Hydrology is the study of water on Earth through its distribution and circulation in different states as part of the hydrologic cycle. The hydrologic cycle involves the continuous movement of water between the atmosphere, land, and oceans through various stages including precipitation, runoff, infiltration, storage, evaporation, and reprecipitation. Hydrology concerns itself with three forms of water: atmospheric water or precipitation above land, surface water or runoff on land, and ground water or percolation below land. The major components of the hydrologic cycle that drive this continuous movement of water are precipitation, interception, evaporation, transpiration, infiltration, percolation, runoff, and moisture storage. Hydrology has many applications including designing

1. UNIT I :
INTRODUCTION TO
HYDROLOGY

2. HYDROLOGY
➢ Hydrology may be defined as applied science
concerned with water of the Earth in all its states,
their occurrences, distribution and circulation through
the unending hydrologic cycle of precipitation,
consequent runoff, stream flow, infiltration and storage,
eventual evaporation and reprecipitation.
➢ Hydrology concerns itself with three forms of water –
1. Above land as atmospheric water or precipitation.
2. On land or surface as stored water or runoff
3. Below the land surface as ground water or percolation
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3. HYDROLOGIC CYCLE
➢ Hydrologic cycle was defined by the National Research Council
(NRC, 1982) the as “the pathway of water as it moves in its
various phases to the atmosphere, to the earth, over and
through the land, to the ocean and back to the
atmosphere”.
➢ Water can occur in three physical phases: solid, liquid, and gas
and is found in nature in all these phases in large quantities.
➢ Depending upon the environment of the place of occurrence,
water can quickly change its phase.
➢ A number of cycles are operating in nature, such as the carbon
cycle, the nitrogen cycle, and several biogeochemical cycles.
➢ The Hydrologic Cycle, also known as the water cycle, is one such
cycle which forms the fundamental concept in hydrology. ground
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4. HYDROLOGIC CYCLE HAS NO BEGINNING OR END AND WATER IS PRESENT IN ALL THE THREE STATES
(SOLID, LIQUID, AND GAS). A PICTORIAL VIEW OF THE HYDROLOGICAL CYCLE IS GIVEN –
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5. COMPONENTS OF HYDROLOGIC CYCLE
➢ The hydrologic cycle can be subdivided into three major
systems:
The oceans being the major reservoir and source of
water, the Atmosphere functioning as the carrier and
deliverer of water and the land as the user of water.
➢ The major components of the hydrologic cycle are
(i) precipitation (rainfall, snowfall, hale, sleet, fog, dew, drizzle
etc.),
(ii) interception, (iii) depression storage,
(iv) evaporation, transpiration,
(v) infiltration, percolation, moisture storage in the
unsaturated zone,
(vi) and runoff (surface runoff, interflow, and base flow).
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6. APPLICATIONS OF HYDROLOGY
1. Design of Hydraulic Structures: - Structures such
as bridges, causeways, dams, spillways etc. are in
contact with water.
Accurate hydrological predictions are necessary
for their proper functioning.
Due to a storm, the flow below a bridge has to be
properly predicted. Improper prediction may cause failure
of the structure.
Similarly the spillway in case of a dam which is
meant for disposing excess water in a dam should also be
designed properly otherwise flooding water may overtop
the dam.
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7. 2. Municipal and Industrial Water supply: -
Growth of towns and cities and also industries around
them is often dependent on fresh water availability in
their vicinity. Water should be drawn from rivers,
streams, ground water.
Proper estimation of water resources in a
place will help planning and implementation of
facilities for municipal (domestic) and industrial water
supply.
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8. 3. Irrigation: - Dams are constructed to store water for
multiple uses. For estimating maximum storage
capacity, seepage, evaporation and other losses should be
properly estimated.
These can be done with proper understanding of
hydrology of a given river basin and thus making the
irrigation project a successful one. Artificial
recharge will also increase ground water storage. It has
been estimated that ground water potential of Gangetic
basin is 40 times more than its surface flow.
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9. 4. Hydroelectric Power Generation: - A hydroelectric
power plant need continuous water supply without much
variations in the stream flow. Variations will affect the
functioning of turbines in the electric plant.
Hence proper estimation of river flow and also
flood occurrences will help to construct efficient
balancing reservoirs and these will supply water to
turbines at a constant rate.
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10. 5. Flood control in rivers: - Controlling floods in a
river is a complicated task. The flow occurring due to a
storm can be predicted if the catchment characteristics
are properly known.
In many cases damages due to floods are high.
Joint work of hydrologist and meteorologists in
threatening areas may reduce damage due to floods.
Flood prone zones maybe demarked to avoid losses.
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11. 6. Navigation: - Big canals in an irrigation scheme can
be used for inland navigation. The depth of water should
be maintained at a constant level.
This can be achieved by lock gates provided and
proper draft to be maintained. If the river water contains
sediments, they will settle in the channel and cause
problems for navigation. Hence the catchment
characteristics should be considered and sediment entry
into the canals should be done.
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12. 7. Pollution control: - It is an easy way to dispose
sewage generated in a city or town into streams and
rivers. If large stream flow is available compared to the
sewage discharge, pollution problems do not arise as
sewage gets diluted and flowing water also has self-
purifying capacity. The problem arises when each of the
flows are not properly estimated. In case sewage flow is
high it should be treated before disposal into a river or
stream.
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