HYDROLOGY DEFINITIONS & TERMS

Of total water on earth

- 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. 

Hydrograph features

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