Subject- Hydrology And Water Resources Engineering.

Year:- 3rd (Civil Engineering)

Complied by :140073106005Mehul B. Dashadiya

Guided By :Prof. J. R. Pitroda sir

Gujarat Technological University

BIRLA VISHVAKARMA MAHAVIDYALAYA

EVAPORATION EVAPOTRANSPIRATION INFILTRATION

TOPIC

Evaporation is a process by which water is converted into gaseous state and is returned to the atmosphere as Vapour.

It is the process by which water in the liquid form transforms into vapour through the transfer of energy.

When water is converted from solid state to vapour state without passing through liquid state then it is called Sublimition.

Due to evaporation loss of water in the form of vapour from soil, snow, lakes, rivers, seas, reservoirs.

EVAPORATION

• Rate of evaporation dependent on

1. Vapour pressure at water surface and air above,2. Air and water temperature3. wind speed4. Atmospheric pressure5. Quality of water6. Size of water body• Rate of evaporation increase with increase of temperature as

well as increase in wind velocity, but decrease with the increase in humidity as well as with increase of dissolved salts in water.

• Further as open area of water and soil is more evaporation also more and vice versa.

John dalton was first scientist to describe the process of evaporation scientifically.

Evaporation rate varies directly with difference of vapour pressure between air and water.E= C (ew – ea)

where,E= rate of evaporation (mm/day)ew & ea=vapour pressure in water and in air.C = constant

This equation is called john dalton’s law of evaporation.

Dalton’s law of Evaporation

i. The vapour pressure at the water surface and air above

ii. Air and water temperaturesiii. Wind speediv. Atmospheric pressurev. Quality of watervi. Size of water bodyvii. Depth of water in the water bodyviii.Humidityix. Radiation

Factor affecting evaporation

1. Empirical formulae2. Water budget method3. Energy budget method4. Mass transfer method5. Actual observations6. Pan observations

Measurement of Evaporation

Transpiration is the process by which water leaves the water body of a living plants and reaches the atmosphere as water vapour. The water is taken up by the plant root system and escape through the leaves.

The soil water used by crops is through the process of evaporation. This loss of water in combined form of evaporation and transpiration is called EVAPOTRANSPIRATION (ET). Water use by evapotranspiration is known as consumptive use of crops.

Evapotranspiration of water by a crop is the depth of water consumed by evaporation and transpiration during crop growth.

EVAPOTRANSPIRATION

When consumptive use of crop is known, the water use of large unit can be calculated.

Consumptive use of for each crop is can be determined which gives water requirement of crop. The value of consumptive use of water varies from crop to crop and also for the same crop it varies with time as well as place.

Detailed study of evapotranspiration is essential in in the design of reservoir, irrigation canals, water balance on earth surface and projects relating to water.

Factors affecting evapotranspiration are same that affect evaporation and transpiration. Some of the factor are as follows.

1. Metrological factors : potential evapotranspiration is controlled essentially in metrological factor. Evapotranspiration increase as temperature, sunshine, and wind velocity increase but decrease as the humidity decrease.

2. Plant and soil factors: Evapotranspiration is less if adjoining land is

cropped because air becomes cool and more humid. The greater density of vegetation, the greater is

the evapotranspiration

Factor affecting Evapotranspiration

1) Direct measurement of consumptive use of water:

a) Tank or lysimeter methodb) Field experimental plotsc)soil moisture studiesd)Integration methode)Inflow and outflow method

2) Use of Empirical Formulae :a) Blaney – Criddle equationb) Thornthwaite equationc) Christiansen equation

Measurement of Evapotranspiration or Consumptive use of water

The downward flow of water from the land surface into the soil medium is called infiltration.

The part of precipitation that is not available as surface runoff is referred as precipitation loss or abstraction.

Infiltration is a dominant abstraction. Infiltration first replenishes the soil moisture

deficiency, the excess water moves downward by force of gravity. This downward movement under gravity is called percolation (seepage).

The rate at with water enters the ground surface and then flow downwards is known as infiltration rate.

INFILTRATION

Infiltration rate is higher in the beginning because it has to meet the requirement of dry soil. It attains steady constant value as passage of time.

Infiltration rate is used for computation of water loss due to infiltration for the determination of surface runoff.

Unit of infiltration rate is mm/hour. Maximum rate at which soil in any given condition

is capable of absorbing water is called its infiltration capacity(f).

Infiltration has great influence on rainfall, runoff, transpiration of plants and evaporation from soil surface.

Infiltration is responsible for subsurface flow and ground water flow.

1. Soil moisture2. Intensity of rainfall3. Temperature 4. Duration of the rainfall5. Texture of soil6. Vegetal cover7. Condition of soil surface8. Compaction of soil9. Human activities10.Depth of surface detention

Factor affecting Infiltration

Infiltration capacity of soil does not remain constant, it decrease with an increase of duration of rainfall.

In hydrological computations for computing surface runoff and flood discharge, use an average constant value of infiltration rate called infiltration index.

Infiltration Indices

Figure is superimposition of a rainfall hyetograph and infiltration rate curve appropriate to the soil of catchment.

The area of hyetograph above infiltration curve represent the surface runoff and that of below represent the infiltration.

If the rainfall intensity is always greater than the infiltration rate, results are satisfactory, but if rainfall fluctuate above and below the problems becomes complicated. In this case is assumed to be constant(average infiltration) throughout the rainfall time.

Infiltration indices are extremely useful for the analysis of major floods when the soil is wet and infiltration rate becomes constant.

1. Ø-index (phi-index)2. W-index3. Wmin.-index.

Types of infiltration indices

Thank you. . .