RUNOFF GENERATION

Riddhi Singh Lecture 6

Email: riddhi@iith.ac.inImage: http://upload.wikimedia.org/wikipedia/commons/5/52/Saturated_ground_due_heavy_rainfall_in_Wagga_Wagga.jpg

CE 4502

Today we will learn about

Runoff generation: conceptualization

Estimating ______ __________

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RUNOFF GENERATIONCONCEPTUALIZATION

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There are many __________ through which water may reach the river

channel.

4Image: http://www.hydrosconsult.eu/s/cc_images/cache_5828534.jpg

Generally runoff generation is conceptualized using two models: Hortonian

Overland Flow model and Saturation Overland Flow model

___________ overland flow occurs when rainfall intensity (i) exceeds the

infiltration capacity (f) of the soil.

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Horton considered surface runoff to take the form of a sheet flow whose depth might be

measured in fractions of an inch.Image: http://geography.unt.edu/~williams/geog_3350/examreviews/exam2images/hydrol1.gif

Rainfall excess i f

Applicable to:

1. _______ surfaces in urban areas

2. Natural surfaces with ____ soil

layers and low infiltration capacity

in semiarid and arid areas.

3. Entire catchment

i>f

i-f

f

overland flow is produced when subsurface flow saturates the

soil near the bottom of a slope and overland flow then occurs as rain falls

onto the saturated soil.

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_________________ are the area of the watershed actually contributing to the stream at

any time. It expands during rainfall and contracts thereafter.

Image: http://ks.water.usgs.gov/pubs/reports/wrir.99-4242.fig02.gif (left)

http://soilandwater.bee.cornell.edu/research/VSA/processes/im_fig2.jpg (right)

Applicable to:

1. ________ surfaces in humid

regions

2. ________ of hill slopes and near

stream banks

A streamflow ______________ is a graph showing the flow

rate as a function of time at a given location on a stream

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A _________ is an integral expression of the physiographic and climatic characteristics that

govern the relations between rainfall and runoff of a particular drainage basin (Chow, 1959)

Image: http://www.xmswiki.com/w/images/2/2a/GSDAImage077.png (left)

http://blackpoolsixthasgeography.pbworks.com/f/1265744759/Storm%20Hydrograph.png (right)

Hydrographs tell the story of the catchment

Image: http://www.xmswiki.com/w/images/2/2a/GSDAImage077.png (left)

http://water.usgs.gov/edu/graphics/wcsnowmeltchart.gif (right)

Baseflow separation: the separation of the _______ runoff from

baseflow

9Image: http://www.engineeringexcelspreadsheets.com/wp-content/uploads/2011/03/Hydrograph-Example.jpg

A. Straight line method

B. Fixed base length method

C. Variable slope method

N

Inflection point

RUNOFF GENERATION

_____________________

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__________ or effective rainfall is that rainfall which is neither retained on the

land surface nor infiltrated into the soil. After flowing across the watershed,

excel rainfall becomes _________ runoff.

11Image: http://upload.wikimedia.org/wikipedia/commons/5/52/Saturated_ground_due_heavy_rainfall_in_Wagga_Wagga.jpg

The graph of excess rainfall vs. time is called the _____________________ hyetograph (ERH).

The difference between observed total rainfall and excess rainfall is termed __________________.

Losses = infiltration + interception + surface storage + evaporation

12Image: http://echo2.epfl.ch/VICAIRE/mod_1a/chapt_1/pictures/fig1.3.gif

Assessing rainfall excess is the first step towards estimating streamflow.

Depending on whether streamflow data is available or not,

rainfall excess can be estimated in two ways:

13Image: http://faculty.ksu.edu.sa/saleh-alhassoun/CE4251/chapter1.files/image004.gif

1. Streamflow is available: index method

2. Streamflow is not available: infiltration equations, or by SCS method

1. The index is that constant rate of abstractions that will yield an excess rainfall hyetograph (ERH) with a total depth equal to the depth

of the direct runoff over the watershed.

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Rai

nfa

ll [in]

Initial

abstraction

Losses

Rainfall excess

index

1. Pick a time interval, t

2. Judge the number of intervals, M of rainfall that actually contribute to direct runoff

3. Subtract t from the observed rainfall in each interval

4. Adjust the values of and M as necessary so that the depths of direct runoff and excess rainfall are equal (equation above)

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M

d m

m

r R t

2a. Abstraction using infiltration equations: is estimated by determining

the ponding time and infiltration under a variable intensity rainfall

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Derivation are based on the following principles:

1. In absence of ponding, cumulative infiltration is calculated from cumulative rainfall

2. The potential infiltration rate at a given time is calculated from cumulative

infiltration at that time

3. Ponding has occurred when the potential infiltration rate is less than or equal to

rainfall intensity

Rai

nfa

ll [in]

Initial

abstraction

Continuing

abstraction

Rainfall excess

index

2b. SCS method for estimating abstractions: developed by the Soil

Conservation Service (1972) for determining rainfall excess using information

of soil type, vegetation, and antecedent moisture conditions

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Fa is the water retained

S is the maximum retention potential

Pe is the excess precipitation

P is the total precipitation

Ia is the initial abstraction

Pre

cipitat

ion r

ate

Ia

Pe

Time

Fa

Through experiments, the data for P and Pe was plotted for many watersheds

and the SCS curves were found. To standardize the curves, a dimensionless

curve number CN is defined such as:

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100010S

CN

Curve number lie between 0 and 100

Curve number of 100 for impervious surfaces

For each soil type (A,B,C, and D) and land use type, a different

curve number exists

Image: http://www.professorpatel.com/uploads/7/6/5/6/7656897/7766438.jpg?606

Corrections can be made for antecedent moisture conditions: for dry (AMC

I) or wet (AMC III) conditions, equivalent curve numbers are calculated by:

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4.2 ( )( )

10 0.058 ( )

23 ( )(II )

10 0.13 ( )

CN IICN I

CN II

CN IICN I

CN II

Image: http://www.professorpatel.com/uploads/7/6/5/6/7656897/7766438.jpg?606

AMC group Total 5 day antecedent rainfall (in)

Dormant Season Growing Season

I Less than 0.5 Less than 1.4

II 0.5 to 1.1 1.4 to 2.1

III Over 1.1 Over 2.1

Table 5.5.1 in Chow et al. (2010)

Curve numbers depend on soil type

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Group Soil type

A High infiltration rates. Deep sand, deep

loess, aggregated soils

B Moderate infiltration rates. Shallow loess,

sandy loam

C Slow infiltration rates. Clay loams, shallow

sandy loam, soils low in organic content,

and soils usually high in clay.

D Very slow infiltration rates. Soils that swell

significantly when wet, heavy plastic clays,

and certain saline soils

Incre

asing cu

rve n

um

bers

And land use

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Table 5.5.2 in Chow et al. (2010)

WATERSHED CONCEPTSTRAVEL TIME, STREAM NETWORKS

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Reading assignment:

Section 5.7 Travel time

Section 5.8 Stream networks

In Chow et al. 2010