HYDROLOGY DEFINITIONS & TERMS
Of total water on earth
- 92.7% of this water is salty and is stored in oceans and seas.
Hydrograph features
- 92.7% of this water is salty and is stored in oceans and seas.
- Only 2.8% of total available water is fresh water.
- Out of this 2.8% fresh water,
- 2.2% is available as surface water and
- 0.6% as ground water. Out Of the 2.2% surface water,
- 2.15% is stored in glaciers and ice caps,
- 0.01% in lakes and streams and the rest is in circulation among the different components of the Earth‘s atmosphere.
Out of the 0.6% ground water only about
- 0.25% can be economically extracted.
It can be summarized that less than 0.26% of fresh water is available for use by humans and hence water has become a very important resource
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
re-precipitation.
Hydrological Cycle
Hydrological cycle is defined as the circulation of water from the sea to the land through the
atmosphere back to the sea often with delays through process like precipitation, interception,
runoff, infiltration, percolation, ground water storage, evaporation and transpiration also water
that returns to the atmosphere without reaching the sea.
TERMS
1. Precipitation- It is the return of atmospheric moisture to the ground in solid or liquid
form. Solid form- snow, sleet, snow pellets, hailstones. Liquid form- drizzle, rainfall.
Forms of Precipitation
1. Drizzle- This is a form of precipitation consisting of water droplets of diameter less than
0.05 cm with intensity less than 0.01cm/ hour.
2. Rainfall- This is a form of precipitation of water drops larger than 0.05cm diameter up to
0.5cm diameter. Water drops of size greater than 0.5 cm diameter tend to break up as
they fall through the atmosphere. Intensity varies from 0.25 cm/ hour to 0.75cm/ hour.
3. Glaze- This is the ice coating formed when a drizzle or rainfall comes in contact with
very old objects on the ground
4. Sleet- This occurs when rain drops fall through air which is below 00
c. The grains are
transparent, round with diameter between 0.1 cm to 0.4 cm.
5. Snow Pellets- These are white opaque round grains of ice. They are crystalline and
rebound when falling onto the ground. The diameter varies from 0.05cm to 0.5cm.
6. Snow- This is precipitation in the form of ice crystals, usually a no. of ice crystals
combining to form snowflakes.
7. Hails- These are balls or irregular lumps of ice of over 0.5cm diameter formed by
repeated freezing and melting. These are formed by upward and downward movement of
air masses in turbulent air currents.
The following are the main characteristics of rainfall
a. Amount or quantity- The amount of rainfall is usually given as a depth over a
specified area, assuming that all the rainfall accumulates over the surface and the unit
for measuring amount of rainfall is cm. The volume of rainfall = Area x Depth of
Rainfall ( m
3
)
The amount of rainfall occurring is measured with the help of rain gauges.
b. Intensity- This is usually average of rainfall rate of rainfall during the special periods
of a storm and is usually expressed as cm/ hour.
c. Duration of Storm- In the case of a complex storm, we can divide it into a series of
storms of different durations, during which the intensity is more or less uniform.
d. Aerial distribution- During a storm, the rainfall intensity or depth etc. will not be
uniform over the entire area. Hence we must consider the variation over the area i.e.
the aerial distribution of rainfall over which rainfall is uniform.
2. Infiltration- Infiltration is the passage of water across the soil surface. The vertical
downward movement of water within the soil is known as percolation. The infiltration
capacity is the maximum rate of infiltration for the given condition of the soil.
3. Overland Flow- This is the part of precipitation which is flowing over the ground
surface and is yet to reach a well-defined stream
4. Surface runoff- When the overland flow enters a well-defined stream it is known as
surface runoff (SRO).
5. Interflow for Sub surface flow- A part of the precipitation which has in-filtered the
ground surface may flow within the soil but close to the surface. This is known as
interflow. When the interflow enters a well-defined stream, then and only it is called run
off.
6. Ground water flow- This is the flow of water in the soil occurring below the ground
water table. The ground water table is at the top level of the saturated zone within the soil
and it is at atmospheric pressure. Hence it is also called phreatic surface.
7. Evaporation- This is the process by which state of substance (water) is changed from
liquid state to vapor form.
8. Transpiration – This is the process by which the water extracted by the roots of the
plants is lost to the atmosphere through the surface of leaves
9. Evapotranspiration : when evaporation and transpiration happen simultaneously
Types of Precipitation
1. Convective precipitation- This is due to the lifting of warm air which is lighter than the
surroundings. Generally this type of precipitation occurs in the tropics where on a hot
day, the ground surface gets heated unequally causing the warmer air to lift up and
precipitation occurs in the form of high intensity and short duration.
2. Orographic Precipitation- It is the most important precipitation and is responsible for
most of heavy rains in India. Orographic precipitation is caused by air masses which
strike some natural topographic barriers like mountains and cannot move forward and
hence the rising amount of precipitation. The greatest amount of precipitation falls on the
windward side and leeward side has very little precipitation.
3.. Cyclonic Precipitation- This is the precipitation associated with cyclones or moving
masses of air and involves the presence of low pressures
4. Turbulent Precipitation- This precipitation is usually due to a combination of the several
of the above cooling mechanisms.
Rain gauging (Measurement of Rainfall):
Rainfall is measured on the basis of the vertical depth of water accumulated on a level surface
during an interval of time, if all the rainfall remained where it fell. It is measured in ‗mm‘. The
instrument used for measurement of rainfall is called ―Rain gauges
These are classified as
a. Non recording types
b. Recording types.
a. Non recording type Raingauges:
These rain gauges which do not record the depth of rainfall,
but only collect rainfall.
Symon‘s rain gauge is the usual non recording type of rain gauge.
b. Recording type Raingauges : These are rain gauges which can give a permanent, automatic
rainfall record (without any bottle recording) in the form of a pen mounted on a clock driven
chart.
Types of recording or automatic rain gauges:
1. Weighing bucket rain gauge: This is the most common type of recording or automatic rain
gauge adopted by Indian Meteorological Department.
2. Tipping bucket rain gauge: This is the most common type of automatic rain gauge adopted by
U S Meteorological Department.
3. Siphon or float type rain gauge:
- This is also called integrating rain gauge as it depicts an
integrated graph of rain fall with respect to time.
- most commonly used recording type raingauge
Factors governing selection of site for rain gauge stations:
1. The site for rain gauge station should be an open space without the presence of trees or any
covering.
2. The rain gauge should be properly secured by fencing.
3. The site for rain gauge station should be a true representation of the area which is supposed
to give rainfall data.
4. The distance of any object or fence from the rain gauge should not be less than twice the
height of the object or fence and in no case less than 30 m.
5. The rain gauge should not be set upon the peak or sides of a hill, but on a nearby fairly level
ground.
6. The rain gauge should be protected from high winds.
7. The rain gauge should be easily accessible to the observers at all times.
Rain gauge Density:
a. Plain areas – 1 station per 520 km2
b. Areas with 1000 m average elevation - 1 station per 260 to 350 km2
c. Predominantly hilly areas with heavy rainfall - 1 station per 130 km2
Determination of average precipitation over an area:
a. Arithmetic mean method: In this method to determine the average precipitation over an area
the rainfall data of all available stations are added and divided by the number of stations to give
an arithmetic mean for the area.
b. Thiessen Polygon method: This is also known as weighted mean method. In this method rainfall recorded at
each station is given a weight age on the basis of the area enclosing the area.
c. Isohyetal Method: Isohyets are imaginary line joining points of equal precipitation in a given
area similar to contours in a given area. Isohyets of different
values are sketched in a manner similar to contours in surveying in a given area. The mean
(average) of two adjacent Isohyetal values is assumed to be the precipitation over the area lying
between the two isohyets.
Estimation of missing precipitation record.
1. Interpolation from isohyetal map
In a isohytal map of the area the passion of the station (rain gauge) where record is
missing is marked by interpolation techniques the missing record is worked out the
factors like storm factor, topography nearness to sea are considered for proper estimation.
2. Arithmetic average method.
Here number of other rain gauge station record surrounding station in question (missing
record) are required. The missing rainfall record at the station is taken as average o fall
available data surrounding station in question. P1, P2, P3-------- etc Pn are rainfall record
from ―n‖ station surrounding a non operative station x the rainfall data for station x is given as
Px= (P1+ P2+ P3------+ Pn) / N
3. Normal ratio method.
This method is applicable when normal annual rainfall at required station differ more
than 10% of annual rainfall at surrounding station.
The Mass Curve of Rainfall
- The mass curve of rainfall is a plot of the accumulated precipitation against time, plotted
in chronological order.
- Mass curve of rainfall are very useful in extracting the information on the duration
and magnitude of a storm.
- intensities at various time interval s in a storm can be
obtained by the slope of the curve.
Hyetograph
- A hyetograph is a plot of the intensity of rainfall against the time interval.
- The
hyetograph is derived from the mass curve and is usually represented as a bar chart.
- The area under
a hyetograph represents the total precipitation received in the period.
EVAPORATION
It is the process by which a liquid changes to gaseous state at the free surface through transfer of
heat energy.
Factors affecting Evaporation
1. Vapour pressure difference: The number of molecules leaving or entering a water body
depends on the vapour pressure of water body at the surface and also the vapour pressure
of air. Higher water temperature leads to high vapour pressure at surface and tends to
increase the rate of evaporation. High humidity in air tends to increase vapour pressure in
air and in turn reduces rate of evaporation.
2. Temperature of air and water: The rate of emission of molecules from a water body is a
function of its temperature. At higher temperature molecules of water have greater energy
to escape. Hence maximum evaporation from water bodies takes place in summer. It has
been estimated that for every 1o C rise in atmospheric temperature increases 5 cm of
evaporation annually.
3. Wind Velocity: When wind velocity is more the saturated air (humid air) is drifted away
and dry air comes in contact with water surface which is ready to absorb moisture. Hence
rate of evaporation is dependent on wind velocity. It has been estimated that 10%
increase in wind velocity increases 2 – 3% of evaporation.
4. Quality of water: The rate of evaporation of fresh water is greater than saline water.
(Specific gravity of saline water is greater than that of fresh water. It is established that
saline water has lesser vapour pressure and it is observed that evaporation from fresh
water is 3 – 4% more than sea water.
5. Atmospheric pressure and Altitude: Evaporation decreases with increase in atmospheric
pressure as the rate of diffusion from water body into the air is suppressed. At higher
altitude the atmospheric pressure is usually lesser and there by evaporation rate is higher.
6. Depth of water body: Evaporation shallow water bodies is greater when compared to
deep water bodies as the water at lower levels in deep water bodies is not heated much
and vapour pressure at lower levels is also reduced.
Dalton‘s law of evaporation states that ―Evaporation is proportional to the difference in vapour
pressures of water and air
Measurement of Evaporation
1. Pan measurement methods
2. Use of empirical formulae
3. Storage equation method
4. Energy budget method
Transpiration:
- It is the process by which the water vapour escapes from living plants from surfaces of leaves,
branches and enter the atmosphere.
- measured using phytometre
Evapotranspiration:
In agricultural fields apart from transpiration, water is also lost due to
evaporation from adjacent soil. The sum of these two losses is often termed as evapotranspiration
(Et) or consumptive use (Cu)
Potential evapotranspiration: When sufficient moisture is freely available to completely meet
the needs of the vegetation fully covering an area, the resulting evapotranspiration is called
potential evapotranspiration.
Actual evapotranspiration: The real evapotranspiration occurring in a specific situation in the
field is called actual evapotranspiration.
Infiltration
- The water entering the soil at the ground surface after overcoming resistance to flow is called
infiltration. The process is also termed as infiltration.
- Infiltration fills the voids in the soil.
- Excess water moves down by gravity and it is known as
percolation.
- Percolation takes place till water reaches ground water table.
- For continuous
infiltration to occur it is essential that percolation should also be continuous, which is also
dependent of ground water movement.
- Usually at the
beginning of a storm infiltration is more and runoff is less and when storm continues infiltration
becomes lesser and runoff become constant.
- The volume of rainfall that will result in runoff is
called Rainfall excess
Infiltration rate (f):
It is actually the prevailing rate at which the water is entering the given soil at any given instant
of time. It is expressed in cm/hr (i.e. depth of water entering soil per unit time).
Infiltration Capacity (fp):
It is the maximum rate at which a soil in any given condition is capable of absorbing water.
Infiltration capacity curve:
It is the graphical representation of variation of infiltration capacity with time, during and a little
after rain many factors affect infiltration capacity of a given soil.
Infiltration indices
The Φ - Index is an average rainfall intensity above which the rainfall volume equalsthe runoff
volume.
The W – Index is refined version of Φ - Index. It excludes the depression storage and
interpolation from the total losses. It is the average infiltration rate during the time rainfall
intensity exceeds the capacity rate.
Run off
When precipitation occurs on land, a part of it is intercepted by vegetation and some part of it is
stored as depression storage. A part of precipitation infiltrates into the ground. If the rate
of precipitation is greater than the rate of infiltration, then the rainfall in excess of infiltration
will start flowing over the ground surface and is also known as over land flow. When over land flow reaches a
well defined stream it is known as surface run off.
Total runoff = Surface run off + Inter flow + Base flow + Channel precipitation
Precipitation = Run off + Evaporation
1. Total Run off: This is the part of precipitation which appears in streams. It consists of
Surface run off, Inter flow, Base flow, and Channel precipitation.
2. Surface run off (SRO): This is the part of overland flow which reaches the streams.
3. Direct run off (DRO): It consists of Surface run off, Inter flow, and Channel precipitation,
but does not include Base flow
4. Hydrograph: A hydrograph is a plot of the run off or discharge in a stream versus time.
Rising limb: It is the curve or line joining the starting point of the raising curve and
the point of reflection. The shape of the raising line is influenced by the rainfall
characteristics.
Peak or Crest: It represents the highest point/position of the hydrograph. Its duration also
depends on the intensity and duration of the rainfall
Falling limb or depletion curve: It is the descending portion or the hydrograph. The
shape of the falling limb it mainly a function of the physical features of the channel alone
and is independent of storm characteristics (it depends on basin characters)
Time to peak (tp): It is the time to peak from the starting point of hydrograph
Lag time: The time interval from the centre of mass of rainfall to the centre of mass
hydrograph is the lag-time.
UNIT HYDROGRAPH
A unit hydrograph is defined as the hydrograph of direct runoff resulting from one cm depth
excess rainfall occurring uniformly over the basin and at a uniform rate for a specified duration
Aquifer : geological formation which stores and transmits water
Aquiclude : stores water but does not transmit
Aquitard : transmits water but doesn't hold
Aquifuge : neither transmit nor store
PREVIOUS YEAR QUESTIONS BASED ON HYDROLOGY