Hydrology and water resources engineering (FM-2) NOTES (B-Tech Civil Engineering)

Explore the fundamental concepts of hydrology and the management of water resources. These notes cover key topics such as the hydrologic cycle, water balance, and sustainable water resource planning, providing a solid foundation for understanding water-related challenges and solutions.

1. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
UNIT-1
Importance of Hydrological Data
Hydrological data is crucial for understanding and managing water resources, as well as making informed
decisions related to water-related infrastructure, environmental protection, disaster management, and more.
Here are some key reasons highlighting the importance of hydrological data:
1. Water Resource Management: Hydrological data provides valuable insights into the availability,
distribution, and movement of water within a specific region. This information is essential for managing
water resources effectively, ensuring sustainable water supply for various purposes such as drinking,
agriculture, industry, and ecosystems.
2. Flood Prediction and Management: Hydrological data, including rainfall patterns, river flow rates,
and water levels, are essential for predicting and managing floods. Accurate data helps in early warning
systems, allowing authorities to take preventive measures and minimize the impact of flooding on
communities and infrastructure.
3. Drought Monitoring and Mitigation: Hydrological data can help identify and monitor drought
conditions by tracking changes in groundwater levels, river flows, and reservoir levels. This
information assists in developing strategies to mitigate the impacts of droughts on agriculture,
ecosystems, and communities.
4. Infrastructure Planning: Reliable hydrological data is crucial for designing and maintaining
infrastructure projects such as dams, reservoirs, bridges, and irrigation systems. It ensures that these
structures are built to withstand various hydrological conditions and contribute to the overall safety and
development of a region.
5. Environmental Conservation: Hydrological data contributes to understanding the health and
functioning of aquatic ecosystems. It helps monitor water quality, habitat conditions, and the impact of
human activities on aquatic environments. This information guides conservation efforts and sustainable
management of aquatic ecosystems.
6. Climate Change Analysis: Hydrological data plays a vital role in assessing the impacts of climate
change on water resources. By analyzing long-term trends and variations in precipitation, temperature,
and river flow, scientists can project how climate change might affect water availability and
distribution.

2. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
7. Policy Formulation and Decision Making: Policymakers and decision-makers rely on accurate
hydrological data to formulate effective water management policies, regulations, and strategies. Data-
driven decisions can lead to more efficient and equitable allocation of water resources.
8. Research and Education: Hydrological data serves as a foundation for scientific research and
education. Researchers use this data to study hydrological processes, model water systems, and develop
innovative solutions for water-related challenges. It also provides valuable teaching material for
students studying hydrology, environmental science, and related fields.
9. Emergency Response and Disaster Management: During extreme weather events or natural
disasters, hydrological data assists emergency responders in making timely and informed decisions.
This includes coordinating evacuation efforts, managing water levels in reservoirs, and deploying
resources to affected areas.
10. Cross-Border Cooperation: Many rivers and water bodies cross international boundaries. Sharing
hydrological data among neighboring countries fosters cooperation, helps resolve disputes, and enables
coordinated management of shared water resources.
In summary, hydrological data is essential for a wide range of applications, from daily water management to
addressing global challenges such as water scarcity and climate change. It provides the foundation for informed
decision-making, sustainable development, and the protection of both human and natural systems.
THE HYDROLOGIC CYCLE
The hydrologic cycle, also known as the water cycle, is a continuous process through which water moves and
changes states between the atmosphere, the Earth's surface, and below the surface. It is a fundamental natural
process that plays a crucial role in regulating the distribution and availability of water on Earth. The hydrologic
cycle consists of several key stages:
1. Evaporation: The cycle begins with the sun's energy causing water from various water bodies (such
as oceans, lakes, rivers, and even soil moisture) to evaporate and transform from liquid to water vapor
in the atmosphere. This process is driven by solar energy and heat.
2. Condensation: As the water vapor rises into the atmosphere, it cools and condenses to form tiny water
droplets, creating clouds. This is the process through which water vapor changes back into its liquid
state.
3. Precipitation: When the water droplets in clouds grow larger and become heavy enough, they fall to
the Earth's surface as precipitation. This can take the form of rain, snow, sleet, or hail, depending on
the temperature and atmospheric conditions.

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Assistant Professor (Department of Civil Engineering)
4. Infiltration: Once precipitation reaches the ground, it can take different paths. Some water flows
directly over the surface as runoff, while some water infiltrates into the soil, moving downward through
the pores and spaces in the ground.
5. Percolation: The infiltrated water continues to move downward through the soil until it reaches the
zone of saturation, where all the spaces between soil particles are filled with water. This forms the
groundwater, which can flow slowly through underground rock formations.
6. Transpiration: Plants take up water from the soil through their roots, and this water travels through
the plant's tissues and eventually evaporates from tiny openings on leaves called stomata. This process
is known as transpiration and is an important component of the water cycle.
7. Runoff: Some of the water that does not infiltrate the soil becomes surface runoff, flowing over the
land and eventually reaching streams, rivers, and other water bodies. Runoff can carry sediment,
nutrients, and pollutants along with it.
8. Subsurface Flow: Water that has infiltrated the soil can also move horizontally beneath the surface
before eventually joining groundwater or contributing to streamflow.
9. Groundwater Flow: The water stored in the ground as groundwater can move slowly through rock
formations and aquifers, eventually discharging into surface water bodies or being accessed through
wells.
10. Surface Water Storage: Water accumulates in surface water bodies like lakes, reservoirs, rivers, and
oceans. These water bodies store and release water, affecting regional water availability and local
climates.
The hydrologic cycle is a continuous and interconnected process, with water moving between different
reservoirs and changing states as it does so. It plays a crucial role in maintaining Earth's ecosystems, supporting
human societies, and shaping the physical landscape. Understanding this cycle is essential for managing water
resources, predicting weather patterns, and addressing water-related challenges.
Precipitation in meteorology refers to any form of water, liquid or solid, that falls from the
atmosphere and reaches the ground. It plays a crucial role in the Earth's water cycle, replenishing water
bodies, sustaining ecosystems, and impacting weather patterns. Precipitation occurs when moist air
cools and reaches its saturation point, causing water vapor to condense into visible water droplets or
ice crystals that fall to the ground due to gravity.

4. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
Forms of Precipitation:
1. Rain: Raindrops are liquid water droplets that form when air cools, and the water vapor condenses into
tiny droplets. Rain is the most common form of precipitation and occurs when temperatures in the
atmosphere are above freezing.
2. Snow: Snowflakes are ice crystals that form around tiny ice nuclei in cold temperatures. These ice
crystals cluster together to create unique and intricate snowflake shapes. Snow forms when the
atmospheric temperature remains below freezing.
3. Sleet: Sleet, also known as ice pellets, forms when snowflakes partially melt as they fall through a layer
of warm air and then refreeze into small ice pellets before reaching the ground.
4. Freezing Rain: Freezing rain occurs when supercooled raindrops—liquid drops that remain in a liquid
state despite temperatures being below freezing—freeze upon contact with cold surfaces, creating a
layer of ice.
5. Hail: Hailstones are solid balls of ice that form within severe thunderstorms. Powerful updrafts carry
raindrops into extremely cold regions of the atmosphere, where they freeze and accumulate layers of
ice as they are lifted and dropped within the storm multiple times. Hailstones can become quite large
depending on the strength of the storm.
Causes of Precipitation:
(I) Cooling and Condensation: Precipitation occurs when moist air is cooled to the point where it
becomes saturated with water vapor. As the air cools, it can no longer hold all the water vapor it
contains, leading to the condensation of water droplets or ice crystals.
(II) Frontal Lift: When warm and cold air masses meet along a weather front, the warm, less dense air is
lifted over the colder, denser air. As the warm air rises and cools, it reaches its dew point, causing
precipitation to form.
(III) Orographic Lift: When moist air is forced to rise over a mountain range, it cools as it ascends. This
cooling leads to the condensation of water vapor and the formation of clouds and precipitation on the
windward side of the mountains.
(IV) Convection: Warm air near the Earth's surface rises due to its lower density. As this air ascends and
cools at higher altitudes, it may reach its dew point, leading to the formation of clouds and precipitation.
(V) Convergence: When air converges at a specific location due to various atmospheric conditions, the
resulting upward motion can lead to cooling and the formation of precipitation.
(VI) Frontal Systems: The interaction of warm and cold air masses along fronts (boundaries) can lead to
the lifting of warm air over cold air, creating conditions conducive to precipitation.

5. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
(VII) Thunderstorms: Intense convective processes within thunderstorms, driven by strong updrafts and
downdrafts, can lead to the rapid formation of precipitation, including heavy rain and hail.
In summary, precipitation is the result of atmospheric cooling, condensation of water vapor, and various
atmospheric processes such as orographic lift, frontal systems, and convection. The type of precipitation that
falls depends on the temperature profile within the atmosphere and the specific conditions that lead to the
cooling and condensation of water vapor.
Mechanics of precipitation
"Precipitation" in meteorology refers to the process by which water vapor in the atmosphere condenses into
liquid or solid water particles and falls to the ground. The mechanics of precipitation involve several key
factors:
1. Condensation Nuclei: Tiny particles, such as dust, pollen, or salt particles, serve as "condensation
nuclei" on which water vapor can condense and form droplets or ice crystals. These nuclei provide
surfaces for water vapor to adhere to.
2. Saturation and Dew Point: As air rises, it cools down. When the air's temperature drops, it may reach
the point of saturation, where it can no longer hold all the water vapor it contains. The temperature at
which this happens is called the "dew point." If the air continues to cool, excess water vapor starts to
condense into visible water droplets or ice crystals.
3. Cloud Formation: As air rises and cools, it can reach a level where the dew point is reached and cloud
formation occurs. Clouds consist of water droplets or ice crystals suspended in the air. When these
particles grow large enough, they can collide and coalesce, forming larger droplets or crystals.
4. Precipitation Formation: In clouds, if water droplets or ice crystals grow to a sufficient size and
weight, they overcome air resistance and fall to the ground due to gravity. The type of precipitation
(rain, snow, sleet, or hail) depends on the temperature conditions within the cloud and as the
precipitation falls through different layers of the atmosphere.
5. Rain: Rain forms when cloud temperatures are above freezing. Water droplets collide and merge as
they fall, eventually becoming large enough to overcome air resistance and reach the ground.
6. Snow: Snow forms in colder temperatures. Water vapor directly freezes onto ice crystals, creating
snowflakes. These flakes accumulate and fall to the ground.
7. Sleet: Sleet occurs when snowflakes partially melt as they fall through a layer of warm air and then
refreeze into small ice pellets before reaching the ground.

6. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
8. Hail: Hailstones form in severe thunderstorms with strong updrafts. These updrafts carry raindrops into
extremely cold regions of the atmosphere, where they freeze and accumulate layers of ice as they are
lifted and dropped within the storm multiple times.
9. Rain Shadow Effect: In some cases, mountains can block the movement of moist air masses, causing
air to rise on one side and descend on the other. As the air rises, it cools and releases precipitation. On
the other side of the mountain (the rain shadow side), the descending air warms and dries out, resulting
in arid conditions.
These mechanics illustrate the complex interplay between temperature, humidity, air currents, and atmospheric
conditions that contribute to the formation and types of precipitation.
Types of Precipitation:
Precipitation is usually classified in the following three categories on the basis of
the basic processes involved in causing the same:
(i) Convective Precipitation:
Convective precipitation is most common in the tropical countries. On a hot day (during
summer) the ground surface becomes heated as does also the air in contact with it. This causes
the air to expand and rise by convection. As it rises it cools dynamically at the dry adiabatic rate
of about 1°C per 100 m which in turn results in condensation and precipitation.
(ii) Orographic Precipitation:
The precipitation caused by lifting of air over mountain barrier is called orographic
precipitation. When moisture bearing winds usually blowing from oceans to land surfaces are
forced to rise far above the ground surface by the presence of the coastal mountain ranges, the
cooling and condensation processes take place and the precipitation occurs on the windward
side of the mountains.
(iii) Cyclonic Precipitation:
Cyclonic precipitation results from lifting of air masses converging into a low pressure area or
cyclone.
The cyclonic precipitation may be classified as:
(a) Frontal precipitation; and
(b) Non-frontal precipitation.

7. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
(a) Frontal Precipitation:
A surface separating the warm air mass and the cold air mass is called a frontal surface or a
front, which may be subdivided as warm front and cold front. A warm front is the one in which
warm air replaces cold air and in a cold front cold air replaces warm air.
Frontal precipitation results from lifting of warm air over cold air and it may be subdivided as
warm front precipitation and cold front precipitation. In warm front precipitation the warm air
moves upwards over a relatively stationary wedge of cold air. In this case the precipitation is
spread over a large area which may extend 300 to 500 kilometres ahead of the front and the
precipitation is generally light to moderate and nearly continuous until after the passage of the
warm front.
On the other hand, in cold front precipitation the warm air is forced upwards by an advancing
wedge of cold air. In this case the precipitation occurs on a small area which may extend only
100 to 150 kilometres ahead of the front and the precipitation is relatively more intense.
(b) Non-Frontal Precipitation:
Non-frontal precipitation occurs when there is low pressure (or barometric depression) caused
in any region. In this case air from an adjacent high-pressure area flows into the area of low
pressure which causes the lifting of the air of the low-pressure area to high altitudes where it
cools down and results in condensation and precipitation.
Measurement of Precipitation:
For almost all the hydrological designs it is necessary to have the records of precipitation for a
long period. Such records may be developed by the measurement of the precipitation occurring
at any place from time to time. All forms of precipitation are measured on the basis of the vertical
depth of water or water equivalent (in the case of snow) which would accumulate on a level
surface if all the precipitation without any loss remained where it fell.
The precipitation is therefore expressed in millimetres (or centimetres). Many types of gages
have been developed for the measurement of rain and snow which constitute the major part of
precipitation. Some of these gages which are commonly used are described below. Since the
amount of precipitation varies from place to place, it would be necessary to install the gages for
the measurement of precipitation at various key points in the area. Especially in the drainage
basin of a stream or river a network of such gage stations evenly distributed over the entire area
of the drainage basin should invariably be provided.

8. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
In the measurement of precipitation it is, however, assumed that the observations of
precipitation made at any gage station is representative precipitation of certain area around the
gage station where the measurement is made. The Indian Standard IS: 4987-1991 gives detailed
specifications for establishing a network of gage stations for the measurement of rainfall.
Measurement of Rainfall:
For the measurement of rainfall rain gages are used which may be classified as
follows:
i. Non-recording type rain gages.
ii. Self-recording type or automatic recording type rain gages.
i. Non-Recording Type Rain Gages:
As the name indicates these rain gages do not record the rainfall directly but only collect the rain
water which when measured gives the total amount of rainfall at the rain gage station during the
measuring interval. In our country until about 1969, for the measurement of rainfall at the
various rain gage stations, the non- recording type rain gage extensively used by the Indian
Meteorological Department (IMD) is the Symon’s rain gage.
However, since 1969 the Indian Meteorological Department (IMD) started using another non-
recording type rain gage which has been standardized by the Indian Standard Institution (ISl)
in collaboration with the Indian Meteorological Department (IMD).
The Indian Standard, IS: 5225- 1992 gives details of this newly standardized non-recording type
rain gage, known as Standard non-recording type rain gage, which is an improvement over the
Symon’s rain gage. Both the Symon’s rain gage and the Standard non-recording type rain gage
are described below.
Symon’s Rain Gage:
The Symon’s rain gage consists of a cylindrical metal case of internal diameter 127 mm (5 inches)
with its Base enlarged to 203.2 mm (8 inches) diameter. At the top of the case a funnel is fixed
which is provided with a brass rim measuring exactly 127 mm (5 inches) inside diameter.
The funnel shank is inserted in a glass bottle placed inside the case. The case of the rain gage is
fixed in masonry or concrete foundation block 600 mm×600 mm ×600 mm which is sunk into

9. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
the ground such that the funnel rim is exactly 304.8 mm (12 inches) above the ground level. The
rain water enters the bottle through the funnel and gets collected in the bottle.
Standard Non-Recording type Rain Gage: The Standard non-recording type rain gage
consists of a collector with a gun metal or aluminium alloy rim, and a receiver consisting of a
base and a bottle. The collector is exposed above ground level while the receiver is fixed partially
below ground level. Both the collector and the base are made of fibre glass reinforced polyester.
The collector has a deep-set funnel and the complete rain gage has a slight taper with the
narrower portion at the top.
The collector and the base are locked to each other by means of two complementary locking
rings, one fixed inside the collector at its lower end and the other fixed at the top end of the base.
The collectors having apertures of either 100 cm
2
or 200 cm
2
area at the top are used. The
collector intercepts the rainfall and the rain water entering the collector is led through the funnel
into the bottle where it is stored.
A rain gage of measuring capacity of 200 mm or rainfall with collector of aperture area 200
cm
2
and bottle of capacity 4 litres is most widely used. However, if a bottle of 2 litres capacity is
used then an additional cylinder is used to collect the overflow, if any, from the bottle.
The rain water collected in the bottle of a non-recording type rain gage is measured with the help
of a standard measuring glass supplied with each rain gage which indicates the millimetres of
rain that has fallen at the rain gage station. The Indian Standard, IS: 4849-1992 gives detailed
specifications for the measuring glasses to be used with the rain gages having collectors of
different aperture areas.
At each rain gage station, the observations for the rainfall are taken daily at 8.30 a.m. (1ST).
However, if the rainfall is likely to exceed the capacity of the bottle then a few intermediate
observations are also taken. The sum of the observations taken will represent the total rainfall
of the past 24 hours of the day on which the observation at 8.30 a.m. is taken.
ii. Self-Recording Type or Automatic Recording Type Rain Gages:
These rain gages automatically record the intensity of rainfall and the time of its occurrence in
the form of a pen trace on a clock driven chart, from which the total amount of rainfall for the
desired duration may also be determined. These gages may be operated over extended period
without attention and the recorded observations may be collected from the gages each time after
a certain fixed duration.

10. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
The most widely used self-recording type rain gages are as given below:
(a) Tipping bucket rain gage
(b) Weighing type rain gage
(c) Float type rain gage
Each of these rain gages are described below:
(a) Tipping Bucket Rain Gage:
It consists of a 300 mm diameter sharp edged receiver at the end of which a funnel is provided.
The rain water enters the receiver and the funnel conducts it to a pair of small buckets pivoted
just below the funnel. The buckets are so designed that when 0.25 mm of rainfall collects in one
bucket, it tips and empties its water into the storage tank below and at the same time the other
bucket is brought under the funnel.
The tipping of the bucket actuates an electric circuit which causes a pen to make a mark on a
record sheet mounted on a clock driven revolving drum. Since each mark on the record sheet
corresponds to 0.25 mm of rainfall, by counting the same the intensity of rainfall may be
determined.
The total rainfall as determined from the records at the end of the day may also be checked by
measuring the rain water collected in the storage tank in the same manner as in the case of a
non-recording type rain gage.
The movement of the tipping bucket may also be transmitted electronically to the control room
so that the rainfall is directly recorded in the control room without any manual assistance. As
such these rain gages are quite suitable for the measurement of rainfall in hilly and other
inaccessible areas where manual recording of rainfall may not be possible.
(b) Weighing Type Ruin Gage:
In this rain gage the rain water passes through a funnel into a bucket, which is supported on the
weighing platform of a spring or lever balance. The increase in the weight of the bucket due to
the addition of the rain water causes the platform to move.

11. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
The movement of the platform is transmitted through a system of links and levers to a pen which
makes trace of accumulated amounts of rainfall on a suitably graduated chart wrapped around
a clock driven revolving drum.
The mechanism is arranged to reverse the travel of the pen after a certain amount of rainfall (say
150 mm) has accumulated and reverse again after another equal amount, so that the gage may
operate unattended for a week at a time, except in regions of very intense rainfall, where the
total rainfall may exceed the capacity of gage (usually 300 mm). The rainfall record produced by
this gage is in the form of a mass curve of rainfall in which as shown in Fig. 3.6. The total rainfall
is plotted with respect to time. The slope of the curve gives the intensity of the rainfall.
(c) Float Type Rain Gage:
In this rain gage the rain water after passing through the funnel enters a chamber which contains
a float. As the level of the rain water collected in the float chamber rises, the float moves up,
which actuates a pen connected to it through a connecting rod. The pen makes a trace of
accumulated amounts of rainfall on a suitably graduated chart wrapped round a clock driven
revolving drum.
Thus, in the case of this gage also the rainfall record is in the form of a mass curve of rainfall,
which is same as in the case of weighing type rain gage. When the float chamber gets completely
filled it is automatically emptied by means of a syphon.
As such this type of rain gage is also known as natural-syphon type rain gage. In our country this
type of rain gage is adopted by the Indian Meteorological Department (IMD) as the Standard
self-recording-type rain gage.
The self-recording type rain gages are installed on a concrete or masonry platform 450 mm
square with the rim of the receiver being kept at a height of exactly 750 mm above the ground
level. The self-recording type rain gage is generally used in conjunction with a non-recording
type rain gage so that the readings of the self-recording type rain gage can be checked and if
necessary adjusted.
In addition to the above noted gages, in the modern times the use of radar as an aid in the
measurement of rainfall is being made. However, the main use of radar is in the determination
of the areal extent, orientation and movement of rain storms.
➢ Hydrograph is a graph that shows the variation in the discharge or flow of water
in a river or stream over a period of time. It provides a visual representation of how

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Assistant Professor (Department of Civil Engineering)
the flow of water changes in response to rainfall, snowmelt, or other hydrological
events. Hydrographs are valuable tools for hydrologists and water resource managers
to understand and analyze the behavior of rivers and streams, predict flooding, and
manage water resources.

Key components of a hydrograph include:
(I) Time Axis: The horizontal axis of the graph represents time, typically in hours, days, or
other time units.
(II) Discharge Axis: The vertical axis of the graph represents the discharge or flow rate of
the river or stream, usually measured in cubic meters per second (CMS) or cubic feet per
second (CFS).

The main components of a hydrograph include:
1) Time Axis: The horizontal axis of the hydrograph represents time, typically in hours, days, or other
time units. It shows the time period over which the variations in river discharge are being observed.

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Assistant Professor (Department of Civil Engineering)
2) Discharge Axis: The vertical axis of the hydrograph represents the discharge or flow rate of the river
or stream, usually measured in cubic meters per second (CMS) or cubic feet per second (CFS). It
indicates the amount of water passing a specific point in the river per unit of time.
3) Baseflow: Baseflow is the continuous flow of water in a river or stream that comes from groundwater
sources. It represents the portion of flow that remains relatively constant even during dry periods.
Baseflow contributes to the gradually sloping portion of the hydrograph before a storm event.
4) Rising Limb: The rising limb of the hydrograph is the portion that shows the increase in river discharge
in response to a hydrological event, such as rainfall or snowmelt. It indicates how quickly the river
responds to the input of water.
5) Peak Flow: The peak flow is the highest point on the hydrograph's rising limb. It represents the
maximum discharge reached during the event and is often associated with the peak of the rainfall
intensity or snowmelt.
6) Lag Time: Lag time is the time difference between the peak of the hydrological input (rainfall or
snowmelt) and the corresponding peak discharge on the hydrograph. It indicates how long it takes for
water to travel from the point of maximum input to the measurement point.
7) Recession Limb: The recession limb of the hydrograph shows the decrease in river discharge after the
peak flow has been reached. It reflects the gradual reduction in water input and indicates the time it
takes for the watershed to drain after the hydrological event.
8) Recession Constant: The recession constant is a measure of how quickly the discharge returns to its
baseflow level after the peak flow has occurred. A steeper recession constant indicates faster drainage.
9) Time to Peak: This is the time interval between the start of the hydrological input (rainfall or
snowmelt) and the occurrence of the peak flow on the hydrograph.
10) Flow Duration: Flow duration is the total time period during which the river discharge remains above
a certain flow rate. It helps in understanding how long the river experiences higher flows.
Direct Runoff and Baseflow
Direct Runoff: Direct runoff, also known as surface runoff, is the portion of precipitation that flows over the
land surface and eventually enters rivers, streams, and other water bodies. It occurs when the rate of rainfall
exceeds the rate at which water can infiltrate into the ground or be stored within depressions. Direct runoff
contributes to river discharge and streamflow, and it plays a significant role in the hydrological cycle.
Key factors influencing direct runoff include:

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Assistant Professor (Department of Civil Engineering)
1) Rainfall Intensity: Higher rainfall intensity increases the likelihood of direct runoff, as the ground
may not be able to absorb water fast enough.
2) Soil Properties: Soil type, compaction, and permeability influence how quickly water can infiltrate
into the ground.
3) Land Cover: Impervious surfaces like roads, pavements, and buildings prevent water from infiltrating
and promote direct runoff.
4) Slope: Steeper slopes tend to generate more direct runoff, as water can quickly flow downhill.
5) Antecedent Moisture Conditions: The amount of moisture already present in the soil before a rainfall
event can affect how much water can infiltrate versus becoming direct runoff.
Baseflow: Baseflow, also referred to as groundwater flow, is the portion of streamflow that originates from
subsurface sources such as groundwater discharge into a river or stream. It represents the sustained flow in a
river during dry periods when there is no recent precipitation or snowmelt. Baseflow comes from water that
has infiltrated into the ground and then gradually flows into the river over time.
Baseflow contributes to the gradual, relatively consistent flow of a river and is influenced by factors such as:
1) Groundwater Levels: The elevation of the water table or the level of the groundwater reservoir affects
the amount of baseflow entering the river.
2) Permeability of Aquifers: The ease with which water can move through underground rock or sediment
layers affects how quickly baseflow can reach the river.
3) Precipitation Patterns: Longer periods of rainfall or snowmelt can lead to increased groundwater
recharge and subsequently higher baseflow.
4) Geology: The type of rocks and sediments present in the aquifer influences the storage and movement
of groundwater.
Baseflow is important for maintaining streamflow during dry periods, supporting aquatic ecosystems, and
sustaining water resources. It's often characterized by its relatively constant flow compared to the more variable
and episodic nature of direct runoff.
Presentation of Rainfall Data
The commonly used techniques for presentation of rainfall data, suitable for
interpretation and analysis in different aspects are given:
1. Mass curve
2. Hyetograph

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Assistant Professor (Department of Civil Engineering)
3. Point rainfall.
MASS CURVE:
It's a graph of cumulative rainfall against time (after it's been arranged in chronological
order). Rainfall is measured in terms of mass-curve by float type and weighing type
rain gauges, but exclusively in terms of depth by symon type rain gauges. To prepare
the mass-curve, the rainfall data obtained by non -recording type rain gauge is
processed (i.e. in the form of cumulative w.r.t. time). The following information about
the rainfall at a specific location can be extracted using mass curves.
1. It furnishes the information on duration of occurring rainfall and its Magnitude.
2. It provides the information on starting and end times of the given rainfall.
3. Of a given storm, it enables to determine the rainfall intensity at different time
intervals. The intensity of rainfall is the slope of mass curve. A mass curve is shown in
figure


HYETOGRAPH:
It is the plot of rainfall intensity and time interval (Fig. 3.9). For development of
hyetograph, the intensity data of rainfall is extracted from the mass curve. The
presentation of hyetograph is in the form of bar diagram. The range of time interval of
hyetograph depends on the purpose. Normally, for urban drainage problems, a small
duration is used whereas for computation of flood-flows in large catchments, the 6-h

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Assistant Professor (Department of Civil Engineering)
time interval is commonly used. A hyetograph furnishes following information about
the rainfall
1. Total depth of rainfall, which is determined by computing the area of hyetograph
2. Effective rainfall depth, which is computed by converting the hyetograph into effective
rainfall hyetograph. The ERH is derived by deducting average loss of rain water during
rainfall (ie index)
3. Predicts the extreme floods by determining the design storm.
4. A hyetograph also provides the information on duration and depth of effective rainfall, both.


POINT RAINFALL:
Station rainfall is another name for it. The point rainfall is the amount of rain that falls
at a certain gauging station. Depending on the necessity, it is expressed in daily,
weekly, monthly, seasonal, or annual terms. Rainfall data is also presented in the form
of a bar graph (i.e. magnitude Vs chronological time). Because there are significant
differences in point rainfall during the given time interval of the bar diagram, this type
of display of point rainfall is incorrect. The moving average plot, in which mean
precipitations of three or five consecutive time intervals are shown through the
midpoints of the selected time intervals, helps correct this variation.
Consistency of rainfall record:
Consistency is assessed for the purpose of testing and correcting existing rainfall data,
particularly when conditions related to the rain guard station have changed

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dramatically, leading the data to become inconsistent. In general, the following factors
contribute to the inconsistency of rainfall data:
1. Because of a shift in the position of the train gauge station
2. Observational error's reputation
3. Whether or if there is a topographical inaccuracy or change


Intensity-Duration-Frequency Relationship
• Intensity-Duration Relationship
Rainfall intensity is the inverse function of rainfall duration i.e. for longer storm
duration, the rainfall intensity is less and vice-versa. It is general phenomena, that the
rainfall intensity is not same throughout the storm period but varies time to time. For
a shorter time, its value can be much higher than the mean rainfall intensity of the
storm. For finding the rainfall intensity at any time ‘t’ during the storm, Richard (1944)
developed a relationship between intensity and duration of rainfall, given as under
I/J = (T + K) / (t + K)

in which, I and I are the rainfall intensity for any time I and 7. Respectively. T is the
storm duration and K is the constant. The value of K is generally taken as 1, except for
extreme events. Thus, after substituting the value of ‘K’ in the equation 3.19, we have,

18. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)
This equation can be used for computing the rainfall intensity for any time during
rainfall, if storm duration and mean rainfall intensity of the given storm are known.
Tejwani et. Al. (1975) developed graphical relationship between one rainfall intensity
and other durations rainfall intensities.
Intensity-Duration-Frequency Relationship
It is general characteristics of the rainfall that as the rainfall duration increases, the
intensity decreases and vice-versa. On contrast, the rainfall intensity increases with
increase in return period and vice-versa. The relationship amongst intensity, duration
and frequency of rainfall is given as under.



Where,
I=Average rainfall intensity, cm/h.
t=duration of rainfall, hour
T= recurrence interval, years.
K, a, b and d = constants, depend on the geographical locations of the area.
The value of recurrence interval can be computed by using the following formula
T = 1/p

For other places, they have also determined the values of K, a, b and d.

19. Prepared by – Er. Chandan Kumar
Assistant Professor (Department of Civil Engineering)


Depth-Area-Duration Relationship
Depth-Area Relationship
The depth-area relationship is very important for determining the variations in rainfall
depth with respect to the variation in area of watershed during a given storm. In this
regard, Horton developed a mathematical mo del for predicting the average rainfall
depth, based on the highest amount of rainfall and area of watershed. The model is
given as under:
P= Po e
-KA^n

Where,
P= average rainfall depth, cm
Po= highest rainfall depth occurred at the storm centre, cm
A = area of the watershed, km²
K and n= constants for given region. Dhar Bhattacharya (1975) determined the values
of K and for North
India on the basis of 42 severe most storms. The values are:
S No. Duration K n
1. One day 8.256 x 10
-4
6.614 x 10
-1

2. Two days 9.877 × 10
-4
6.306 x 10
-1

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Assistant Professor (Department of Civil Engineering)
3. Three days 1.745 × 10
-3
5.961 × 10
-1

Depth-Area-Duration Relationship
The depth-area-duration connection is particularly important for determining the
rainfall depth throughout storm duration over the watershed owing to any storm. From
the storm's centre to its perimeter, this relationship is derived by showing the steadily
decreasing average rainfall depth over a progressively greater watershed area. In other
words, the depth-area-duration curve is calculated by graphing the average rainfall
depth against the equivalent area up to the enclosing isohyets. The following is a
description of the derivation procedure:
1. Find out 1-day, 2-days. 3-days up to 5 consecutive days maximum average rainfall.
2. Prepare the isohyetal map of maximum average rainfall for 1- to 5-days rainfall durations,
separately for each day.
3. Divide each isohyetal map into different zones, representing the centre of main storm.
4. Determine the enclosed area by each isohyet. It is determined by using the planimeter or
any suitable method, Measurement of area is started from centre of the storm of each zone.
5. Calculate the rainfall volume between two isohyets. It is determined by multiplying the
enclosed area between them and mean of the two adjacent isohyet value.
6. Compute the total rainfall volume by adding the incremental volume to the previous
accumulated rainfall volume.
7. Find out the average rainfall depth over the area, which is obtained by dividing the total
rainfall volume computed in step (6) with the total area of isohyetal map.
8. Determine the average rainfall depth for each zone by repeating the procedure up to step 7
and then combine the enclosed area by the common isohyets.
9. Plot the values of highest average rainfall depth and corresponding area and join all the
points by smooth curve. The obtained curve is the depth area-duration curve for maximum
rainfall of a particular period.

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Assistant Professor (Department of Civil Engineering)

Maximum Depth -Area-Duration Curves
Many hydrological studies involving the estimation of severe floods require data on the
maximum quantity of rainfall over various time periods and over various region sizes.
DAD analysis is the creation of a link between maximum depth -area-duration for a
region and is a key part of hydro-meteorological research. For further information on
DAD analysis, see References 2 and 9. The storm's isohyetal maps and mass curves
are compiled. A depth-area curve for a specific storm duration is created. Following
that, several durations and the greatest depth of rainfall in these durations are
observed based on a study of the rainfall mass curve. During a given duration D, the
maximum depth-area curve is calculated by assuming that the area distribution of
rainfall for shorter durations is similar to the whole storm. The technique is then
repeated for other storms, yielding the maximum depth -area envelope curve for
duration D. DAD curves are the name for these curves.
The preparation of DAD curves necessitates a significant amount of computer effort as
well as regional meteorological and topographical data. Detailed information about past
strong storms is required. The development of design storms for use in computing the
design flood in the hydrological design of big buildings such as dams requires DAD
curves.

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Assistant Professor (Department of Civil Engineering)
Estimation of messing rainfall data
Estimating missing rainfall data is a common task in meteorology and hydrology, especially when
dealing with historical or incomplete datasets. There are several methods and techniques you can
use to estimate missing rainfall data, depending on the available information and the specific
characteristics of the dataset. Here are some common approaches:
1. Interpolation:
• Spatial Interpolation: If you have rainfall data from nearby weather stations or rain gauges,
you can use spatial interpolation techniques like Inverse Distance Weighting (IDW), Kriging,
or Thiessen polygons to estimate missing values at a specific location.
• Temporal Interpolation: For missing values within a time series dataset, you can use
temporal interpolation methods like linear interpolation, spline interpolation, or time-series
modeling (e.g., ARIMA) to fill in the gaps.
2. Climatology:
• You can use historical climate data to estimate missing rainfall values. This involves calculating
long-term averages (climatology) for the specific location and time of year. For example, you
might use the average rainfall for the month of June over the past 30 years to estimate a
missing value for June of the current year.
3. Neighboring Values:
• Sometimes, using neighboring observed values (both spatially and temporally) can provide a
reasonable estimate for missing data. For example, you can take the average of the rainfall
values from the days before and after the missing day.
4. Machine Learning Models:
• You can use machine learning models like regression, neural networks, or decision trees to
predict missing values based on other available meteorological variables, such as temperature,
humidity, and atmospheric pressure.
5. Satellite Data:
• Satellite-based rainfall estimation, such as data from remote sensing instruments like TRMM
(Tropical Rainfall Measuring Mission) or GPM (Global Precipitation Measurement), can be used
to fill in missing data, especially in regions with limited ground-based observations.
6. Rainfall-Radar Data Fusion:
• In areas with radar coverage, you can use radar data in conjunction with rain gauge data to
estimate missing rainfall values. Radar data can provide high-resolution information about
rainfall intensity and distribution.
7. Data Imputation Techniques:
• Statistical imputation techniques like mean imputation, median imputation, or multiple
imputation can be used to estimate missing values based on statistical properties of the
available data.
8. Hydrological Models:
• In some cases, hydrological models can be used to estimate missing rainfall data by simulating
how precipitation interacts with the land surface, rivers, and other hydrological features.

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Assistant Professor (Department of Civil Engineering)
It's essential to consider the quality and reliability of the available data when choosing an estimation
method. Additionally, the choice of method may vary depending on the specific requirements of your
analysis or application. It's often a good practice to document the method used for estimating
missing data and the sources of data used in your analysis to ensure transparency and reproducibility.

Test for consistency of record of rainfall data
Testing the consistency of rainfall data records is crucial to ensure the reliability and accuracy of the
data. Here are some specific tests and checks you can perform to assess the consistency of rainfall
data records:
1. Range Checks:
• Verify that rainfall values fall within plausible ranges. Extreme or unrealistic values can indicate
errors. For example, check for negative rainfall values or extremely high values that may be
outliers.
2. Temporal Consistency:
• Check that the temporal aspects of the data are consistent. Ensure that time intervals between
data points are uniform and that there are no gaps or overlaps in the time series.
3. Data Formatting:
• Ensure that the data is formatted consistently, with the same units and time scales used
throughout the dataset.
4. Duplicate Data:
• Identify and remove any duplicate entries in the dataset, as they can skew analyses and create
inconsistencies.
5. Missing Data:
• Check for missing data points. Missing data can be inconsistent with a continuous rainfall
record. Decide on an appropriate method for handling missing data, such as interpolation or
imputation.
6. Data Type Consistency:
• Ensure that the data types used for recording rainfall values (e.g., float, integer) are consistent
across the dataset.
7. Spatial Consistency:
• If you have rainfall data from multiple stations or locations, check for spatial consistency. Verify
that rainfall amounts make sense concerning the geographic distribution and topography of
the area.
8. Quality Control Flags:
• Many rainfall datasets include quality control flags that indicate the reliability of each data
point. Check these flags to identify data points that have been flagged as potentially
erroneous.
9. Extreme Event Analysis:

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• Analyze extreme rainfall events to check for consistency. Extreme rainfall events should be
consistent with the historical climate and rainfall patterns for the region.
10. Comparison with Nearby Stations:
• If you have data from multiple nearby stations, compare their records to identify any
significant discrepancies. Inconsistent readings between neighboring stations may indicate
data quality issues.
11. Temporal Trends:
• Examine long-term temporal trends in the data. Sudden, unexplained shifts or changes in the
rainfall pattern can be indicative of data inconsistencies.
12. Visual Inspection:
• Plot the data graphically to visually identify any irregularities, outliers, or patterns that may
suggest inconsistencies.
13. Rainfall Intensity-Duration-Frequency (IDF) Analysis:
• Perform IDF analysis to check if the rainfall data conforms to expected intensity-duration-
frequency relationships for the region. Inconsistent data may not align with established IDF
curves.
14. Hydrological Modeling:
• Use hydrological models to simulate runoff or streamflow based on the rainfall data.
Inconsistent data may lead to unrealistic model results.
15. Data Documentation:
• Ensure that the metadata and documentation for the dataset are complete and accurate. This
includes information about measurement instruments, calibration, and any corrections
applied to the data.
Consistency checks for rainfall data should be part of routine data quality assurance practices in
meteorology and hydrology. Addressing inconsistencies and errors in the data is essential for reliable
hydrological modeling, flood forecasting, and climate studies.

Analysis of rainfall data
Analyzing rainfall data is a crucial part of meteorological, hydrological, and environmental studies. It helps in
understanding climate patterns, water resource management, flood prediction, and more. Here are the key steps
and methods for analyzing rainfall data:
1. Data Collection and Preprocessing:
• Collect historical rainfall data from reliable sources, such as weather stations, satellites, or rain gauges.
• Preprocess the data by cleaning, formatting, and validating it. Address missing values, outliers, and
data inconsistencies.
2. Descriptive Statistics:
• Calculate basic statistics to describe the data's central tendency, variability, and distribution. Common
statistics include mean, median, standard deviation, skewness, and kurtosis.
•

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3. Time Series Analysis:
• Plot the rainfall data as a time series to visualize temporal patterns.
• Analyze long-term trends and seasonality in the data using techniques like moving averages,
decomposition, and autocorrelation analysis.
4. Frequency Analysis:
• Conduct frequency analysis to examine rainfall events and their characteristics.
• Calculate rainfall return periods and intensity-duration-frequency (IDF) curves to assess the likelihood
of extreme events.
5. Spatial Analysis:
• If you have data from multiple locations or weather stations, perform spatial analysis to understand
spatial variability in rainfall patterns. Techniques like spatial interpolation can help generate rainfall
maps.
6. Drought and Wet Spell Analysis:
• Identify droughts and wet spells using methods like the Standardized Precipitation Index (SPI) or the
Palmer Drought Severity Index (PDSI). These indices quantify rainfall anomalies over time.
7. Hydrological Modeling:
• Use rainfall data as input for hydrological models to simulate river discharge, streamflow, and
groundwater recharge. Models like the Soil and Water Assessment Tool (SWAT) and Hydrologic
Engineering Center-Hydrologic Modeling System (HEC-HMS) are commonly used.
8. Extreme Event Analysis:
• Analyze extreme rainfall events to assess their impact on local hydrology, infrastructure, and the
environment. This is vital for flood risk assessment.
9. Temporal Trends:
• Detect and analyze long-term temporal trends in rainfall data, such as increasing or decreasing rainfall
patterns, which can have significant implications for water resource management and climate change
studies.
10. Statistical Analysis:
• Use statistical tests like Mann-Kendall trend tests or t-tests to determine if there are significant changes
or differences in rainfall patterns over time or between different locations.
11. Climate Indices:
• Calculate climate indices like the Southern Oscillation Index (SOI) or the North Atlantic Oscillation
(NAO) to assess the influence of large-scale climate phenomena on regional rainfall patterns.
12. Visualization:
• Create various plots and charts, such as time series plots, bar graphs, histograms, and spatial maps, to
visualize and communicate your findings effectively.
13. Risk Assessment:
• Assess the risk associated with different levels of rainfall, especially concerning flood risk. Develop
flood risk maps and models based on historical rainfall data.
14. Statistical Modeling:
• Build statistical models, such as regression models, to predict rainfall based on various meteorological
variables, such as temperature, humidity, and atmospheric pressure.
15. Reporting and Interpretation:
• Compile your findings into reports, presentations, or dashboards to communicate results to
stakeholders, policymakers, or the public.
Rainfall data analysis is a complex and multidisciplinary task, often involving expertise in meteorology,
hydrology, statistics, and data science. The specific methods and techniques you use will depend on your
research objectives and the characteristics of the dataset you are analyzing.

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Intensity depth area relationship
The Intensity-Duration-Frequency (IDF) relationship is a fundamental concept in hydrology and
rainfall analysis. It provides a mathematical relationship between the intensity (rate) of rainfall, the
duration of rainfall events, and their frequency of occurrence. The IDF relationship helps in estimating
the expected rainfall intensity for a given duration and return period (frequency) at a specific location.
Here's how the IDF relationship works:
1. Intensity (I): This represents the rainfall rate, typically measured in millimeters per hour (mm/hr) or
inches per hour (in/hr). It is the amount of rainfall that falls in a given time period, usually per hour.
2. Duration (D): Duration refers to the time period over which rainfall occurs. It can be expressed in
hours, minutes, or any other suitable time unit.
3. Frequency (F): Frequency represents the probability or likelihood of a rainfall event of a certain
intensity and duration occurring within a specified time frame, often expressed as a return period
(e.g., 2-year, 10-year, 100-year).
The IDF relationship is typically expressed in the form of an equation or a set of curves. The equation
relates intensity (I) to duration (D) and frequency (F) as follows:
I = C / (D^k)
Where:
• I is the rainfall intensity (mm/hr or in/hr).
• C is a constant specific to a location and return period.
• D is the duration of the rainfall event (hours).
• k is an empirical exponent that varies with location and return period.
To develop IDF curves for a specific location, hydrologists and meteorologists use historical rainfall
data to calculate the values of C and k for various durations and return periods. These values are then
used to estimate the intensity of rainfall events for different combinations of duration and return
period.
IDF curves are often presented graphically, showing how rainfall intensity varies with duration for
different return periods. These curves are valuable tools for engineers, urban planners, and
hydrologists when designing infrastructure like stormwater drainage systems, flood control
measures, and water resource management strategies.
The IDF relationship is an essential component in assessing the risk of flooding, designing drainage
systems, and making informed decisions regarding land use and development in areas susceptible
to heavy rainfall events. It helps ensure that infrastructure can handle the expected rainfall conditions
for a given location and return period.

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