ABSTRACTION FROM PRECIPITATION: EVAPORATION

- Lian Gilongos

Evaporation

It is the process in which a liquid changes to the gaseous state at the free surface, through the transfer of heat energy.

Heat

Evaporation is a cooling process since it requires energy to change the water into vapor, taking _______ from the water.

Vapor pressure

This can be defined as the pressure exerted by the vapour at a given temperature in equilibrium state with liquid.

Saturation Vapor Pressure in mm of mercury

𝐄𝐋 = 𝐂(𝐞𝐰 − 𝐞𝐚)
What is “ew’?

Actual Vapor Pressure in mm of mercury

𝐄𝐋 = 𝐂(𝐞𝐰 − 𝐞𝐚)
What is “ea’?

The rate of evaporation

this is proportional to the difference between the saturation vapor pressure at the water temperature, 𝑒𝑤 and the actual vapor pressure in the air, 𝑒𝑎.

Relative humidity

It is the ratio of actual vapor pressure and saturation vapor pressure.

Increase

The rate of evaporation decreases with an __________ in the Relative Humidity

Kinetic Energy

Temperature has a direct effect on the rate of evaporation because it determines the _________ of liquid molecules.

Wind

This aids in removing the evaporated water vapor from the zone of evaporation and consequently creates greater scope for evaporation.

Lower

(Lower or Higher) atmospheric pressure increases the rate of evaporation because there is less external force opposing the escape of liquid molecules into the vapor phase.

High

(Low or High) atmospheric pressure suppresses evaporation by exerting greater resistance on escaping molecules.

reduction

When a solute is dissolved in water, it causes __________ in the rate of evaporation.

more

Deep water bodies have (lesser or more) heat storage than shallow ones.

Seasonal Evaporation Shift

The phenomenon where lake depth dictates when evaporation peaks; deep lakes shift their peak evaporation toward winter, while shallow lakes evaporate more readily in summer.

Evaporimeters

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

Solar Energy

These are noted along with evaporation measurement: (Find the one that doesn’t belong)
- humidity
- wind movement
- air and water temperatures
- precipitation
- solar energy

CLASS A EVAPORATION PAN

is a standard, 120.7 cm diameter, 25.4 cm deep, galvanized iron or stainless steel cylinder used to measure daily evaporation rates, typically installed on a wooden platform to allow air circulation.

ISI STANDARD PAN

a type of evaporimeter: Similar to the Class A pan but follows Indian Standards Institute (ISI) specifications. • Typically smaller than Class A, with exact material, dimensions, and placement requirements. Standardized measurement for hydrological studies and irrigation planning.

COLORADO SUNKEN PAN

a type of evaporimeter: a 920 mm (3 ft) square, 460 mm (18 in.) deep, unpainted galvanized iron pan used for measuring evaporation. It is buried in the ground to within 5–10 cm (2–4 in.) of its rim, causing its evaporation rates to better mimic large lake surfaces compared to above-ground pans.

The pan coefficient (Kp)

It is a correction factor used to convert the evaporation measured from a pan into the actual evaporation from a natural water body (like a lake, reservoir, or river)

0.70

What is the value of the Pan Coefficient of Class A Land Pan?

0.80

What is the value of the Pan Coefficient of USGS Floating Pan and ISI Pan (modified Class A)?

0.78

What is the value of the Pan Coefficient of Colorado Sunken Pan?

30,000 km^2

It is usual to install evaporation pans in such locations where other meteorological data are also simultaneously collected. The WMO recommends the minimum network of evaporimeter stations as below:

1. Arid zones-One station for every __________

50,000 km

It is usual to install evaporation pans in such locations where other meteorological data are also simultaneously collected. The WMO recommends the minimum network of evaporimeter stations as below:

2. Humid temperate climates-One station for every ___________

100,000 km^2

It is usual to install evaporation pans in such locations where other meteorological data are also simultaneously collected. The WMO recommends the minimum network of evaporimeter stations as below:

3. Cold regions-One station for every ___________

Lake Evaporation

what is the “EL” in this equation?

monthly mean wind velocity at about 9 m above ground

what is the “u9” in this equation?

0.36 and 0.50

in KM, give me the value of the coefficient accounting for large deep water and for small shallow waters, respectively.

0.6 m

in 𝜇0 = mean wind velocity in km/h at ground level, which can be taken to be the velocity at __________ height above ground

uh = Ch^1/7

This equation can be used to determine the velocity at any desired level if uh(wind velocity at a height h above the ground) is known

WATER-BUDGET METHOD

an analytical method of evaporation: it is the simplest of the three analytical methods and is also the least reliable. It involves writing the hydrological continuity equation for the lake and determining the evaporation from a knowledge or estimation of other variables.

ENERGY-BUDGET METHOD

an analytical method of evaporation: It is an application of the law of conservation of energy. The energy available for evaporation is determined by considering the incoming energy, outgoing energy and energy stored in the water body over a known time interval.

MASS-TRANSFER METHOD

an analytical method of evaporation: this is based on theories of turbulent mass shift in boundary layer to calculate the mass water vapor transfer from the surface to the surrounding atmosphere. However, the details of the method are beyond the scope of this book and can be found in published literature. With the use of quantities measured by sophisticated (and expensive) instrumentation, this method can give satisfactory results.