A presentation engineering hydrology

Hydrological Cycle
and Measurement of
precipitation
Prepared by
Shashikant Verma
(Lecturer)
Department of Civil Engineering
Pietech,Raipur (C.G.)

Contents
• Introduction
• Global water resource
• Hydrological cycle (water cycle) and its components
• Common hydrological units
• Global water budget
• Measurement of rainfall
• References
28-08-2017 2Dept.of Civil Engg,Pietech,Raipur

Outcome of the lession
• To know the principles behind the sources of water.
• To study about hydrologic cycle in general aspect.
• To further understand the hydrological components
like evaporation,precipitation,infiltration, runoff,
rainfall and sub surface flow.

Introduction
• Earth contains enormous amounts of water in the form
of reservoirs.
• Water exists in the atmosphere, lithosphere, hydrosphere
and biosphere. Among all these segments, water masses are
in continuous circulation. It is in general, called as the
hydrologic cycle. It is also called as the World’s Great Water
Cycle, since it is the driving wheel for all the
movements of available water resources on the planet earth.

Global Water Resources

Types/Categories of water uses
Domestic water demand
Public water use
Commercial water use
Industrial water use
Irrigation water demand
Losses and wastes
Global Water Usage

Global hydrological cycle
• Most of the Earth’s water masses reside in the oceans
Continental water makes up about 3.5 percent of the
Earth’s water. About three-quarters of this amount (29
million cubic kilometres) is present as polar ice caps and
glaciers. About 5.3 million cubic kilometres as deep
groundwater.
• Thus, only the remaining fraction can take part in the
water exchange between the oceans, the atmosphere,
and the continents
28-08-2017 Dept.of Civil Engg,Pietech,Raipur 7

Global hydrological cycle
• This remaining part includes shallow groundwater
and soil moisture, water in lakes, reservoirs, and
swamps, water storage in river channels, biosphere
water.
• The amount in the atmosphere is only 0.013
million cubic kilometres But it plays a very crucial
role in the global water cycle.

Hydrologicalcycle (Water cycle)
• Water never leaves the Earth. It is constantly being cycled
through the atmosphere, ocean, and land. This process, known
as the water cycle, is driven by energy from the sun.
• The hydrologic cycle has a profound effect upon climate
prediction
• Water is vital so we must understand where to find water
hydrological-cycle supplies water through the Earth.

Hydrological cycle

Evaporation
Evapotranspiration
Condensation
Précipitation
Interception
Infiltration
Percolation
Runoff
Storage
Component of Hydrological cycle

Component of Hydrological Cycle
• Evaporation: Heat from the sun starts the hydrologic
cycle causing water into water vapour that is held in the air
of the atmosphere.
• Transpiration occurs when plants take in water through
the roots and release it through the leaves, a process that can
clean water by removing contaminants and pollution.
• Evapotranspiration is water evaporating from the ground
and transpiration plants. Evapo-transpiration is also the way
water vapour re-enters the atmosphere.

• About 90% of atmospheric water comes from
evaporation, while the remaining 10% is from
transpiration
• Water is evaporated from lakes, streams, oceans, and
plants. In addition, water is released by animals'
breathing and perspiration.

Evapotranspiration:-is water
evaporating from the
ground and transpiration by
plants.
• Evapotranspiration is also
the way water vapour re-
enters the atmosphere.

Condensation:
As water (in the form of gas)
rises higher in the atmosphere, it
starts to cool and become a
liquid again. This process is
called condensation. When a
large amount of water vapour
condense, it results in the
formation of clouds.

• Condensation can be a form of fog, dew, and clouds
• Fog: Fog forms when air near the surface is cold and
nearly saturated with water. Now when water from the
ground evaporates, it condenses immediately forming tiny
water droplets that create a low-lying cloud we call fog.
• Dew: Dew forms at night when air becomes saturated
with water vapour. When this saturated air comes in
contact with plants or other objects it condenses, leaving
tiny water droplets behind on the object.

Clouds: When the air containing water vapour is heated
by the sun, it rises into the atmosphere by convection.
The water vapour in the air is then cooled by the colder
air higher in the atmosphere causing the relative
humidity to increase. As the relative humidity increases,
the air eventually becomes saturated. The water vapour
then condenses into tiny water droplets around particles
of dust or salt in the air. These tiny water droplets make
up clouds.

When the water in the clouds gets too heavy, the
water falls back to the earth is called precipitation.
Types of precipitation
Drizzle
Rain
Freezing
Sleet
Snow
Hail
Precipitation

Types of precipitation

Interception:
refers to precipitation that
does not reach the soil,
but is instead intercepted
by the leaves and
branches of the plants and
forest floor.

Infiltration: Some precipitation
seeps into the groundwater and
is stored in layers of rock below
the surface of the Earth.
This water stays there for
varying amounts of time. Some
water may evaporate into the
hydrologic cycle within days,
while other water will stay in
the ground for centuries or
more.

This process of precipitation seeping into the
groundwater is called infiltration.
Groundwater percolation: It is a part of
infiltrated water that percolates into deeper strata
and become part of ground water.

• If the terrain is
sloping, infiltrated
water starts to flow
under gravity. This
flow is termed as
Subsurface flow.
• It can be further
categorized into.

Interflow: Shallow ground water flow-joins stream
within few days of rain.
Base-flow: Deeper ground water flows-becomes part of
ground water.
Subsurface flow: incorporates movement of water within
the earth either within the recharge zone or aquifier.After
infiltration subsurface water may return to the surface or
eventually seep into the ocean.

On hard or frozen ground, most of the precipitation is unable to
seep below ground. This precipitation then flows down slopes and
hills, eventually stopping in rivers, lakes, streams, and oceans Some
of this water will then evaporate and rejoin the hydrologic cycle
while other water will remain in body of water.
This process of water travelling over the ground and collecting in
a body of water is called Surface runoff.

Storage: Natural lakes, man-made reservoir or ground
water.
Lake Geneva or Lake Léman is a lake in
Switzerland and France.

Table:Typical Resident time of water found various reservoirs

Common Hydrological Units
• Precipitation - inches (or) mm (or) cm
• Runoff - inches (or) mm (or) cm
• Runoff Volume - acre feet (or) cubic feet
• Runoff Rate - Cubic feet per second
• Evaporation / Interception - inches (or) cm.
• Infiltration - inches (or) cm / hour
• Storage - cubic feet, acre feet

Water Budget
Water balance equation in its most fundamental form is given by
Where, P = precipitation, E = evaporation = runoff and
∆S = change in storage.

Measurement of Rainfall

Introduction
• Rainfall is expressed in terms of the depth to which
rainwater would stand on an area, if all the rain were
collected on it.
• Thus, 1cm of rainfall over a catchment area of 1km2
represents a volume of water equal to 104m3.

The precipitation is
collected and
measured in a Rain
gauge. In India, the
rain gauges commonly
used are Symons's
gauge.
Rain gauge

Average Rainfall over an area
To convert the point rainfall values measured by various
rain gauge stations into an average value over a
catchment, following methods are used.
Arithmetic Mean Method
Thiessen Polygon Method
Isohytel Method

Arithmetic Mean Method

If there are small variations in the rainfall values
measured by the stations, this method can be used.
If P1, P2, . . . , Pn are the rainfall values obtained from n
rain gauge stations within a catchment, then the average
precipitation is given by
1 2 1.....
n
i
n i
P
P P P
P
n n
  
 


Thiessen Polygon Method

In this method, the rainfall recorded at each station is
given a weightage on the basis of an area closest to that
station.(1.) Draw the catchment area to a scale and mark
the rain gauge stations on it.

2. Join each station by straight line to create a
triangulated network.

3. Draw perpendicular bisectors on each sides of each
triangles.Extend the bisectors to meet the other
bisectors and the catchment boundary.

4. The polygons formed by the perpendicular bisectors
(and part of catchment boundary) are the influence
areas of each stations.

If there are n number of rain gauge stations in and
around the catchment and if A1,A2, . . . ,An are the
respective influence areas of Thiessen Polygon, then
the average rainfall is given by

Isohytel Method

Isohyet: An Isohyet is a line joining points of equal
rainfall magnitude.
If P1, P2, . . . , Pn are the values of isohyets and if A1,A2, .
. . ,An−1 are the inter-Isohyet area respectively, then the
mean precipitation over the catchment is given by

Example
The isohyets due to a storm in a catchment were drawn and the
area of the catchment bounded by isohyets were tabulated as
below. Estimate the mean precipitation due to the storm.

Solution

References
• Engineering Hydrology K.Subramanya , Mc Graw Hill Publication.
• Engineering Hydrology Rajesh Shrivastav and Ashu Jain, Mc Graw Hill
Publication.
• Rainfall measurement 150312100947.
• Water cycle and water budget 15031601222.
• The hydrological cycle 170731134722.

A presentation engineering hydrology

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