LECTURE 5 EVAPORATION

Evapotranspiration

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

evaporation, transpiration, infiltration, surface detention and storage.

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

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.

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)

Wind

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

atmospheric pressure

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

increase

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

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.

Deep water

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

effect of heat storage

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

Evaporimeters

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

Meteorological data

noted along with evaporation measurement in evaporimeters

Class A Evaporation Pan

A pan of diameter 1210mm and depth 255mm

1210mm and depth 255mm

diameter of Class A Evaporation Pan

18 and 20cm

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

unpainted GI sheet

Class A Evaporation Pan is made of ____________

15cm

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

Evaporation

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

ISI Standard Pan

A pan of diameter 1220mm and depth 255mm

1220mm and 255mm

diameter of ISI Standard Pan

0.9mm

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

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

A fixed-point gauge

it indicates the level of water

Colorado Sunken Pan

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

Colorado Sunken Pan

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

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.

US Geological Survey Floating Pan

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

900mm sides and 450mm deep

dimension of US Geological Survey Floating Pan

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

75mm

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

Diagonal baffles

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

US Geological Survey Floating Pan

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

Evaporation Pan

are not exactly models of large reservoirs

Water–Budget Method

simplest but the least reliable.

Energy–Budget Method

application of law of conservation of energy.

Reduction of surface area

as the area increases the rate if evaporation also
increases

Mechanical covers

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

Chemical films

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