LECTURE 5 EVAPORATION

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Evapotranspiration

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Evapotranspiration

Evaporation from water bodies and soil masses together with transpiration from vegetation

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evaporation, transpiration, infiltration, surface detention and storage.

All abstractions from precipitation are those due to ____________, ___________, _____________, ________________, and ____________

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Evaporation

process in which a liquid change to the gaseous state at the free surface, below the boiling point through the transfer of heat energy.

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Rate of Evaporation

proportional to the difference between the saturation vapor pressure (SVP) at the water temperature, (ew ) and the actual vapor pressure in the air (ea)

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Wind

it helps in removing the evaporated water vapor from the zone of evaporation, thereby creating greater scope for evaporation.

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atmospheric pressure

a decrease in _________________ (as in high altitudes areas) increases the evaporation rate.

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increase

The rate of evaporation increases with an __________ in the water temperature

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solute, less than

When a ______ is dissolved in water, the vapor pressure of the solution is _______ that of pure water and hence it causes reduction in the rate of evaporation.

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Deep water

________ bodies have more heat storage capacity than shallow water bodies.

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effect of heat storage

The _________________ is to change the seasonal evaporation rates and the annual evaporation remains more or less unaltered.

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Evaporimeters

water-containing pans which are exposed to the atmosphere and the loss of water by evaporation measured in them at regular intervals.

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Meteorological data

noted along with evaporation measurement in evaporimeters

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Class A Evaporation Pan

A pan of diameter 1210mm and depth 255mm

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1210mm and depth 255mm

diameter of Class A Evaporation Pan

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18 and 20cm

Depth of water in Class A Evaporation Pan is maintained between _____________

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unpainted GI sheet

Class A Evaporation Pan is made of ____________

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15cm

placed on a wooden platform of height _____ above ground level to allow free air circulation below the pan

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Evaporation

measured by measuring the depth of water in a stilling well with a hook gauge.

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ISI Standard Pan

A pan of diameter 1220mm and depth 255mm

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1220mm and 255mm

diameter of ISI Standard Pan

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0.9mm

The ISI Standard Pan is made of copper sheet _____ thick, tinned inside and painted white outside

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1225mm, 100mm

The ISI Pan is placed on a square wooden platform of width _______ and height ________ above ground level to allow free air circulation below the pan

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A fixed-point gauge

it indicates the level of water

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Colorado Sunken Pan

920mm square pan made of unpainted GI sheet, 460mm deep, and buried into the ground within 100mm of the top

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Colorado Sunken Pan

this pan's main advantage is its aerodynamic and radiation characteristics are similar to that of a lake

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Colorado Sunken Pan

this pan's disadvantages are difficult to detect leaks, expensive to install, extra care is needed to keep the surrounding area free from tall grass, dust, etc.

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US Geological Survey Floating Pan

A square pan of 900mm sides and 450mm deep.

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900mm sides and 450mm deep

dimension of US Geological Survey Floating Pan

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US Geological Survey Floating Pan

Supported by drum floats in the middle of a raft of size 4.25m x 4.87m, it is set afloat in a lake with a view to simulate the characteristics of a large body of water

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75mm

Water level in the pan is maintained at the same level as that in the lake, leaving a rim of _____.

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Diagonal baffles

provided in the pan to reduce surging in the pan due to wave action

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US Geological Survey Floating Pan

this pan's disadvantages are High cost of installation and maintenance, difficulty in making measurements.

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Evaporation Pan

are not exactly models of large reservoirs

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Water–Budget Method

simplest but the least reliable.

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Energy–Budget Method

application of law of conservation of energy.

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Reduction of surface area

as the area increases the rate if evaporation also increases

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Mechanical covers

permanent roods over the reservoir, temporary roods and floating roof such as rafts and light – weight floating particles

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Chemical films

application of cetyl alcohol (hexadecanol) and stearyl alcohol (octadecanol)

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