Water Resources Engineering
Prof.

R.Srivastava

Department Of Water Resources Engineering
Indian Institute Of Technology, Kanpur
Lecture

No. # 07

In today's
lecture, we are going to talk about runoff. Runoff is the componentof precipitation
which runs off from the catchment through the streamsor other drains. We will start by looking
at the hydrologic cycle with runoff components in detail. So let us look at the cycle once again.
(Refer Slide Time: 00:43)

This is the ground surface. There may be drains, trees;there may be small depressionslike lakes.
So when precipitation occurs, part of it will evaporate directly, part of it will fall on the treesand
other structures like roof of the buildings. From the roof of the buildings and trees, some part
will evaporate directly without reaching the ground and some part will fall down to the ground.
Now the part which falls on the ground, part of that will again evaporate from the ground surface
and a part of it will go below the ground level and this is known as inltration.
Now once this water goes underground, there is below the ground, a saturated soil layer and this
is known as the ground water which moves very slowly. So once the water reaches the ground
water, it will move again towards the drain but at a slow rate. Some part of inltration, if the soil
is pervious and there is an impermeable layer, will go to the stream or will appear on the surface.
If the impermeable layer intersects the surface, this part is known as interow. So this is the part
which inltrates and then comes back again on the surface or in the stream. So this interow is

also an important component. Now the part of precipitation which does not evaporate or inltrate
runs on the land. This is known as the over land ow. Over land ow will go into the lakes and
once the depressionsand lakes are filled then the rest of it runs off to the stream. What we will
look at today is this component. How much water runs off into the stream from a particular
precipitation?
(Refer Slide Time: 03:54)

If we look at various components of the runoff, let us say a river cross section and let us say the
ground surface is like this and the ground water is here. The part which comes over the surface is
known as surface runoff and the part which comes from the ground water is called base ow.
The difference between these two is that surface runoff comes immediately or very soon after the
precipitation while base ow for the same precipitation to appear in the stream as base ow will
take a longer time because,it will inltrate into the water into the ground, reach the ground water
and moves very slowly. This may take a few months or sometimes even years to reach the
stream. So there is a fast component and there is a slow component.

(Refer Slide Time: 05:06)

We can write down various components in two forms. Direct run off and base ow depending on
how fast or how slow they reach the stream. In the direct run off, we can again have various
components. For example surface runoff which comes over the surface. There is some rainfall
which occurs directly on the stream surface so precipitation on stream, and there is some part of
interow which comes very fast and back to the stream and we can call this Prompt inter ow or
rapid interow. So the component of inter ow which goes in to the ground and then reappears
in a short time is known as the rapid or prompt inter ow and that is also counted towards direct
runoff. So this entire direct runoff component is the one which occurs a little while after the
precipitation. The base ow component can again have two different parts. One, this will be a
delayed interow and two; this would be a base ow. So the delayed interow is the component
of interow which goes into the ground and appears on the surface again, but not very fast, it
takes a while sometimes a few days. Base ow is the component which comes from the ground
water and typically it takes a few months or sometimes even years. So we will look at various
aspects of these components and see how we can predict or estimate the runoff with or for a
given event of precipitation.

(Refer Slide Time: 07:32)

So let us look at a river which has its origin at some point. Now let us say that this point is A.
This entire area is the one which contributes to the ow at the point A in the river. This area is
known as the catchment area. So this is the catchment of the river at the point A. This means that
that all the precipitation falling in this area will ultimately reach the point A and it will be taken
out of this area through the stream at point A. So this catchment area has its boundary which is
known

as water

divide

or sometimes

the catchment

area is also called

water

shed.

So the idea

of

this catchment area is that any precipitation falling anywhere in this area would go to the river
and then will be carried out of this area or runoff from this area through point A. Now if we take
another point B then naturally the catchment area of B would be larger and it may look like this.

(Refer Slide Time: 09:14)

If we look at the elevation, then supposethe ground is like this and this is the river section then
any water precipitation falling on this side may come to the river and a precipitation falling on
this side may go out of this area. So this will be a water divide, dening the catchment area. Now
how do we study the runoff in a river?

(Refer Slide Time: 09:47)

Typically what we do is plot what is known as a hydrograph, which tells us how the ow in a
river or a stream varies with time. The ow typically would be in meter cube per second. The
time may be in hours, days or even months depending on what scale we are talking about. If you
look at the ow, let us say over the scale of a year we might say that it is a January, February,
and March and so on. The ow in a river may be very small during the months of winter and
until may be May. But when the monsoon arrives, the ow will increase and then again it will go
down and then in January it will become very small. So this is the hydrograph of the stream and
it is very important to know the hydrograph becauseit will tell us what is the maximum ow in
the stream, what is the minimum ow in the stream and both these are very important because
the maximum ow will affect the design of any ood control works. The minimum ow will
affect the storage requirement or whatever water we can take out of the river on a consistent
basis, it will affect that. So both these are important. What is the minimum ow? What is the
maximum ow? If we look at typical streams,there may be three different kinds of streams.
One which runs throughout the year, i.e., perennial for which the hydrograph will look like this
(Refer Slide Time: 11:56). Because it will run through the year and there is no part of the year
where there is no ow in the river, so in this all the time there is at least some ow in the river.

But the other kind of a stream is intermittent and ephemeral. They may not have ow during
some part of the year. For example the intermittent stream may have a hydrograph which would
look like this. So in some part of the year, there will be no ow and during most part of the year
there would be some ow in the stream. In the absenceof storm, when there is no precipitation
falling on the area, ground water ow or the base ow are the ones which sustain the ow in the
stream. So in a perennial stream the ground water level will be above the stream bed and
therefore it will contribute water towards the stream ah throughout the year. But in intermittent
streams the ground water level sometimes may fall below the stream bed and there will be no
ow in the stream. But again during monsoons the water in the ground water level may rise and

therefore it will have a continuous period of 6-7 months or so in which there will be always the
ow. Contrasting behavior is observed in ephemeral streams in which there is almost 0 ow in
most part of the year and there will be some isolated ows in responseto storms. When there is
rainfall, then there is some ow in the river and then there is no rain there will be no ow.

So in

these kinds of streams, the pattern of a stream is important to know in order to design storage or
a ood

control

measure.

(Refer Slide Time: 14:36)

Now the runoff process or this hydrographs shifts it may have a at peak it may have a very
sharp peak the time base may be small the time base may be large this depends on a lot of
factors. So next, we will see some of the factors which affect the shape of the hydrograph. We
would start with a hydrograph which is of a smaller duration. So it is in response to some
precipitation over the area and then we will go to the higher duration, behavior for example
animal hydrograph. The hydrograph which is in response to a storm is typically called storm
hydrograph or simply hydrograph. So in this uh lecture we will just use hydrograph for this term
hydrographs. So let us see how the storm hydrograph shape is affected by different factors. The
factors which affect the shape depend on the basin or the catchment basin and catchments that
are used, they are synonymous. So some basin or catchment properties affect the shape of the
hydrographs. So when a rain occurs how does the runoff change?

(Refer Slide Time: 16:26)

So over a certain area let us say, this is the catchment area. A river originating here and moving
to the point A and now supposethere is some rain over this entire area, (we will assumethat the
rain is of uniform intensity), the hydrograph of the rainfall can be plotted as the intensity versus
time and it may be of uniform intensity for certain duration D. Now if this intensity is less than
the evaporation and inltration from the basin, then there will be no runoff. So first it has to
satisfy the evaporation, the infiltration and also depending on the presence of lakes and other
small depressions, the depression storage. So some part of it will go in to satisfying the initial
obstruction and we have already seen this in the chapter on inltration that we can define an
inltration index phi which is taken out of the rain and then the effective rainfall is reduced. This
hydrograph is called ERH, the effective rainfall hydrograph. So our interest is in looking for a
given ERH, what will be the runoff hydrograph or DRH direct runoff hydrograph? In a
hydrograph there will be some component which will have a base ow and that is not a direct
result of the effective rainfall, therefore we must remove that component and then study the
relation between effective rainfall and the direct runoff. So if you look at this catchment the one
important factor is known as the time of concentration. Now this time of concentration typically
denoted by to, is the maximum time it takes for a drop of water anywhere on the catchment to
reach

the outlet

A.

Now if we look at this catchment area which does not have any tributaries, there is just one main
channel and there is no tributary to this channel then a water drop falling on this point will rst
move over the land and then join the stream. A water drop falling here may move here and then
follow the stream. Typically the velocity in the stream is much higher than the velocity over
land. So what we say is that, over land ow typically moves at a smaller velocity compared to
the stream ow, therefore a point which is farther from the outlet A, this point which is at a
distance, is farther from the point A. It may reach A sooner then let us call this point B and this
C. So a rain drop falling at B will first move over the land to the river and then it will follow the

river to the point A. A rain drop falling at C will rst have to travel over the land to this point on
the stream and then move towards the point A in the stream. Now since the stream velocity is
very high, the rain drop at B may reach A sooner than C. So it is not the distance which affects
the time of concentration but it is how far it has to go over land and that how far than it will have
to go to a stream. So this is the maximum time. In this case it may be a point somewhere here or
here (Refer Slide Time: 21:15). So it will have to cover a lot of area over land and then will

reach the stream and move to the point A. So time of concentration is an important parameter in
the hydrographs. So let us now look at these factors.
(Refer Slide Time: 21 :36)

For example how does the basin shapewill affect the hydrograph? Now let us look at two basins
in which have everything identical. For example same length of river, same size, but the shapes
of the catchment are different.

So in one of them, the catchment is wider, near the outlet and

narrower as we go away and in one, the catchment is narrow near the outlet and it is wider as we
move away. If you look at the hydrograph, considering everything to be the same except the
basin shape, we would expect the peak to come sooner. If you look at the curve, becausemost of
the rain occurring near the outlet point will reach very fast at A and also since this area is large
the peak will be higher. But in the other case, the hydrograph will be more at because the
rainfall which is occurring in the distant area would take some time to reach the point A. So the
shape of the hydrograph will be affected by the shape of the basin. The next factor may be the
basin size. Size of course decides the volume of rain occurring over the catchment.

(Refer Slide Time: 23:50)

So if the size is more, the volume of rain will be more and therefore the runoff will be higher. So
if you have a smaller size of catchment, the hydrograph may look like this, for a bigger size of
catchment, the hydrograph may look like this (Refer Slide Time: 24:27) for the same rain
becausethe same rain is occurring over the larger area, so it will contribute more volume to the
runoff. Typically the peak ow is proportional to the basin area to some power n and n will vary
from area to area. There are some typical values given in their S books for different areas in
India

and outside

India.

(Refer Slide Time: 24:57)

The next factor we would look at is basin slope. Now basin slope will affect the time of over land
ow. So if there is a channel and the catchment area. The time it takes for rain to drop or a water
to travel from B to C will be dependent on the slope of the basin. Higher the slope, smaller will
be the time taken in travelling from B to C. Therefore the time base of the hydrograph will
depend on basin slope, typically larger slope will have a smaller tb. Similarly the stream slope
also is important because the stream slope decides the stream velocity. So higher slope again
means higher velocity and therefore it will affect the falling part of the hydrograph. This part
which is typically known as falling limb or recession limb denotes the water coming from the
storage. It may be channel storage or it may be storage inside lakes or small depressions in the
area. So this part will depend on what is the stream slope. If the stream slope is large, this part
will be showing a rapid decline. If the stream slope is small it will show a at decline. The next
factor

is land

use.

(Refer Slide Time: 27:09)

For the same rainfall, if the land is open, the soil is porous then most of the water will go inside
the ground. Lot of infiltration and runoff will be smaller but if the land is paved, we have lot of
roads, parking lots and other paved areas, and then there will be less inltration and more
runoffs. Similarly vegetation also affects the runoff. If there is more vegetation there will be
more interception. There will be more transpiration and also the over land ow velocity will be
reduced. So if there is more vegetation, typically the runoff volume will be smaller. Drainage
density is another important parameter which affects the hydrograph. By drainage density we
mean in the catchment area, what is the length of channel and again this is channel network
which includes all the tributaries also. So if you have a tributary joining in here, and tributary
joining in here, these tributaries may themselves have some smaller channels joining them. So
this is the length of the entire channel network per unit area of the catchment. So divided by the
area of catchment, this is known as the drainage density.
So this should also be straight forward to look at if we look at two hydrographs with different
drainage density, for example this may correspond to a network which has a smaller drainage
density and this may correspond to a network which has a larger drainage density. Now smaller
drainage density means that most of the ow will be over land ow and higher density means
that most of the ow will be through the streams. So it will reach the point A faster and the peak
also would be affected by the drainage density in the area. Now when the rain occurs initially,
there will be some ow becauseof the base ow component. So if we want to study the effect of
the rain, we must separate out the base ow and the direct runoff. So let us look at the typical
hydrograph and seehow we can distinguish these components and separatevarious components.

(Refer Slide Time: 30:32)

So if you look at this typical hydrographs, the rain has started and it is conventional to show the
rain on the hydrograph itself. So the rain hydrograph is shown on the hydrograph itself to tell that
this hydrograph is because of this rain. Now if you see from this figure, the ow in the river is
decreasing even after starting of rainfall. So initially the ow in the river is base ow, the base
ow will be decreasing with time becauseas water is taken from the ground water depending on
the surrounding area and conductivity, the water level will be going down and therefore the ow
in the channel will be reducing. Now once the rain occur the stream ow does not immediately
start to increase because initially some part of the rain will be going into satisfying the river
pollution and infiltration requirements. Let us say this is point A. At point A, the rainfall has
satisfied

the initial

abstraction

and now

the runoff

has started.

So we call

this

as the start

of the

direct runoff. It will start increasing and will reach the peak. So this point is known as the peak
point Qp. Now at Qp, we have reached a maximum ow and this will depend on the time of
concentration. Suppose our rainfall is more than the time of concentration, after reaching Qp it
might continue at Qp for some time and then come down but typically the rainfall is not for that
long duration because the time of concentration is typically quite high and rainfall generally is
for a shorter duration. So once it reaches Qp, it will not continue at that level. But it will fall
down.

Now on the rising limb, this portion of the curve is called a rising limb or sometimes even
concentration curve. If you look at this part, it is concave upwards at this point A and at the peak,
it is convex upwards, so somewhere in the middle there is a point of inection where the nature
of the curve changes from concave to convex. Let us call this point of inection as point B.
Similarly when the rainfall has stopped and the ow is now reducing on the falling limb, which
is also known as recession curve, because water is receding recession curve. So on the falling
limb again the nature of curve is changing from convex upwards to concave upwards and there

will be another point of inection here where, before that the curve was convex upwards and
after that was concave upwards.
(Refer Slide Time: 34: 13)

The portion from B to C includes the peak, so this BC is typically called crest segment. So AB is
the rising limb, BC is the crest segment and then from C to a point D is the falling limb. The
point D typically will be a point, where there will be ow. So after D we will have only base
ow and there is no direct runoff beyond the point D. One method of separating the base ow is
we join A and D. So the curve ABCD would be the direct runoff hydrograph DRH and also it is
conventional to show not the total rainfall here, so instead of showing total rainfall, we would
show only the effective rainfall and the effective rainfall would look like this. So effectively the
rainfall starts at the point A because before that we say that whatever rain has occurred is not
going as runoff, it is satisfying the inltration and portion requirement of the basin and if there is
some depression storage, it will satisfy the initial abstraction requirements. From the point A, the
effective rain starts, it stops somewhere and the peak will occur after the stoppage of the rain,
becausewhen the rain stops, the areas which are removed from the point A, it would take some
time for that water to reach the point A and therefore the runoff will continue to increase even
after stoppage of rain. So let us now look at some of the methods which are used to separatethe
base ow.

(Refer Slide Time: 37:45)

So this is the effective rainfall, point A is easy to spot because point A is the point from which
the hydrograph starts rising that means our rainfall has started contributing. So let us see how to
spot various points on this. We have already seen that point A it is very easy, becausethis is the
point at which the hydrograph is starting to rise. B and C we do not need but I will just mark
here, these are the points of inection which denote change from concave to convex or in other
words, the slope of this curve ah dq/dt will reach a maximum here. If you look at the slope of this
curve at point A, it would be 0 at the peak. Also the slope of this curve is 0 in between the slope,
is increasing. It will reach a maximum at point B and then it will start to decreaseand become 0
at the peak. Point C also we do not need for base ow separation but I will show it here. The
point D here is the one where we see the direct runoff has ended and beyond that we have only
base ow. So before this we have base ow and beyond D also we have base ow.
The region from C to D is recession that denotes the falling or decreasein the ow with time
now. This part C to D is not affected by storm characteristics becausestorm has already stopped.
C to D denotes the decreasein channel storage or some other small depressions which may be
draining. This recession limb is generally affected by three different parametersand what we say
is that Q during recession is typically given by a power Q0 kr to the power t where Kr is
dependant on three different kinds of factors, Krs, Kri and Krb which are for the surface stream
interow

and base ow,

surface interow

base ow.

K denotes how fast the stream or the

surface is, the discharge from the surface stream will decay, how fast the interow is decaying
and how fast the base ow is decaying. In most the basins, inter ow component may not be
very large and typically Kri can be taken as one. Krs denotes how fast the water is coming out of
storage from the channel and as we have seen earlier, it dependson the channel slope. If channel
slope is large water will come out of it faster and base ow it depends on how the ground water
contribution is decreasing. There are some typical values which are generally used for these
coefcients. So Krs is typically taken as 0.1 and Krb is typically taken as 0.9, so this decreases

very slowly. This decreasesat a quite faster rate so this tells you about the recession length. Point
D is the point at which we say that the direct runoff has ended and base ow has started ah
typically whatever is the base ow at the start of the hydrograph we can use a similar pattern
here or we can plot this curve on a log or semi log scale and see where the curve departs from ah
this equation Q 0 Krt. So we will look at some methods which do the base ow separation.
(Refer Slide Time: 43:14)

The easiestmethod is to join A and D with a straight line. Supposethis is the hydrograph, we say
that the base ow is given by this line joining A and D and whatever area is above this line, will
give the direct runoff ordinate at that time and then we can plot this direct runoff hydrograph
separately, which will have a peak equal to this value and a time base which is equal to AD.
Time base of course will remain same for all the methods, because we have already xed the
points A and D.
This method which is known as the straight line method is the simplest. But there are some
drawbacks to this method. One is, some people say that the base ow does not remain constant
throughout this period. Once this storm occurs, the ground water level will also go up and the
base ow should increase. So there are different methods which can be used to improve on this.

(Refer Slide Time: 45:11)

One of the methods which have been used is extending the curve on the rising limb or before the

rising limb. So if you have this point A and D and this is the time of the peak, let us call this tp
then we assume that the base ow will continue to decrease beyond A till the peak of the
hydrograph is reached. Let us call this point C. We have already used C, so let us call this point
E and join E and D. So A E D will represent the base ow contribution and then the ordinate
above this, can be taken as direct runoff. So you can see that compared to the previous straight
line method which used to join A and D, this method typically gives a higher peak. So in this
what

we have

done

is extension

of base ow

forward

from

A. So this

is method

two

in which

we

extend base ow forward. So from A we have extended it till the peak and then join A and D
with a straight line.

(Refer Slide Time: 47:01)

The third method is the one in which we extend the base ow backwards from D. So again let us
draw the hydrograph. Find out the points A and D, these B and C are the points of inection. So
what

we do in this

method

is extension

of base

ow

backward

from

D.

So we extend

the base

ow component, so this is going at a curve like this, so we extend it till we reach the point of
inection and then we use another smooth curve to join. Let us call this point F, so A F D will
now give the base ow component and the peak. This case turns out to be smaller and these are
the direct runoff components at various points. So these are the methods which can separatethe
base ow from the total runoff in order to give us a direct runoff hydrograph or DRH.

(Refer Slide Time: 48:47)

So the DRH will start from 0 because the base ow is separated,it will reach a peak and will
again come back to 0 because again the base ow is separated. So there is only direct runoff
component after this point. Beyond this we have base ow in the river but we are not showing it
and the corresponding hydrograph can be shown like this. So this is a very simple description of
the hydrograph for uniform rain occurring over the catchment. If the rain is not uniform
sometimes the hydrograph may look like this (Refer Slide Time: 49:34). In that case the
hydrograph may have a very different shape,it may go like this.

(Refer Slide Time: 49:50)

So for a complex storm in which various intensities of rain occur for different times, the shape
of the hydrograph will be very different. But if we assume, there are two assumptions. If we
make them then it makes the analysis much easier and those assumptions are called time
invariance and linearity. In general rainfall runoff relationship is very complex. If we have two
different catchments, the same rainfall will cause two different or very different hydrographs or
for the same catchment if we have two different kinds of rainfalls they will also cause very
different hydrographs. Sometimes even for the same catchment and same rainfall we may get
different hydrographs. It depends on what the starting initial condition is. For example if the soil
is very dry, rainfall occurs. Lot of it will go into inltration and runoff will be smaller. For the
same rainfall if the soil is wet initially, for example if there is a rain before that day, soil will be
wet. So if the soil is wet there will be more runoff, so not even catchment to catchment but even

from event to event one rainfall to other rainfall event, the hydrograph will be different. So it is
very difficult to theoretically analyze this unless we make some assumptions. So these two
assumptions, time invariance and linearity help us in analyzing the hydrograph and to estimate
what would be the runoff for a very complex rain. So time invariance means that with time the
hydrographs are not changed. If the same rainfall occurs no matter whether it is occurring at time
t equal to 0, like this or if it is occurring at time t equal to 3 days, it will produce identical
hydrograph of course it has to be shifted.

(Refer Slide Time: 52:23)

So let us look at t versus Q. So if there is rain occurring here and we are generating the
hydrograph, direct runoff hydrograph like this and supposewe have another rain occurring let us
say starting from this point, same intensity, same duration then the hydrograph for the second
rainfall event, second storm would be exactly same as the rst one. Only the base will be shifted
like this. So these two hydrographs will be the same if the effective rainfall hydrographs ERH is
same. So that is the principle of time invariance, no matter where in time, the effective rainfall
occurs it will produce an identical hydrograph.

(Refer Slide Time: 53:36)

The second principle which we have is the principle of linearity which says that if some ERH
causesa DRH, this DRH direct runoff hydrograph is for this effective rainfall hydrograph. If the
same durational rain occurs with a larger intensity, suppose this is intensity I1 and there is
another one let us say within these 211.The principle of linearity says that the ordinate of direct
runoff hydrograph becauseof the second rain where the intensity is 211will be exactly twice the
ordinate of the first one. So everywhere if you take the ordinates, this ordinate will be twice this
ordinate, if the rainfall intensity is twice. So the principle of linearity and time invariance help us
in analyzing lots of different kinds of storm patterns in order to arrive at corresponding DRH.

(Refer Slide Time: 55:17)

One of the most important theories which are used is known as the unit hydrograph theory. So
this unit hydrograph theory creates a hydrograph for a unit rainfall and then using the principles
of time invariance and linearity, it tries to estimate for any other kind of rainfall pattern, what
would be the direct runoff hydrograph. So before we discuss unit hydrograph, let us look at DRH
and look at the area under this curve becausethis will help us in nding out or deriving the unit
hydrograph. This Q is in meter cube per second and t is in hours. For area of this curve would
give us some volume V. If we take both of them in same unit like meter cube per second, the
area will come out to be in meter cube. If they are in different units, meter cube per second and
hours then we have to accordingly multiply it with a constant to get the volume in meter cube. So
this volume would be let us say, we have obtained this volume in meter cube. So what does this
volume

indicate?

This DRH is because of some ERH which has intensity I and duration D. What we have said
earlier is that this rainfall ERH is occurring uniformly over the entire catchment area. So if the
catchment area is A and the rainfall intensity is I, duration is D, I into D will give us, suppose
this I is in millimeter per hour and D is in hours then I in to D will give us amount of rain in
millimeter units. Now this is occurring uniformly over the whole catchment area. So if we
multiply A into I into D, we would get a volume and since this DRH is a result of this ERH these
two volumes should be same. So this I into D is the amount of rain which is falling in terms of
depth. This is the depth of rain falling over the whole catchment area and this will be different
for different duration and different intensities. So when we talk about unit hydrograph we say
that we normalize the depth of rain to a unit quantity. So the depth of rain should be the unit. Let
us call it depth of precipitation. The unit which is used typically in India is 1 cm. In some
countries depth of the precipitation is taken as 1 inch for the unit hydrograph, but we will take
1cm as the unit rainfall. So what it means is that if we have 1 cm of depth falling over the

catchment in let us say D hours, then the direct runoff hydrograph which results from direct rain
will be called a unit hydrograph for D hours. So let us take an example.
(Refer Slide Time: 59:47)

t, Q and let us say we have a rain falling for 3 hours then its intensity must be 1 over 3 cm per
hour, so that the total depth of effective rainfall is 1 cm. Now because of this ERH whatever
DRH we get, would be called a unit hydrograph. Since duration is very important, 3 hours is
called as 3 hour unit hydrograph. Similarly if we have rain which is falling for 2 hours intensity
will be 1 by 2 centimeter per hour and the resulting hydrograph would be called a 2 hour unit
hydrograph. The area under the unit hydrograph representsA, the catchment area into 1 cm.