainfall (in)(in)

Excess RainfallExcess Rainfall

Hyetograph (in)Hyetograph (in)

PP IaIa FaFa PePe

00 00

11 0.20.2

22 0.90.9

33 1.271.27

44 2.312.31

55 4.654.65

66 5.295.29

77 5.365.36

Example (SCS Method – 2)Example (SCS Method – 2)

Calculate storageCalculate storage Calculate initial abstractionCalculate initial abstraction Initial abstraction removes Initial abstraction removes

0.2 in. in 10.2 in. in 1stst period (all the period (all the precip)precip)

0.3 in. in the 20.3 in. in the 2ndnd period (only period (only part of the precip)part of the precip)

Calculate continuing Calculate continuing abstractionabstraction

in50.21080

100010

1000

CNS

a

ea IP

PSF

in5.05.2*2.02.0 SIa

aae FIPP )0.2(

)5.0(5.2

)(

)(

P

P

SIP

IPSF

a

aa

in34.0)0.29.0(

)5.09.0(5.2hr)(2

aF

Time Time (hr)(hr)

CumulativeCumulative

Rainfall Rainfall (in)(in)

PP

00 00

11 0.20.2

22 0.90.9

33 1.271.27

44 2.312.31

55 4.654.65

66 5.295.29

77 5.365.36

Example (SCS method – Example (SCS method – 2)2)

Cumulative abstractions can now be calculatedCumulative abstractions can now be calculated

Time Time (hr)(hr)

CumulatCumulativeive

Rainfall Rainfall (in)(in)

Cumulative Cumulative

Abstractions Abstractions (in)(in)

PP IaIa FaFa

00 00 00 --

11 0.20.2 0.20.2 --

22 0.90.9 0.50.5 0.340.34

33 1.271.27 0.50.5 0.590.59

44 2.312.31 0.50.5 1.051.05

55 4.654.65 0.50.5 1.561.56

66 5.295.29 0.50.5 1.641.64

77 5.365.36 0.50.5 1.651.65

)0.2(

)5.0(5.2

P

PFa

Example (SCS method – Example (SCS method – 2)2)

Cumulative excess rainfall can now be calculatedCumulative excess rainfall can now be calculated Excess Rainfall Hyetograph can be calculatedExcess Rainfall Hyetograph can be calculated

Time Time (hr)(hr)

CumulatiCumulativeve

Rainfall Rainfall (in)(in)

Cumulative Cumulative

Abstractions (in)Abstractions (in)CumulativeCumulative

Excess Rainfall Excess Rainfall (in)(in)

Excess RainfallExcess Rainfall

Hyetograph (in)Hyetograph (in)

PP IaIa FaFa PePe

00 00 00 -- 00 00

11 0.20.2 0.20.2 -- 00 00

22 0.90.9 0.50.5 0.340.34 0.060.06 0.060.06

33 1.271.27 0.50.5 0.590.59 0.180.18 0.120.12

44 2.312.31 0.50.5 1.051.05 0.760.76 0.580.58

55 4.654.65 0.50.5 1.561.56 2.592.59 1.831.83

66 5.295.29 0.50.5 1.641.64 3.153.15 0.560.56

77 5.365.36 0.50.5 1.651.65 3.213.21 0.060.06

aae FIPP

Time of ConcentrationTime of Concentration

Different areas of a watershed Different areas of a watershed contribute to runoff at different contribute to runoff at different times after precipitation beginstimes after precipitation begins

Time of concentrationTime of concentration Time at which all parts of the watershed Time at which all parts of the watershed

begin contributing to the runoff from begin contributing to the runoff from the basinthe basin

Time of flow from the farthest point in Time of flow from the farthest point in the watershedthe watershed