UNIT 1

Introduction

1.1 Introduction

Hydrology is the science that deals with the occurrence, circulation and distribution of water of the earth and its atmosphere.

1.2 Hydrological Cycle

The word hydrology has been derived from two Greek words ‘Hydro’ I .e. Water and ‘logos’ i.e. science so it is a science of water.

It is defined as “A science which deals with the properties, distribution and circulation of water on the surface of the land, in the soil, in the underlying rocks and also in the atmosphere mainly in the form of moisture or water atmosphere mainly in the form of vapour and in the form of water droplets during rainfall or in the solid form of snow particles during snows fall.”

C.O. Sister and E.F barter have defined the science of hydrology as “A science which deals with the process of governing the depletion and replacement of water resources of the land areas of the earth.

Hydrology is an applied science used by many persons working in different fields for the development of the human beings.

First Step

• Due to solar radiation called asinsulation i.e. in coming solar radiation.
• The oceanic and the water on the land surface get heated and get converted into vapour.

Second step

• The vapour being lighter is lifted up in the atmosphere so the air have this vapour called as moisture starts cooling down and so its start controlling.
• It reduces the carrying capacity of air to hold the moisture and it becomes moisture saturated i.e. no more moisture it can hold. This level is called as level of condensation.

Third Step

• Due to the pushing of air from the earth surface, this saturated air is still pushed in the upward direction to make it still cooler and still more compact to reduce its capacity lesser than the actual available moisture

• This extra moisture is converted into small rain drops. Which attract the dust particles to make it visible is called as clouds

Fourth step

• The moisture carrying clouds are carried by the winds towards the land surface.
• If it is a hilly region the clouds are still lifted up to increase the size of the raindrops and they start falling down called as rainfall.

Fifth step

• The rainwater received on the surface starts Moving towards the slopes, to form the running water or streams, which finally meet the oceans
• The rainwater received on the surface same times percolates (it depends upon the type of rock, if it is porous the water percolates) to form the ground water. It also comes back on the surface in the form of springs to form a surface stream to reach the ocean.
• The rain water received on the surface, which is plain and have non porous rock: forms lakes or tanks.
• The cycle is completed once the ocean or surface water comes back can the earth surface to start the first stage of Hydrology ice. It starts getting converted from the liquid state to the gaseous state. When the water is received in the form of solid state i.e. snow, it also undergoes the same cycle.
• When the process is fast the water vapour directly gets converted into solid snow particles. It is called as sublimation. This happened when the temperature of the moisture saturated air is less than O Cie. Less than the freezing point temperature. In the temperature and in the arctic zones on the earth it causes rainfall.

Fig. 1: The process involved in the hydrological cycle

As stated above the hydro cycle is completed in six states. Which can express by using technical terminology these processes are given

• Evaporation : It is process of conversion of the liquid or solid water bodies into the sea state

• Precipitation: Process of the conversion of the water vapour in the
  atmosphere in the liquid form water of the solid form Le hail or snow or frost.
• Interception: Interceptions the short-term retention of rainfall by the foliage of vegetation.
• Infiltration: Infiltration is the movement of water into the soil of the earth’s surface.
• Percolation: Percolation is the movement of water from one soil zone to a lower soil one.
• Transpiration: Transpiration is the soil moisture taken up through the roots of a plant and discharged into the atmosphere through the foliage by evaporation.
• Storage: Storage is the volume of water which gets stored in natural depressions of a basin.
• Runoff: Runoff is the volume of water drained by a river at the outlet of a catchment.

Key takeaways

• Hydrological cycle is complete in 5 steps the process where start is the same point where the process gets stop
• In hydrological cycle the processes involved is evaporation, precipitation, interception, infiltration, percolation, transpiration, storage, run off.

1.3 Water Budget Equations

For a given catchment in a time interval ∆t,

Infow - Outfow = Storage [continuity equation]

This continuity equation expressed in terms of various phase of hydrological cycle is called water budget equation/hydrological budget equation.

For Surface Flow

P+R1+Rg-R2-Es-Ts-I = ∆Ss (change in storage)........... (1)

P = Ppt.

R1 =Surface water inflow

Rg=Ground water appearing as surface water.

R2 = Surface water outflow.

Es = Evaporation

Ts = Transpiration

I = Infiltration.

For Underground Flow

I+ G1- G2- R g- E g- T g= ∆S g.     (Storage change).......... (2)

I = Infiltration

G1 =Ground water inflow

G2= Ground water outflow

Rg =Ground water appearing as surface water

Eg=Evaporation

Tg = Transpiration

Combined hydrological budget (water budget equation) is obtained by adding equation (1) and equation (2)

P-(R2 -R1) - (Es + E g)-(Ts + T g) - (G2 - G1Z= ∆ (Ss + S g)

P-R-E-T-G = ∆S..........Water budget equation.

Where,

P = Precipitation

R = Net runoff

E =Net evaporation

T =Net transpiration

G = Net ground water flow

∆S = Net storage increase

Note:

• For large river basin ground water system boundary often follow surface divides in such case

• Over a long period of time (5 or more yr). Seasonal excesses and deficit in storage tend to balance out in large catchments. Thus ∆S=0
• Under above assumptionsP-R-ET =0 (Water Budget Equation)

• In terms of rainfall runoff relationship water budget equation can be represented as

                  R = P-L

L =Losses =water not available to runoff due to (I, E, T and depression storage)

Key takeaways

• Water budget equation is simply inflow – outflow = storage

P-(R2 -R1) - (Es + E g)-(Ts + T g) - (G2 - G1Z= ∆ (Ss + S g)

P-R-E-T-G = ∆S..........Water budget equation.

• For surface flow

P+R1+Rg-R2-Es-Ts-I = ∆Ss

• For underground flow

I+ G1- G2- R g- E g- T g= ∆S g.  

HISTORY OF HYDROLOGY

• There is no readily available source of information about the history of hydrology and hydrologists. This site is one of a number of initiatives (see the links below) to try and remedy that situation.
• All hydrologists are welcome to contribute information about the development of the subject (and its overlaps with hydraulics, water resource engineering, geomorphology, ecohydrology, sociohydrology, hydrometeorology, water quality, etc)
• The site is intended to provide information on hydrologists who are no longer active but who have made a valuable contribution to the history of hydrology.

• Hydrology has been a subject of investigation and engineering for millennia. For example, about 4000 BC the Nile was dammed to improve agricultural productivity of previously barren lands.
• Mesopotamian towns were protected from flooding with high earthen walls. Aqueducts were built by the Greeks and Ancient Romans, while the history of China shows they built irrigation and flood control works.
• The ancient Sinhalese used hydrology to build complex irrigation works in Sri Lanka, also known for invention of the Valve Pit which allowed construction of large reservoirs, anicuts and canals which still function.
• Marcus Vitruvius, in the first century BC, described a philosophical theory of the hydrologic cycle, in which precipitation falling in the mountains infiltrated the Earth's surface and led to streams and springs in the lowlands.
• With the adoption of a more scientific approach, Leonardo da Vinci and Bernard Palsy independently reached an accurate representation of the hydrologic cycle.
• It was not until the 17th century that hydrologic variables began to be quantified.
• Point of the modern science of hydrology includes Pierre Perrault, Edme Mariotte and Edmund Halley.

• By measuring rainfall, runoff, and drainage area, Perrault showed that rainfall was sufficient to account for the flow of the Seine. Mariotte combined velocity and river cross-section measurements to obtain a discharge, again in the Seine.
• Halley showed that the evaporation from the Mediterranean Sea was sufficient to account for the outflow of rivers flowing into the sea.

• Advances in the 18th century included the Bernoulli piezometer and Bernoulli's equation, by Daniel Bernoulli, and the Pitot tube, by Henri Pitot.
• The 19th century saw development in groundwater hydrology, including Darcy's law, the Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.

• Rational analyses began to replace empiricism in the 20th century, while governmental agencies began their own hydrological research programs. Of particular importance were Leroy Sherman's unit hydrograph, the infiltration theory of Robert E. Horton, and C.V. Theis' aquifer test/equation describing well hydraulics.
• Since the 1950s, hydrology has been approached with a more theoretical basis than in the past, facilitated by advances in the physical understanding of hydrological processes and by the advent of computers and especially geographic information systems (GIS). (See also GIS and hydrology)

1.4 World Water Balance

1. Introduction

• Water balance is the ratio between water inflow and outflow estimated for different space and time scales, i.e. for the Earth as a whole, for oceans, continents, countries,  natural-economic regions, and river basins, for a long-term period or for particular years and seasons.
• Water balance is the most important integral physiographic characteristic of any territory, determining its specific climate features, typical landscapes, possible water management and land use.

• Analysis of water balance components for individual territories and time intervals is of great importance for studies of the hydrological cycle or water circulation in the atmosphere-hydrosphere-lithosphere system, as well as the underlying processes influenced by natural factors and human activities.

• Precipitation, evaporation, river runoff and ground water outflow not drained by river systems are basic components determining water balance.

• Besides these components, there are minor components, too, e.g. Moisture due to atmospheric water vapor  condensation, deep artesian water outflow, or, conversely, recharge of deep aquifers,
• Water losses for animal survival, etc. According to investigations, however, these components are very small if related to large river basins, regions and the globe—they are of no importance for water balance computation, so they can be ignored.
• It should be noted that much fresh water is used in many regions for different human needs.
• Some of this is returned to water bodies as surface and subsurface runoff, but  some water is lost, particularly to evaporation (from irrigated lands, reservoirs, etc.),  thus increasing evapotranspiration in the region.
• This must be taken into account in the appropriate water balance components.

• Thus, the assessments of water balances of large regions with sufficient accuracy is reduced to reliable determination of the main water balance components, i.e. precipitation, evaporation and runoff (surface and subsurface).

• Quantitative characteristics of these components for different regions of the Earth presented in this chapter are mainly based on the results of the global hydrological cycle studies carried out in Russia at the State Hydrological Institute (St Petersburg) and at the Institute of Water Problems (Moscow).
• More detailed information about individual water balance components of the Earth is given in Atmospheric Precipitation of the Earth, Evaporation from the Surface of the Globe, River Runoff to Oceans and Lakes and Groundwater Discharge to the Oceans.
  

2. Water Balance Equations

• Water balance equation for any land area and any time interval (without taking account of the above minor components) is as follows:

  P + R’s + R’ u n = E + Rs + Run ± S.   ..................... (1)
Where: P is precipitation;

E is evapotranspiration;

R’s and R’un indicate surface and subsurface runoff from some land area andsubsurface water inflow to the land area, respectively;

S is water storage change in the area.

• All terms in equation (1) are in mm of water layer, which is a water volume for time unit divided by the area of the land.
• If the water balance equation is considered for a long-term period it is simplified, because S = 0.

• If the water balance is considered at the global scale, it should be noted that there are regions on each continent which differ greatly in their water balance structure.
• Most territories on the continents are the zones of so-called external runoff—river runoff from these zones discharges to the World Ocean directly.
• There are also rather large areas on the continents (probably except Antarctica) which have internal runoff. These endorheic areas are not connected to the World Ocean.
• River runoff formed in such regions is completely lost to evaporation.

• For oceanic slopes and large river basins related to areas of external runoff from the continents (when it is possible to neglect surface and subsurface inflow from adjacent areas) the water balance equation for a long-term period is as follows:

Pext =Eext + Rext + Run              .................. (2)

In equation (2) Pext is a precipitation;

Next is river runoff (from an oceanic slope) to the ocean (sea, lake);

Run is subsurface runoff not drained by river systems and directly discharging to the ocean (sea, lake);

Exert is evapotranspiration including additional evaporation due to human activities.

• In the areas of internal runoff (endothecia regions) the whole quantity of precipitation is ultimately lost to evaporation, so the water balance equation for a long-term period for such regions is as follows:
  Pint = Eint                    ......................(3)

In equation (3) Pint is precipitation;

Eint is evapotranspiration from endorheic areas, including runoff formed within these areas and water losses for different human needs.

• For a continent with available zones of external runoff and endorheic areas, the water balance equation would probably consist of equation (2) and equation (3) joint together:

Pext + Pint = Eext + Eint + Rext + Run          .............. (4)

For the World Ocean, as well as for individual oceans (and seas) the freshwater balance equation for the long-term period (without taking account of water exchange between the oceans) will be as follows:

Eoc = Poc + Rext + Run.                     ............. (5)

Where: Eoc and Poc are evaporation from the ocean and precipitation onto the ocean surface, respectively;

Rext and Run are river water inflow and subsurface water inflow to the ocean.

• For the whole Earth for a long-term period and a steady climate situation it is evident that the total precipitation should be equal to evaporation from the water surface plus evapotranspiration from land, i.e. for the world water balance the water balance equation similar to that for endorheic areas is valid:

            Pgl = Poc + Pext + Pint = Eoc + Eext + Eint = Egl       ........... (6)

Where: Pgl and Egl are global values of precipitation and evaporation from the Earth as a whole.

It should be noted that equation (6), just like equations (1) to (5), is valid if we assume that water coming from outer space is balanced by the amount of water vapor lost to space and deep water inflow (or juvenile water) is equivalent to water used for hydration of minerals in the lithosphere.

3. Application in Engineering

• In hydrology we apply scientific knowledge and mathematical principles to solve water-related problems in society: problems of quantity, quality and availability.

• Mathematical models of all Hydrological phenomena are made. They may be concerned with finding water supplies for cities or irrigated farms, or controlling river flooding or soil erosion. Or, they may work in environmental protection:
• Preventing or cleaning up pollution or locating sites for safe disposal of hazardous wastes.

• Hydrology is used to find out maximum probable flood at proposed sites e.g. Dams.

• The variation of water production from catchments can be calculated and described by hydrology.

• Engineering hydrology enables us to find out the relationship between a catchments’ surface water and groundwater resources

• The expected flood flows over a spillway, at a highway Culvert, or in an urban storm drainage system can be known by this very subject. It helps us to know the required reservoir capacity to assure adequate water for irrigation or municipal water supply in droughts condition.

• It tells us what hydrologic hardware (e.g. Rain gauges, stream gauges etc) and software (computer models) are needed for real-time flood forecasting Used in connection with design and operations of hydraulic structure Used in prediction of flood over a spillway, at highway culvert or in urban storm drainage Used to assess the reservoir capacity required to assure adequate water for irrigation or municipal water supply during drought. Hydrology is an indispensable tool in planning and building hydraulic structures.

• Hydrology is used for city water supply design which is based on catchments area, amount of rainfall, dry period, storage capacity, runoff evaporation and transpiration.Dam construction, reservoir capacity, spillway capacity, sizes of water supply pipelines and affect of afforest on water supply schemes, all are designed on basis of hydrological equations.

• Hydrology provides guidance for undergoing proper planning and management of water resources. Calculates rainfall, surface runoff, and precipitation.

• It determines the water balance for a particular region.

• It mitigates and predicts flood, landslide and drought risk in the region.

• It estimates the water resource potential of the river basins

• Enables real-time flood forecasting and flood warning.

• Hydrology analyses the variations observed in the catchments by bringing a relationship between the surface water and groundwater resources of the catchment.

• Hydrology studies the required reservoir capacity that is necessary for irrigation and municipal water supply purpose during drought conditions.

• It is used in the design and operation of hydraulic structures

• It is used for hydropower generation.

• Brings measures to control erosion and sediments.

Key takeaways

Water balance equation for any land area and any time interval (without taking account of the above minor components) is as follows:

P + R’s + R’ u n = E + Rs + Run ± S.   ..................... (1)
Where: P is precipitation;

E is evapotranspiration;

• P ext =E ext + R ext + Run              .................. (2)
• Pint = E int                    ......................(3)
• P ext + Pint = E ext + E int + R ext + Run          .............. (4)
• E oc = P oc + R ext + Run.                     ............. (5)
•  P gl = P oc + P ext + Pint = E oc + E ext + E int = E gl       ........... (6)

1.5 Precipitation: Forms of Precipitation, Measurement

• Precipitation is the fall of water in various forms on the earth from the clouds. The usual forms area is, snow, sleet, glaze, hail, dew etc.

• Before studying the phenomenon of precipitation let us consider water vapour air in atmosphere can easily absorb moisture in the form water vapours. The amount of water vapours absorbed by air depend upon the temperature of air, them or is the temperature the more water vapour sit can absorb.

• The water vapour exerts a partial pressure on the water surface called vapour pressure.  The amount do water vapour present in air is in directly expressed in terms of vapour pressure.

• If the evaporation continues, a state of equilibrium is reached when the air is fully saturated with vapour and therefore it cannot absorb more vapours. The vapours then exert a pressure which is known as saturation vapour pressure (e s).e s increase with increase in temperature.

• Let us consider a parcel of air as temperature T and a vapour pressure (ea) indicated by pt. A. The saturation vapour pressure at that temperature is indicated by pt. B. The intercept BA= (e s-ea) is called saturation deficit.

• If vapours are added to the parcel of air, the pt .A will move to pt. B when air is fully saturated.

• If the parcel of air is cooled at constant pressure but with out the addition of more vapours, the pt. Moves horizontally towards pt. D and the air would be saturated when pt. D is reached. At that stage, the air would have a temperature called dew point temperature (T). Cooling of air beyond this pt. would result in condensation or formation of mint.

• If neither the temperature not the pressure remains constant, the water evaporates freely and the pt. Moves to pt. C.  In this case, water vapour is but temperature falls. The temperature at pt C. Is called wet temperature (T). The saturation vapour pressure is indicated by e w.

• Air in atmosphere can be cooled by many processes. However which occurs by a reduction of pressure through lifting of air masses is the main natural process.

1.5.1 Forms of Precipitation

1. Rain: Precipitation in form of water drops of size greater than 0.5 mm and less than 6mm.

2. Snowfall: The fall of larger snow flakes from the clouds on the ground surface is called snowfall. In fact, snowfall is precipitation of white and opaque grains.  The snowfall occurs when the freezing level is so close to the ground surface (less than 300 m from the surface) that aggregation of ice crystals reaches the ground without ground being melted in a solid form of precipitation as snow. Average density=0.1gm/cc.

3. Sleet: Refers to a mixture of snow and rain but an America   terminology sleet means falling of small pellets of transparent or translucent ice having a diameter of 5 mm or less.

4. Hail: Consists of large pellets or spheres of ice. In fact, hail is a form of solid precipitation where in small balls or pieces office, known as hails tones, having a diameter of 5 to 50 mm fall downward known as hail storm. Hails are very destructive and dreaded form of solid precipitation because they destroy agricultural crops and claim human and animal lives.

5. Drizzle: The fall of numerous uniform minute droplets of water having diameter of less than 0.5 mm is called drizzle. Drizzles fall continuously but the total amount of after received on the ground surface is significantly low. Intensity is usually less 0.1 cm/hr.

6. Glaze: It is a form of precipitation which falls as rain and freezes when comes in contact with cold ground at around 0'c. Water drops freeze of or man ice coating also called freezing grain.

1.5.2 Measurement

• The total amount of precipitation on a given are a is expressed as the depth of water if accumulated over the horizontal projection of the area. Thus 1 cm of rainfall over a area of 1 km2 represents a volume of water equal to 104 m3.

• Any part of this precipitation, if falling as snow or ice, is to be accounted for in its melted form.

• Since it is not physically possible to catch all the rainfall or snowfall over a drainage basin, it is only sampled by rain gauges in a perfect. Exposure, represent the precipitation falling on the irrespective surrounding are as.

TYPES OF GAUGES

The various types of precipitation gauges are broadly classified as

(A)Non-recording gauge sand (b) Recording gauges

Non-Recording Gauges

• The non recording gauge extensively used in India is the Symons rain gauge. It is installed in an open area on a concrete foundation. The distance of the rain gauge from the nearest object should be at least twice the height of the object. The gauge may defence with a gate to prevent animal and unauthorized person from entering the premises.

• Measurements are to be. Made at a fixed time. Every day normally at 08:30 hrs which is considered as the daily rainfall? In case of heavy areas, measurements are made a soft as possible. However, the last reading must be taken at 8:30AM. So that last 24 hrs data may be added up to get the rainfall of that day.

Recording Gauges

• The recording gauges produce a continuous plot of rainfall against time and provide valuable short duration data on intensity and duration of rainfall for hydrological analysis of storms. The commonly used recording gauges are:

(a)Tipping bucket type

(b)Weighing type, and

C) Natural siphon type

The weighing type is suit able form ensuring all kinds of precipitation (rain, sleet etc.).

Tipping-Bucket Type:

• The catch from the funnel falls on too of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it sand brings the-other one in position. The tipping electrically driven record one lock work-driven chart. There cord from tipping bucket gives data on the intensity of rainfall. The main advantage of this type of instrument is that it gives an electronic pulse out put that can be recorded data distance from the rain gauge.

Weighing-Bucket Type:

• The catch from the funnel empties in to a bucket mounted on a weighing scale. The weight of the bucket and its contents are recorded on a clockwork-driven chart.
• That instrument plate of the accumulated rainfall against the time, i.e. the mass curve of rainfall (accumulated precipitation against time).

Natural Siphon Type:

• This type of recording rain-gauge is also known as float type gauge here the rainfall collected by a funnel shaped collector is leading to a float chamber causing a float of rise as the float rises, a pen attached to the float through a lever system record the elevation of the float on a rotating drum driven by a dock work mechanism. A siphon arrangement empties the float chamber when the flat heads reached a pre- set maximum level which resets the pent zero, level. This type of rain gauge is adapted as the standard recording type rain gauge in India.

1.6 Introduction to Characteristics of Storm

• A storm is any disturbed state of an environment or in an astronomical body's atmosphere especially affecting its surface, and strongly implying severe weather.

• It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstorm), heavy precipitation (snowstorm, rainstorm), heavy freezing rain (ice storm), strong winds (tropical cyclone, windstorm), or wind transporting some substance through the atmosphere as in a dust storm, blizzard, sandstorm, etc.

• Storms have the potential to harm lives and property via storm surge, heavy rain or snow causing flooding or road impassibility, lightning, wildfires, and vertical wind shear.

• Systems with significant rainfall and duration help alleviate drought in places they move through.

• Heavy snowfall can allow special recreational activities to take place which would not be possible otherwise, such as skiing and snowmobiling.

• Storms are created when a center of low pressure develops with the system of high pressure surrounding it.

• This combination of opposing forces can create winds and result in the formation of storm clouds such as cumulonimbus. Small localized areas of low pressure can form from hot air rising off hot ground, resulting in smaller disturbances such as dust devils and whirlwinds.

• A strict meteorological definition of a terrestrial storm is a wind measuring 10 or higher on the Beaufort scale, meaning a wind speed of 24.5 m/s (89 km/h, 55 mph) or more; however, popular usage is not so restrictive. Storms can last anywhere from 12 to 200 hours, depending on season and geography.

• In North America, the east and northeast storms are noted for the most frequent repeatability and duration, especially during the cold period. Big terrestrial storms alter the oceanographic conditions that in turn may affect food abundance and distribution: strong currents, strong tides, increased siltation, change in water temperatures, overturn in the water column, etc.

1.7 Abstraction from Precipitation

• In engineering hydrology, runoff is the prime subject of study. Before the rainfall reaches the outlet of a basin in the form of runoff, apart of the rainfalls lots thorough various processes, such as Evaporation (E), Transpiration (T), Interception (I), Depression Storage (DS) and Infiltration (1L).

• These processes E, T, I, DS and IL are termed as abstraction from precipitation.

• This transpiration is studied in conjunction with evaporation from an area; hence the term evapotranspiration is more commonly used.

Key takeaways

• There are different forms of precipitation

Rain, snowfall, sleet, hail, drizzle, glaze

• Type of gauges

Non-recording gauges

Recording gauges

1. Tipping bucket type
2. Weighting type
3. Natural siphon type

1.8 Evaporation – Process, Measurement and Estimation

The process of transformation of liquid water into gaseous form is called evaporation.

1.8.1 Process

The rate of evaporation is dependent on

(1) The vapour pressures at the water surface and air above,

(2) Air and water temperatures,

(3) Wind speed

(4) Atmospheric pressure,

(5) Quality do water,

(6) Depth of water body an

(7) Shape and size of water body

1.8.2 Measurement

EVAPORATION MEASUREMENT

The amount of water evaporated from a water surface is estimated by the following methods:

(1) Using evaporimeter data

(2) Using empirical evaporation equations

(3) Analytical methods

Evaporimeters are water-containing pans which are espoused to the atmosphere, and the loss of water by evaporation in the m is measured at regular intervals.

TYPES OF EVAPORIMETER

• Class A Evaporation pan (US Weather Bureau)

• IS Standard Pan(Used in India)

• Colorado Sunken Pan (Pan is sunk below ground such that water level in Pan is at ground level)

• US Geological Survey Floating Pan (Simulates the characterization of large water body. The evaporimeter is kept floating in lake).

Pan Coefficient (Cp):

• Evaporation pans are not exact models of large reservoirs.
• In the above, the evaporation observed from a pan has to be corrected to obtain the value of evaporation from a lake under similar climatic and exposure conditions. Thus, a coefficient (C) is introduced as shown below.
• Lake evaporation=Cp ×pan evaporation.

• Where, Cp= pan coefficient. The values of Cp in use for different pan are given in Table.

Evaporation Stations:

• It is usual to install evaporation pans at such locations where other meteorological data are also simultaneously being collected. As per WMO recommendation.

1. Arid zones- Mino tone station for every 30,000 km2,

2. Humid temper at climates- Mino tone station for every 50,000 km2

3. Cold regions- Mino tone station for every 100000 km2

1.8.3 Estimation

Methods of estimation of evaporation may be grouped into two categories:

(1) Empirical formulae, and

(2) Analytical methods (water budget method, energy balance method, mass transfer method).

1. Empirical Formulae:

• Empirical Formulae are based on Dalton’s law

Meyer's Formula

EL=K M (we-ea) [1+ (u9/16)]

• Where, EL=lake evaporation in mm/day;
• E w=saturated vapour pressure at the water-surface in mm of mercury; ea=actual vapour pressure of over lying air at a specified height in mm of mercury; u9=monthly mean wind velocity in km/ ha t about 9 m above the ground,

=0.36 for large deep and 0.50 for small, shallow waters.

• Often, the wind-velocity data would be available at, an elevation other than that needed in the particular equation. However, it is known that in the lowest part of the atmosphere, up to a height of about 500 m above the ground level, the wind velocity can be assumed to follow the 1/7 power law as

                         Uh=Ch1/7

• Where u h= wind velocity at a height h (in meter) above the ground in km/hr and C= constant. This equation can be used to determine the velocity at any desired level if uh is known.

2.     Analytical Methods:

• The analytical methods for the determination of lake evaporation can be broadly classified in to three categories as:

(1) Water-budget method

(2) Energy-balance method and

(3) Mass-transfer method

(1) Water-Budget Method

The water-budget method is the simplest of the three analytical methods an also the least reliable. In this method we write the hydrological continuity equation for the like and determine the evaporation from the knowledge or estimation of other variables.

P + Vis + Vig = Vos + Vog + EL + ∆S + TL.

Where, P=daily precipitation

Vis=daily surface inflow into the lake.

V i g=daily ground water inflow

V o s=daily surface outflow from the lake

V o g=daily seepage outflow

E L=daily lake evaporation

∆S=increase in lake storage in a day

TL=daily transpiration loss

(2) Energy-Balance Method

• The energy budget method uses the law of conservation of energy. The energy available for evaporation is determined by considering the incoming energy out going energy, and energy stored in the water body over a known time interval.
• From the energy available for evaporation, though value of evaporation rate is calculated.

HN=Ha + He + Hg + Hs + Hi

Where, H N=net energy received by the water surface,

HN=H c (1-r)-Ho

Hc(1-r) = incoming solar radiation into a surface

Hb = back radiation

r = reflection coefficient.

Ha = sensible heat transfer from water surface to air,

Hg = heat flux in to the ground,

Hs = heat stored in water body.

Hi = net heat conducted out of the system by water flow (adverted energy)

He = heat energy used up in evaporation =p L E L

Where, P=density of water

L = latent heat of evaporation, and

E = evaporation in mm

Key takeaway

Measurement on evaporation

1. Using evaporimeter data
2. Using empirical evaporation equation
3. Analytical methods

Estimation of evaporation

1. Empirical formula

EL=K M (we-ea) [1+ (u9/16)]

2.     analytical methods

• Water budget methods – P+ Vis + Vi g= Vo s+ Vo g+ EL+∆S+TL.

• Energy balance methods – HN=Ha+ He+ Hg+ Hs+ Hi

• Mass transfer methods -

1.9 Evapotranspiration: Measurement and Estimation

• Transpirations have been defined as the process by which the water vapor eichpes from the living plant, principally through its leaves then into the atmosphere. In an area covered with venation it will be difficult and also unnecessary from practical view point to separately evaluate evaporation and transpiration. It will be more convenient to estimate the evapotranspiration directly. Only over those areas of the earth's surface where no vegetation is present will purely evaporation occur. Evapotranspiration represents the most important aspect of the water loss in the hydrologic cycle.
• The term consumptive use will also used to denote this loss by the evapotranspiration. If decent wet is often on the market to the fully meet the wants of vegetation absolutely covering the area, the ensuing evapotranspiration is termed as potential evapotranspiration (PET). Potential evnpotranspiration doesn't depend upon the soil and plant factors however depends basically on environmental conditions factor. The important evapotranspiration occurring in the prevailing/actual condition is called as actual evapotranspiration (AET).
• Field capability is that the most amount of Water that the soil can retain against the force of the gravity. Permanent weakening purpose is the moisture content of the soil at that the plant wilts and does not recover during a wet climate. At this stage, even if the thusil contains some wet, it'll be so command by the soil grains that the roots of area don't seem to be able to extract it in decent quantities to sustain the plants and consequently a plants wilt. The fled capability and permanent wiling purpose rely on the varied soil characteristics. The distinction between these 2 moisture contents is termed as on the market water (moisture available for plant growth).
• If the soil moisture is at the sector capacity AET =PET If the water system is also below PET, the soil dries place and therefore the magnitude relation AET/PET would be below unity.
• The decrease of the ratio AET PETith on the market wet depends upon the sort of soil and the rate of drying generally, for the clayey soils, ABT/PET= 1.0 for nearly 50% drops during a available moisture. Once a soil moisture reaches the permanent weakening point, the AET are going to be reduces to zero.

1.9.1 Measurement

The measuring of the evapotranspiration for given vegetation sort got to be dispensed in 2 ways:

Either by mistreatment lysimeter or by a use of the sphere plot.

Phytometer

• Measures solely transpirations

Lysimeter

• A lysimeter (also called evapotranspirometer) consists of the circular will crammed with the soil and individual crops or natural vegetation, that the evapotranspiration is needed. It's buried in order that its high flush with the encompassing ground surface. The perimeters of the lysimeters are runproof whereas very cheap will be pervious. Water passing through the soil column would be collected at the bottom Associate in Nursingd conducted through atiny low tube to the activity gauge in an adjacent pit. Evapotranspiration are calculated as;

                 AET = W s i +Wad –W c –W s f.

           Wsi = original wt of instrumentation+soil+plant+water moisture

           Wad = water value-added

           W c = water collected at bottom

           Wsf = final wt of container content

Field Experimental Plots:

In this methodology an irrigation plot is choosen and also the amounts of water added to the irrigation plot by the approach of precipitation and irrigation are measured at the side of the runoff.

The wet content in Associate in varied layers of the soil inside the foundation zone depth are measured each at starting and finish of the crop season. Then the evapotranspiration will be computed as:

                             ET = I -Q - ∆S

Where i'll be the entire flow in millimeter as well as precipitation and irrigation water, Q will be the total surface runoff in mm and ∆S will be the rise in soil moisture storage in mm.

1.9.2 Estimation

Potential evapotranspiration would be observed mistreatment Penman's equation and some empirical formulae.

Penman's equation will be based on combination of both the energy balance and mass transfer approaches.

             PET=A (H n) +Ea Y/ (A+Y)

PET = daily potential evapotranspiration, in mm per day,

A = slope of a saturation vapor pressure vs temperature curve at the mean air temperature, in mm of mercury per 0C

H n = net radiation, in mm, of evaporable water per day,

Ea = parameter including wind velocity and saturation deficit, and

Paychometric constant = 0.49 mm of mercury/°C.

Empirical Formulae

Blaney-Criddle formula

This is a purely empirical formula wich is based on data from arid western United States. This formula assumes that the PET is said to the hours of sunshine and temperature, which can be taken as a live of the radiation on the given area. The potential evapotranspiration within the crop-growing season is given by the subsequent equation,

             Er =2.54 K

Er = PET in a very crop season, in cm,

K= empirical coefficient, counting on the sort of the crop, month and locality

=sum of monthly consumptive use factors for the p

Ph = monthly p.c of annual day-time hours, depending on the latitude of the place and

Tf = mean monthly temperature, in °F.

Isopleths

The lines on the map through places having equal depth of evapotranspiration.

Key takeaways

For measure the evapotranspiration there are 2 ways

1. Phytometer
2. Lysimeter

AET = W s i +Wad –W c –W s f.

For estimation of evapotranspiration

Penman equation

             PET=A (H n) +Ea Y/ (A+Y)

Blaney-criddle formula

             Er =2.54 K

1.10 Initial Losses – Interception & Depression Storage

1.10.1 Interception

• Interception is that portion of total precipitation that, where as falling on the surface of the earth, is intercepted by the surfaces of buildings vegetation cowl on the bottom, road sand pavement and soon and after lost by evaporation.

• The 3 main elements of interception by vegetal cover are outlined below:

Interception loss: Water which is preserved on a surface, as mentioned above, and which is later gaseous away.

Through fall: Water which can drip through comes down from the leaves, etc. on to the ground surface.

Stem Flow: Water which trick lesson the branches and eventually down the pain trunk onto the bottom surface.

• Thus, it's solely the interception loss that doesn't reach the ground primary water loss.

• It was found that ever green trees have additional interception loss than that decide us ones. Also, dense grasses have nearly same interception loses as full-growth trees and might account for nearly 20% of the entire precipitation with in the season. Agricultural crops in the season additionally contribute high interception losses.

1.10.2 Depression Storage

• When the precipitation of the storm reaches a ground, it must first fill up all depressions before it can be flow over the surface. The volume of the water trapped in this deprecation is called as depression storage.

• Rainfall held in the depressions does not have other surface runoff unless the sea refilled to capacity. This amount is eventually lost through processes of infiltration and evaporation and thus forms apart of a initial loss.

1.11 Infiltration – Process, Capacities Indices, Measurement & Estimation

Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration.

1.11.1 Process

• Infiltration is that process by which precipitation moves own ward through the surface of the earth and replenishes soil moisture, recharges aquifers, and ultimately supports stream flows during dry periods. A distinction is to be made between the terms in filtration and percolation, the latter being used to describe the downward flow of water through the 20 of aeration towards the watertable, the former being restricted to the entry of water through the surface layers of the soil.

• Along with interception, depression storage, and storm period evaporation (evaporation during rainfall), infiltration determines the availability, if any, of the precipitation in put for generating, over land flows.

(Infiltration+ Depression storage+ Interception)= (Rainfall-Runoff)

• During a major storm, capable of producing a flood, evapotranspiration loss is generally negligible and losses by interception and depression storage are small compared to infiltration.

Hence infiltration+0+0= Rainfall-Runoff

• Infiltration continues as long as there is a supply of water at the soil surface either by direct precipitation or by a flowing sheet of water If the intensity of rainfall is lees, all the water infiltrating in to the soil gets stored as soil moisture and does not contribute other ground water flow. If how ever the rainfall intensity is more, the soil gets saturated and thereafter contribution to ground water flow starts.

1.11.2 Capacities Indices

• In hydrological calculations involving floods, it is found convenient to use constant value of infiltration rate for the duration of the storm. The average infiltration rate is called infiltration index and two types of indices, in these regards are in common use.

(1)  -index

• The index is the average rainfall above which the rainfall volume is equal to the runoff volume

• Index is average infiltration rate during the period of rainfall excess. Rainfall excess is the rainfall contributing to runoff and the period during which such a rainfall takes place is called period- of rainfall excess.

• The index is derived from the rainfall hyetograph with the knowledge of the resulting runoff volume. The initial loss is also considered as a part of infiltration

If I <index, f= I and f>index, f=index.

Where, I is rainfall intensity and f is infiltration rate.

• In estimating the maximum floods for design purposes, in the absence of any other data, an index value of 0.10 cm/h can be assumed.

      (2) W-Index

• The W-index are fined version of index. It excludes the depression storage and interruption from the total losses. It is the average infiltration rate during the time rainfall intensity exceeds the capital rate. That is

          W=F/t= (P-Q-S)/t

Where F is the total infiltration,

 t is time during which rainfall intensity exceeds infiltration capacity,

 R is total precipitation corresponding to t,

 Q is the total storm runoff and

 S is the volume of depressions to rage and interception.

W-index is always less than index (W-index<index).

Factors Affecting Infiltration

• Infiltration capacity (fc) is defined as the maximum rate at which rain can be absorbed by a soil in a given condition.

• The infiltration process is affected by a large number of factors as discussed below.

Rainfall Characteristics:

• The actual rate of infiltration, f, at a given time can be expressed as

f = fc when i>fc

f = I, when i<f c

• Where, I is intensity of rainfall and
• f c is the infiltration capacity at a given time;
• i, f and f c are expressed in cm/hr or mm/minute. The infiltration capacity (f c) of as oil is high at the beginning of a storm and has an exponential decay as the time elapses.

Characteristics of Soil:

• A loose permeable sandy soil will have a larger infiltration capacity than a light clayey soil. Clayey soils can be rendered virtually impermeable due to raindrop compaction, where as clean sandy soils are much less susceptible to rain compaction.

• A dry soil can absorb more water than the soil whose pores are already full of water.

Surface Cover:

• A vegetation cover tends to increase infiltration by:

(1)Retarding surface flow, and thus allowing more time for water to enter the soil,

(2)Shielding the soil surface from direct impact of raindrops, there by reducing surface compaction,

Spread of building sand paved surfaces in urban a reason effectively reduces the infiltration capacities, of various patches of ground to zero and thus contributes significantly to the frequency off flood peaks in such areas.

Characteristics of infiltrating Water:

• Viscosity of water and, therefore, the ease with which it may move through so pore spaces, varies with water temperature. It is therefore, expected that temperature will tend to exert some influence another at of infiltration.

Key takeaways

Indices has 2 types

1. Index
2. W index

Factor affecting infiltration

1. Rainfall characteristics
2. Characteristics of soil
3. Surface cover

1.11.3 Measurement & Infiltration

Infiltration characteristics of soil, at a given location, can be obtained by conducting controlled experiment small areas. The experiment all set- up is called an infiltrometer. There are two kinds of infiltrometers:

(1) Flooding type infiltrometers, and

(2) Rainfall simulator

Note - Infiltration can also be found from hydro graph analysis

Flooding Type Infiltrometer

• This is a simple instrument consisting essentially of a metal cylinder. This cylinder is driven in to the ground to a depth of 50 cm

• Water is poured into the top part to a depth of 5 cm and a pointer is set to mark the water level. As infiltration proceeds, the volume is made up by adding water from a burette to keep the water level at the tip of the pointer. Knowing the volume of water added at different time intervals, the plot of the infiltration capacity us time is obtained.

• A major objection to this simple infiltrometer is that the un filtered water spreads at the out let from the tube (as shown by dotted lines in the figure), such the tube area is not represent active of the are a into which infiltration takes place.

• To overcome this lacuna a ring infiltrometer costing of a set of two concentric rings is used.

• In this infiltrometer two concentrating rings are inserted into the ground and the water depth is maintained on the soil surface as discussed above, in both the rings, to a common fixed level. The outer ring provides a water jacket to the infiltrating water of the are ring, and hence prevent, to a large extent, the spreading out of the infiltrating water with respect to the inner tube. The measurement of water volume is done for the inner ring only.

The main disadvantages of a flooding type infiltrometer are:

• The raindrop- impact effect is not simulated, and

• The driving of the tube or rings disturbs the soil structure

Rainfall Simulator

• Here, as small plotland, of about 2mx4m size is provided with a series of nozzle son the longer side, with arrangements to collect and measure the surface run off rate.

• The specially designed nozzles produce raindrops falling from a height of 2m, and a real so capable of producing various intensities of rainfall

• Experiments are conducted under controlled conditions with various combinations of intensity and duration, and the consequent surface runoff is measured in each case.

• Using the water budget equation involving the volume of rainfall, infiltration and runoff, the infiltration rate and its variation with time is calculated.

• If the rainfall intensity is higher than the infiltration rate, infiltration capacity values are obtained.

• Rainfall simulator type infiltrometers give lower values than the flooding type infiltrometers .This is due to the effect of rainfall impact and turbidity of the surface water present in the former.

Empirical Infiltration Equations

• Under given soil type and antecedent moisture conditions, there will be an initial infiltration rate, f0 This rate will decrease as more water gets infiltrated, finally achieving a constant, f e I e, ultimate infiltration capacity. This infiltration capacity rate prevails when the soil is saturated. The parameter s f o, f e and the decay of infiltration capacity are functions of the soil moisture conditions, vegetation, rainfall, intensity and soil surface conditions.

• Several empirical equations incorporating the above mentioned factors, affecting the behaviour of the soil have been proposed.

Key takeaways

• Basically, for measurement there are 2 types of instruments

1. Flooding type infiltrometer
2. Rainfall simulator

References

1. Techmax
2. Groundwater Hydrology by Todd D K Wiley
3. Irrigation Theory and Practice by Michael A M Vikas Publication House
4. Engineering Hydrology by Ojha Oxford University