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THE HYDROLOGIC CYCLE
Technical Report · February 2015
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THE HYDROLOGIC CYCLE
By
Prof. A. Balasubramanian & Prof.D.Nagaraju
Centre for Advanced Studies in Earth Science,
University of Mysore,
Mysore-6
Objectives:
Several processes and factors are involved in driving the global water circulation. This lesson is aimed at
highlighting the world’s water cycle and its major components and contributions. The objectives of this
report are to provide the details about the following aspects:
1. The basic concepts of the hydrologic cycle
2. The components of the cycle
3. The role of individual components
4. The water balance estimation concepts
5. The role of hydrological cycle in promoting the other environmental and biogeochemical cycles
on the earth.
1.0 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. It
is necessary to identify the cyclic and circulatory routes of water masses in all the spheres of the earth.
Several processes and factors are involved in driving the global water circulation. This lesson is aimed to
highlight the world’s water cycle with all its components and contributions.
2.0 The Earth as a Water Planet
The planet earth is called as the water planet. It is due to the fact that the earth is probably the only planet, in
the solar system, which has a huge mass of water. On the planet earth, the primitive life got originated only
from the water mass. Most of the earth’s surface is covered with water. Water exists in all the segments of
the earth. The hydrosphere is the sphere of water on earth. It is a discontinuous layer containing both fresh
water and saline water. The subject hydrology deals with all aspects of the hydrological cycle.
2.1 The World’s Water
The world’s water is not in static from. It is in continuous motion. It also transforms from one state to the
other. The water gets temporarily stored and moved in the form of surface water in rivers, lakes, ponds, ice
caps, and below the surface as groundwater. Oceans form the biggest and largest water reservoirs. All these
water bodies are called as water reservoirs. The duration of stay and storage of water in every reservoir
varies due to varying geological, environmental and other conditions. This is called as the residence time of
water. Water moves from one reservoir to the other. The sun’s radiant energy plays a very significant role in
this movement.
Atmospheric pressure, mind blows, temperature and the amount of water vapour present in the air, all play a
dominant role in these processes. The rates of movement of water and the quantities involved the cyclic
processes are the major aspects involved in the hydrological sciences. There is an endless circulation of
water among all the spheres of the earth. It is popularly known as the hydrologic cycle. It is necessary to
learn about the hydrologic cycle, when we intend analyse the water resources of the region and the world.
3.0 Concept of Hydrologic Cycle
1
Water gets transformed from liquid to solid, solid to liquid, liquid to vapour, vapour to liquid and vapour to
solid states. The sun’s radiation, acceleration due to gravity, ability of the water to flow and several other
properties of water, make this transformation more effective and regular.
The basic input to the world’s water masses comes from precipitation.
Precipitated rain (or) snow falls overland. Processes like infiltration and peroration moves the water down to
the groundwater systems. Some amount of water flows towards the sea as runoff. The surface water
collected in lakes, ponds, swamps, seas and oceans get evaporated into the atmosphere. The vegetation
transpires the water collected from the soil moisture. Evaporated and transported water enters into the
atmosphere as vapour. Collected water vapour gets condensed to form the clouds.
Clouds more towards the land and starts precipitating again. These processes continue. This endless
circulation of water is known as the hydrologic cycle. It is of two kinds as terrestrial hydrologic cycle and
global hydrologic cycle. The terrestrial hydrological cycle is of a special interest as the mechanism of
formation of water resources on a given area of the land, like a river basin or a watershed. The global
hydrological cycle deals with its role on the global climate and other geological and physical processes. It is
obvious that the role of different processes involved in the hydrological cycle and their description have to
depend on the chosen spatial- temporal scales.
3.1 Components of the Hydrologic Cycle
The circulation of water masses seen in all spheres of the earth involves several causative factors and
components. The major components (or) elements of the hydrologic cycle are :
1. Precipitation
2. Evaporation
3. Transpiration
4. Evapotranspiration
5. Surface Runoff
6. Condensation
7. Infiltration
8. Groundwater base flow
9. Sublimation
10. Interception.
3.2 Global hydrologic 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 kilometers) is present as polar ice caps and
glaciers. About 5.3 million cubic kilometers as deep groundwater. Thus, only the remaining fraction can take part
in the water exchange between the oceans, the atmosphere, and the continents. 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.
3.3 Terrestrial hydrologic cycle
The key component of the terrestrial hydrological cycle is generation of river runoff and movement of water
in the river networks. As stated previously, the main land area units may be river basins and watersheds.
They are ideal hydrologic units. The sizes of these areas vary from tens of sw.kms to several thousand square
km. Within these hydrologic units, distinct spatial differences, in topography, climate, geology, structure,
vegetation, soil properties, land use, land cover and other features may occur. The terrestrial hydrological
cycle consider all aspects of the main processes, and factors that control the processes as well.
3.4 The Underlying Factors
2
The following are the major factors involved in controlling the movement of water masses in the hydrologic
cycle:
1. Application of energy that promotes transformation from one state of matter to the other state.
2. Inherent properties of matter, i.e., water.
3. Dimension and geo environmental settings of the reservoirs.
4. Gravity which promotes flow.
5. Air which promotes mobility of water molecules
6. Earths revocation and rotation, which are responsible for climate and weather cycles.
4.0 Condensation
The sun’s radiant energy evaporates water from surface water resources, including seas and oceans. This
water rises upwards as water vapour and reaches the atmosphere. The process of evaporation continues until
the air becomes fully saturated with maximum amount of moisture. It is called as Saturation Humidity. It is
directly proportional to the temperature of the air. It is expressed in grams/ cubic metre of air. At 0 degree C,
humidity of air is 4.874 gm/ cu.m and at 30 degree C it is 30.38 gm/cu.m. Relative humidity is a term used
to denote the ratio of the water vapor content of the air to the water vapor capacity of the air. The term
absolute humidity is used to denote the number of grams of water per cu.m of air. If the air mass reaches the
dew point, then the condensation process begins. Humidity is measured using hygrometers. Through the
process of condensation clouds are formed and moved towards the land frontiers.
Condensation in the atmosphere takes place around small particles of substances present in the air, which
have the affinity for water. These form the hygroscopic nuclei surrounding the condensation nuclei. These
condensed water vapour masses get precipitated the form of water molecules. Precipitation is the process by
which this transformation happens in the atmosphere, under certain specific conditions.
Condensation depends on several factors. The notable areas are:
a) Adiabatic cooling.
b) Mixture of two air masses of different temperatures
c) Contact cooling.
d) Radiational cooling.
Among these, the most important condensation process is the adiabatic cooling which leads to all kinds of
precipitation. Condensation leads to cloud formation. Condensed clouds of smaller sizes may quickly
disappear on a hot day.
Cloud condensation nuclei, which vary in size from 10-5 to 10-1 mm, are required to condense water vapour
at dew point. Typical cloud condensation nuclei are meteoric dust, windblown clay and silt, volcanic
material, sea salt and combustion products. Natural concentrations of cloud condensation nuclei vary from
100 to 300 cm-3, but may locally become 10 to 100 times larger due to human activities.
When a vapour condenses, heat is released. Condensation also leads to the formation of fog. Before
precipitation to happen, the cloud droplets sized from 0.001 to 0.2 mm have to grow to 0.4 to 4 mm in
diameter to reach a fall velocity that exceeds the rate of uplift. Cloud droplets may grow at temperatures
above 0 degree Centigrade. The droplet collision occurs due to differences in fall and lift velocities caused
by the differences in droplet size.
The world’s water vapour is equal to about 2.5 cm thickness, on an average, if it is distributed all over the
surface of the globe.
5.0 Precipitation
Precipitation is the process of transforming the water vapour into a liquid or solid form, depending upon the
temperature of air near the clouds. The term precipitation is a common term. It includes a variety of forms of
precipitation. It includes mist, rain, hail, sleet and snow. The term precipitation and rainfall are always used
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synonymously. Precipitation mainly depends on the water vapour present in the atmosphere. When the air
temperature is well below the freezing point, clouds may form tiny ice crystals.
For precipitation to form, a sequence of four processes must occur:
1) atmosphere must have sufficient water vapour present, which is cooled to dewpoint,
2) condensation of water vapour on cloud condensation nuclei
3) growth of water droplets, and
4) importation of water vapour.
Whenever the water vapor in the air is cooled down below the temperature corresponding to the pressure of
saturated vapor, condensation occurs on dust particles, globules of water, grass, or other foreign objects.
Condensation of moisture out of the atmosphere above the immediate surface of the earth occurs on dust
particles or suspended globules of water and takes the form of fog, cloud, rain, snow or hail. During warm
weather, cyclonic areas are usually accompanied by thunderstorms. A thunderstorm, as the name implies, is
a storm accompanied by lightning and thunder and, usually, precipitation. The condensation which results in
the precipitation of moisture from clouds is caused by what is known as "dynamic cooling," i.e., the cooling
resulting from the consumption of heat in the work of expansion of the rising vapor, as previously explained.
These conditioned are fulfilled in the atmosphere every time during the monsoons. Though there are several
mechanisms active in the atmosphere which cool air, only the process of adiabatic cooling due to vertical
uplift is able to produce precipitation of any significance. Three types of lifting mechanism(of lifting moist
air) to a level where condensation of water vapour to take place are available as convective, cyclonic, and
orographic.
Under the convective process the lifting occurs due to differential heating of a region when the warmer
moist air rises in relation to colder surroundings. Lifting in a cyclonic process occurs due to convergence of
moist air into a low pressure area. Orographic rain occurs due to mechanical lifting of moist air on mountain
slopes.
5.1 The Rainfall:
Rainfall is most common form of precipitation occurring in almost all parts of the world. In tropical regions,
precipitation is expected completely as rainfall. In the polar regions, precipitation is expected to be
completely as snowfall. In mid latitudes, at high altitudinal zones, precipitation occurs as snowfall, sleet and
ice. All these are called as forms of precipitation.
Four conditions are necessary to get sufficient amount of rainfall. They are:
a) A mechanism to produce cooling of the air
b) A mechanism to produce condensation
c) A mechanism to produce growth of cloud droplets
d) A mechanism to produce accumulation of moisture of sufficient intensity to generate rainfall.
The amount of rainfall occurring in a place is measured using a raingauge. A network of rain gauges is
needed to analyse the rainfall of a larger region.
The total rainfall on the earth’s land surfaces amounts to 110,000 km3. It returns to the atmosphere via
evaporation and evapo-transpiration.
5.2 The Snowfall:
The snowfall is another form of precipitation. It comes as a percentage of annual precipitation. It accounts
for 5% globally. The snow melts and creates stream flow. On the prairies, the snowmelt accounts for about
80% of the stream flow and water stored in sloughs. The snow falls are measured using snow gauges. The
snow gauges are shielded and mounted on brackets such that the gauge can be raised as the snow
accumulates inside and can be measured. The most accurate method of determining snowfall amounts,
requires frequent measurement of changes in the depth of snow on ground( because snow tends to quickly
settle and undergo metamorphosis). It is measured in depth of snow.
4
The hydrological significance of snow are:
 total annual snowfall is only one factor determining the contribution of snow to a water budget
 snow is first stored for days to months before participating in the hydrological cycle.
6.0 Evaporation
Evaporation is the process of converting a liquid (or) solid into a gas, through the transfer of heat energy. In
hydrologic cycle this conversion is towards water vapour. Heat energy can convert water mass (or) ice into a
vapour. Evaporation occurs more rapidly when there is increase in temperature and also flow of wind. It
also depends on the boiling point and vapour pressure. The greater a substance’s vapour pressure, the more
rapid the substance gets evaporated and escape into the air.
Open water evaporation is the theoretical evaporation flux from a smooth shallow water surface (with no
storage of energy) when subjected to the ambient meteorological conditions. Pan evaporation is the
evaporation flux from an evaporation pan. Soil evaporation is the evaporation flux from the soil.
Under the conditions of constant temperature, humidity and wind, evaporation from the large water surfaces
of the earth must remain constant. A temporary increase in temperature results in increased evaporation
and also increased precipitation. Water gets evaporated more rapidly in dry air. This is due to the fact that
dry air has only a fraction of the maximum vapour pressure of water. It is also essential to note that the
evaporating molecules absorb heat from the surroundings. Huge volume of water evaporates from the
ocean surfaces. The amount varies from place to place. It is called as evaporation discharge. The greatest
amount is around the equator, where the intensity of solar radiation is more.
The factors affecting evaporation are
a) Air Temperature
b) Relative Humidity
c) Incoming radiation
d) Wind speed
e) Duration of bright Sun shine
f) Geomorphic conditions of the region.
The amount of water getting evaporated from a free water surface is measured using evaporimeters or pans.
6.1 Evaporation from soil surface
The water molecules present in the soil matrix also get evaporated with great difficulty, unlike those which
are released from the free water surfaces. The amount of water vapor present in the atmosphere varies
greatly from time to time, but the dry gases do not change materially in quantity from season to season. It
may be remarked at this point, however, that about half of the total moisture present in the atmosphere is
found below an elevation of about 6000 feet, and less than one tenth of it occurs above an elevation of
20,000 feet.
7.0 Transpiration
Transpiration is the process of releasing the water absorbed by the plants through their root system after
utilizing the nutrients for building their tissues, in a specified time. Vegetation including numerous growing
plants, play a significant role in the hydrologic cycle. The water which is drawn into the plants rootlets from
the soil moisture, owing to osmotic pressure moves up through the plants stems and leaves. Through the
stomatal openings, the water is released out as water vapour. The amount of transpiration depends on the
density and size of the vegetation existing in place. The amount of water used for irrigating the crops get
transpired into the air. Transpiration is dominant during the growing season of crops in agricultural lands.
Most of this happens during day time, when photosynthesis is active in plants. Transpiration is limited due
to the shortage of soil moisture is some places. The controlling factors of transpiration are:
a) Temperature
b) Solar radiation
c) Wind and
5
d) Soil moisture.
8.0 Evapotranspiration
Evaporation is the evaporation flux from intercepted water and from the soil. Transpiration is molecular
diffusion of water vapour through the stomatal aperture of leaves. Evapotranspiration is the combined
effect of both evaporation of water from the soil, surface water bodies, snow, ice and transpiration from
vegetation. In a well-irrigated land, it is difficult to separate evaporation from transpiration. The total water
loss due to both evaporation and transpiration is called as evapotranspiration. Majority of the water loss
due to evapotranspiration happens during summer months and growing seasons. There will be no (or) little
loss expected during winter months / period. Two terms as potential evapotranspiration and actual
evapotranspiration are used to denote these conditions. Actual evapotranspiration is the total evaporation
flux of a cropped surface. The actual evapotranspiration depends mainly on
a) Atmospheric factors
b) Soil water characteristics and
c) Physiological factors of the crops and other vegetation.
The rate of evapotranspiration is controlled by several climatic, hydrologic, soil and geomorphological
conditions of a region. Soil texture and permeability play the major roles in this process. Potential soil
evapotranspiration is the theoretical evaporation flux from the soil, if the soil would sufficiently be supplied
with water. Potential evapotranspiration is the sum of the potential soil evaporation and transpiration.
The major use of potential evapo(transpi)ration data is generally for:
1. water resources planning,
2. agricultural and irrigation management, and
3. research.
9.0 Surface Runoff
Runoff is the quantity of water that is discharged (“runs off”) from a drainage basin during a given time
period. Runoff data may be presented as volumes in acre-feet, as mean discharges per unit of drainage area
in cubic feet per second per square mile, or as depths of water on the drainage
basin in inches. It is measured by establishing stream gauges at selected places of the river courses.
The term runoff refers to the overland flow of water, after every rainfall or snowmelt. The overland flow
starts when the rate of rainfall is greater than the rate of infiltration of the soil and increase in the amount of
slope. Initially, Runoff starts as small streams and the water gets added from many such streams. Finally, all
of these reach and confluence with a lake or stream or directly with seas. The volume of water leaving
through a river is known as river discharge. It is considered as precipitation returning to the sea.
Stream flow is the discharge that occurs in a natural channel. Although the term “discharge” can be applied
to the flow of a canal, the word “stream flow” uniquely describes the discharge in a surface stream course.
The term “stream flow” is more general than “runoff” as stream flow may be applied to discharge whether or
not it is affected by diversion or regulation.
9.1 Factors Controlling Runoff
The flow of any stream is determined by two major groups of factors. The first set belongs to the
geomorphological factors of the drainage basin. The second set of factors depend on the climatological
variables. The climatological factors are :
1. Rainfall – Intensity and Type.
2. Duration of Rainfall.
3. Distribution of Rainfall.
4. Direction of Storm Movement.
5. Soil Moisture Conditions.
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The geomorphological factors include land use land cover, type of soil, area, shape, elevation, slope,
network of drainages and indirect influences for runoff.
10.0 Infiltration
Infiltration is the downward percolation of rainwater (or) snow melt water into the soil horizons. The
downward movement of water happens in the top soil layer, especially through the smaller pore spaces
present in the soils. Infiltration is governed by two forces as gravity and capillary action. The smaller pores
offer greater resistance to gravity, very small pores pull water through capillary action in addition to and
even against the force of gravity. Tensiometer is used for the measurements of soilwater properties.
After every rain, the zone of soil moisture obtains the first part of the rainfall. The rate of percolation is
known as infiltration rate. It is measured using infiltrometers or lysimeters. Infiltration rates are measured by
conducting infiltration tests. The rate of infiltration varies from soil to soil. It depends on the hydrologic
properties of soils, like porosity and permeability. The process is also known as percolation. The percolating
rainwater ultimately reaches the groundwater zone. The process of infiltration happens only when there is
space available for addition of water within the soil surface. This depends on the porosity of the soil and the
rate at which previously infiltrated water can move away from the surface through the soil.
The maximum rate that water can enter into a soil layer in a given subsurface condition is known as the
infiltration capacity. Infiltration rate in soil science is a measure of the rate at which a particular soil is able
to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases
as the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will usually occur
unless there is some physical barrier.
The factors that affect infiltration are:
a) Porosity and permeability of the soils
b) Structure and texture of the soils
c) Surface entry possibilities of the soils
d) Transmission through the soil
e) Already available soil moisture and its depletion
f) Characteristics of the fluid.
All terrestrial plants and forest vegetation thrive due to the presence of infiltrated water in the soil horizon.
The maximum rate at which a soil is capable of absorbing water is called as its infiltration capacity.
Percolation is favoured by a a) slow and steady precipitation, b) permeable soil, and c) a flat or very gentle
slope. Steep slope may leave the water over the surface.
11.0 Interception
Interception is the process of retaining water on the leaves of vegetation. A small amount of rainfall is
intercepted by vegetation. The rainfall which is not intercepted is known as through fall. The water which
reaches the ground via steps and thanks is called as stemflow. These are the three Main Components of
Interception. When rain falls onto a forest land, a proportion of it is intercepted by the canopy and
evaporates back into the atmosphere. This water is not playing any role in the terrestrial portion of the
hydrologic cycle. This is called as canopy interception loss. Interception can amount up to 15-50% of
precipitation, which is a significant part of the water balance. Although interception storage is generally
small, the number of times that the storage is filled and depleted can be so large that the interception flux is
generally of the same order of magnitude as the transpiration flux(flow accounted as loss). Snowfall is also
intercepted by trees, especially, the coniferous trees can store much snow for interception.
The storage capacities of vegetation while intercepting rainfall vary with the type and structure of vegetation
and also with the meteorological factors. The experiments have shown that about 8mm of rainfall can be
intercepted by the vegetative zones. This water is evaporated back to the atmosphere. The rate at which it goes
back to the atmosphere is known as interception rate. The controls on interception rate are:
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a) vegetation characteristics
1. growth form: trees, shrubs, grasses, forbs
2. plant density
3. plant structure: number, size, flexibility, strength and pattern of branches; texture,
surface area and orientation of leaves
4. plant community structure
b) meteorological factors
1. precipitation intensity
2. precipitation duration
3. wind speed
4. type of rainfall: rain versus snow
5. precipitation frequency
12.0 Groundwater baseflow
The rainwater that is falling over the ground surface percolates down through the soil and reaches the
groundwater zone. Depending upon the slope of the groundwater system, the groundwater moves towards the
rivers, lakes or the oceans. This flow of groundwater is known as baseflow. It depends on the hydrologic
properties of rocks forming the groundwater systems. This is an invisible flow of water. A river may become
a loosing stream (or) a gaining stream depending upon the contribution of stream water down the earth (or)
groundwater into the stream respectively. The subject of groundwater hydrology deals with all aspects of
groundwater. Computation of groundwater balance is made to utilize the available groundwater resources,
when there is no surface water facilities in a region.
13.0 Sublimation
Snow covered zones also experience losses of water through direct evaporation. The process of direct
evaporation of snow into water vapour is known as sublimation. The solid does not pass through a liquid
state for evaporation. It is very difficult to distinguish between evaporation and sublimation from snow. Air
temperature plays a significant role in addition to wind, in sublimation to happen.
14.0 Global Water Balance
The direct input sources of water for the lands and seas are from precipitation. The major output sources are from
evaporation, transpiration, sublimation, interception and evapotranspiration. There is a balance of water existing
as storage in the form of groundwater, surface water bodies as lakes and streams, ice caps and glaciers and as seas
and oceans. These components can be analysed using a simple mass balance equation called as water balance
equation. This equation considers the inflow, outflow and changes in storage reservoirs of fresh and saltwater.
The basis of the equation is
Inflow = outflow  changes in storage.
This equation can be expanded as
P – E – T – RO = S
Where
P
=
Precipitation
E
=
Evaporation
T
=
Transpiration
RO
=
Runoff
S
=
Changes in storage
This equation balances the availability of water for a specific period of time in any region of the world.
Computation of a hydrological budget is needed for all practical purposes of water resources consumption
and management.
15.0 Earth’s Other Cyclic Process
8
The hydrologic cycle is the primary cycle of the planet earth. It becomes the focal concept and dining
mechanism for part of the rock cycle, geochemical cycle and the sedimentary cycles of biogeochemical
cycle. In the rock cycle, weathering, mass wasting, erosion, transportation and depositional aspects of
sediments are carried out by the components of hydrologic cycle. The sulphur and phosphors cycles of
biogeochemical cycles are driven by the components of hydrologic cycle. All trace elements and their
motilities among rock, soil, water, plant and animal are all controlled by the hydrologic cycle.
16.0 River Basin as a Hydrologic Unit
Drainage basin is a part of the Earth’s surface that contains a drainage system with a common outlet for its
surface runoff. Hydrologic unit is a geographic area representing part or all of a surface drainage basin.
Drainage area of a stream at a specific location is that area upstream from the location, measured in a
horizontal plane, that has a common outlet at the site for its surface runoff from precipitation that normally
drains by gravity into a stream. Drainage areas given herein include all closed basins, or non-contributing
areas, within the area unless otherwise specified.
The total volume of water on the earth in its liquid, solid and vapour forms has been the same since the
formation of the planet.
Rivers are natural watercourses. River water flows over the land surface in small channels after draining
discrete areas. The existence, size and flow of a river water are influenced mainly by the availability of
surface water, a river channel in the ground, and an inclined surface. A river basin refers to the fall
catchment zone of a river and its tributaries with a clear cut water divide separating it from other basins. All
network of streams belonging to the main river and its tributaries are enclosed well within the river basin.
The boundary of thee basin is denoted as basin boundary. There will be a single outlet for any drainage
basin. The main river may confluence with a reservoir, a lake or an ocean. The stream of a large river basin
may have a very wide delta at the point of confluence. The catchment zone of tributaries and their streams
are referred to as sub-basins. The small network of streams joining the tributaries may have their own minicatchment zones. These are called as watersheds.
A watershed covers area above and below the earth’s surface. The small channels and streams drain water
towards a surface water body like a pond or a river. Bigger watersheds may contain numerous tributaries
and their corresponding catchment areas.
The watersheds are considered to be the smallest hydrological units of a large river basin.
The contributions of all watersheds are cumulatively considered for the yield (outflow) of the river basin.
A quantitative estimation of water balance of each washed is possible as the parameters are measureable,
suitable and ideal for computations.
17.0 Hydrologic Cycle and Natural Hazards
Floods are the major natural hazards arising out of the surface runoff of large basins and their rivers. Floods
cause server damage to life and properties. They also damage the grown up crops and rural/ urban
settlements. It is a part of the hydrologic process that are happening in a short span of time after a heavy
rainfall, along the river courses. Floodplains are the zones severly affected due to this hazard.
Thunderstorms are yet another hazard coming due to hydrologic processes in the atmosphere. These are
issues requiring proper disaster management methods.
Sudden and heavy downpour of precipitation can cause severe damage to the population.
Drought is a major issue arising out of continuous failure of monsoons in any area or a river basin.
Landslides are hazards occurring after heavy rainfall in hilly regions.
Hailstorms and Snowfalls create a lot of problems in the mountainous and polar regions for transport,
cultivation and other activities.
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Cyclones are depressions created in bays and oceans due to atmospheric pressure belts. They move towards
the condensed water masses.
Both wind and storms create the damage to the coastal belts.
Proper disaster management plans are necessary to mitigate the effects of these water-related natural
hazards.
18.0 Units and measurements
While discussing about the components of hydrologic cycle, it is necessary to know about the technical
terms which are used. The terms stream flow, runoff, discharge and yield of drainage basin are almost used
synonymously. Runoff, here, refers to the surface runoff. Groundwater flow is referred to as base flow. The
term groundwater discharge refers to base flow and pumping of water from the groundwater system. The
units in which all these quantities are expressed are always in relation to volume per unit of time.
The flowing are the common units used while referring to the components of the hydrological cycle
1. Cubic metre per second (m/s).
2. Cubic kilometer per day (or) month (or) year.
3. Acre feet per day, month, (or) year.
4. CM depth on drainage basin per day
5. Million gallons per month (or) day or year (mgd).
6. Million metre cube per day (mmc).
18.1 Volume units
Gallon is also the standard unit of measuring liquids. It is used to express the storage and flow capacity in
cubic feet / cubic metre. It is a unit for expressing volume.
Acre – feet is yet another unit. It refers to the quantity of water required to cover one acre to a depth of 1
foot. It is the unit of volume for expressing the storage in reservoirs also. The forms are Inches / cm per area
depth per unit area.
18.2 Common Hydrological Units
Precipitation
Runoff
Runoff Volume
Runoff Rate
Evaparation / Interception
Infiltration
Storage
-
inches (or) mm (or) cm
inches (or) mm (or) cm
acre feet (or) cubic feet
Cubic feet per second
inches (or) cm
inches (or) cm / hour
cubic feet, acre feet
Rainfall and evapotranspiration are expressed as cm depth on drainage area. Acre feet per day refers to the
rate of flow of a stream. Volume of water flow is normally expressed in Cu. m. per second or Cu. Km per
second or in million gallons per day (or) million metre cube. Appropriate conversion factors are available
for converting these components from one unit to the other. In all the old text books, fps (feet – pound –
second) system might have been used by the authors. Now, the SI units (Systems International) are
invariably used by all.
19.0 Conclusion:
The planet earth contains enormous amount of water on its surface. Water exists in all the
spheres of the earth. The hydrosphere is the sphere of water on earth. It is a discontinuous layer
containing both fresh water and saline water. Water has the unique ability to transform into
different states of matter as liquid, solid and vapour. Water moves from one reservoir to the other.
The sun’s radiant energy plays a significant role in this movement. Atmospheric pressure, mind
blows, temperature and amount of water vapour play a dominant role in these processes. The rates
of movement of water and the quantities involved the cyclic processes are the major aspects
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involved in the hydrological sciences. The hydrologic cycle is the driving wheel of all the available
water resources on the planet earth. It is necessary to identity the cyclic circulation routes of water
in all spheres of the earth. Several processes and causative mechanisms are involved in driving the
global water circulation, which is called as the great water cycle. This lesson highlighted the basic
concepts of the hydrologic cycle, the components of the cycle, the role of individual components,
the water balance estimation concepts and the units of measurements. The role of hydrological
cycle in promoting the other environmental and biogeochemical cycles on the earth has also been
explained.
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