The hydrological cycle involves the continuous movement of water on, above, and below the surface of the Earth. Water evaporates from surfaces into the atmosphere, condenses to form clouds, and precipitates as rain or snow back onto the Earth's surface, where it collects in lakes, oceans, soil, and underground aquifers before returning to the atmosphere through further evaporation or transpiration from plants. The sun drives the hydrological cycle by providing the energy needed for evaporation and transpiration. This cyclic movement of water is crucial to life on Earth and is known as the hydrological cycle.

1. HYDROLOGICAL CYCLE

2. r masses comes from precipitation.
rland. Processes like infiltration and peroration moves the water down to the groundwater systems. Some
sea as runoff. The surface water collected in lakes, ponds, swamps, seas and oceans get evaporated into the
res the water collected from the soil moisture. Evaporated and transported water enters into the atmosphere as
condensed to form the clouds.
starts precipitating again. These processes continue. This endless circulation of water is known as the hydrologic
l hydrologic cycle and global hydrologic cycle. The terrestrial hydrological cycle is of a special interest as the
sources on a given area of the land, like a river basin or a watershed. The global hydrological cycle deals with its

3. 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.

4. 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,

5. 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

6. The Underlying Factors 3
• 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.

7. 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

8. • 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

9. 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 4
• synonymously. Precipitation mainly depends on the water
vapour present in the atmosphere. When the air temperature

10. • 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.

11. 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

12. 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

13. 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.

14. 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.

15. 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

16. 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.
• 7
• The geomorphological factors include land use land cover, type of soil, area, shape, elevation, slope, network
of drainages and indirect influences for runoff.

17. 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

18. 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:

19. 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.

20. 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

21. 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.

22. 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
• Evaparation / Interception - inches (or) cm
• Infiltration - inches (or) cm / hour
• Storage - 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

23. 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