Evaporation

Question 1

the hydrologic definition of evaporation is restricted to the net rate of vapor transport to the atmosphere, hence it is common to define evaporation as ________________.

Answer 1

the net rate of vapor transfer

Question 2

Although there is always continuous exchange of _______________ to and from the atmosphere, the ___________ definition of evaporation is restricted to the net rate of vapor transport to the atmosphere, hence it is common to define evaporation as the net rate of vapor transfer.

Answer 2

water molecules, hydrologic

Question 3

_______________is the process by which water is transferred from the land and water masses of the earth to the atmosphere.

Answer 3

Evaporation

Question 4

__________________is the evaporation counterpart for plants. It is the process by which ______________is taken up by ____________ is eventually evaporated as it exits at plant pores.

Answer 4

Transpiration, soil moisture, vegetation

Question 5

Evaporation and transpiration combined (evapotranspiration) generally constitute the __________________________________________.

Answer 5

largest component of losses in rainfall-runoff sequences.

Question 6

Evaporation from a particular surface is directly related to the opportunity for evaporation (availability of water) provided by that surface. For open bodies of water, evaporation opportunity is ________ percent, while for soils it varies from a ___________percent when the soil is highly saturated- for example, during storm periods- to essentially ___________at stages of very low _____________.

Answer 6

100, high of 100, zero , moisture content

Question 7

Evaporation is important in all areas of water resources because it affects:

Answer 7

-the capacity of the reservoir
-the yield of river basin
-the consumptive use of water by plants

Question 8

Factors Affecting Evaporation

Answer 8

-Meteorological factors
-The nature of the evaporating surface

Question 9

Meteorological Factors

Answer 9

Solar Radiation
- evaporation is a process of energy exchange. Solar radiation supply the energy necessary for the liquid water molecules to evaporate.

Relative Humidity
- as the humidity of air increases, its ability to absorb more water decreases, and the rate of evaporation becomes slower.

Air Temperature
- temperature increases saturation of vapor pressure (increases saturation deficit). High temperature implies that there is energy available for evaporation.

Wind
- as the liquid water vaporizes from a water body, the air adjacent to this body will be saturated. For continuous evaporation, this saturated air must move, this is possible by wind. A change in wind in the surrounding body of water will likely change the rate of evaporation.

Atmospheric Pressure
- an increase in atmospheric pressure prevent the movement of molecules out of water.

Question 10

Nature of the Evaporating Surface

Answer 10

Temperature of liquid water
- high liquid temperature, high molecular motion in the water, meaning the number of molecules leaving the water body will also be high.

Salinity
- is the saltiness or amount of salt dissolved in a body of water, called saline water. Adversely affects evaporation, decreases the rate of evaporation.

Aerodynamic characteristics of the surface
- roughness, texture or size of the surface.

Reflection Coefficient of the Surface
- also known as albedo (whiteness), the higher the albedo, the lower the rate of evaporation from the surface.

Question 11

Measurement Methods

Answer 11

The water loss from a standard saturated surface is measured with evaporimeters, which may be classified as atmometers and pan or tank evaporimeters. These instruments do not directly measure either evaporation from natural water surfaces, actual evapotranspiration or potential evapotranspiration. The values obtained cannot, therefore, be used without adjustment to arrive at reliable estimates of lake evaporation or of actual and potential evapotranspiration from natural surfaces.

Question 12

ATMOMETERS

Answer 12

An atmometer is an instrument that measures the loss of water from a wetted, porous surface. While it may be possible to relate the loss from atmometers to that from a natural surface empirically, a different relation may be expected for each type of surface and for differing climates. Atmometers are likely to remain useful in small-scale surveys. Their great advantages are their small size, low cost and small water requirements.
One of the major problems in the operation of atmometers is keeping the evaporating surfaces clean. Dirty surfaces will affect significantly the rate of evaporation.

Question 13

EVAPORATION PANS AND TANKS

Answer 13

Evaporation pans or tanks have been made in a variety of shapes and sizes and there are different modes of exposing them. Among the various types of pans in use, the United States Class A pan, the Russian GGI-3000 pan and the Russian 20 m2 tank. These instruments are now widely used as standard network evaporimeters and
their performance has been studied under different climatic conditions over fairly wide ranges of latitude and elevation.

Question 14

United States Class A pan - The United States Class A pan is of cylindrical design,______________________________. The bottom of the pan is supported ____________ above the ground level on an open-frame wooden platform, which enables air to circulate under the pan, keeps the bottom of the pan above the level of water on the ground during rainy weather, and enables the base of the pan to be inspected without difficulty.

Answer 14

25.4 cm deep and 120.7 cm in diameter,
3 to 5 cm

Question 15

Russian GGI-3000 pan - The Russian GGI-3000 pan is of cylindrical design, with a surface area of _________________________________. The bottom of the pan is cone-shaped. The pan is set in the soil with its rim _____________________ In the centre of the tank is a metal index tube upon which a __________________ is set when evaporation observations are made.

Answer 15

3 000 cm2 and a depth of 60 cm,
7.5 cm above the ground,
volumetric burette

Question 16

Russian 20 m2 tank - This tank has a surface of _____________________________; it is cylindrical with a flat bottom and is ______ deep. It is made of ____________ thick welded iron sheets and is installed in the soil with its rim 7.5 cm above the ground. The inner and exposed outer surfaces of the tank are painted white. The tank is provided with a replenishing vessel and a stilling well with an index pipe upon which the volumetric burette is set when the water level in the tank is measured. Inside the stilling well, near the index pipe, a small rod terminating in a needle point indicates the height to which the water level is to be adjusted.

Answer 16

20 m2 and a diameter of about 5m,

2m,

4-5mm

Question 17

The rate of evaporation from a pan or tank evaporimeter is measured by the change in level of its free water surface. Several types of automatic evaporation pans are in use. The water level in such a pan is kept constant by releasing water into the pan from a storage tank or by removing water from the pan. when precipitation occurs. The amount of water added to, or removed from, the pan is recorded. In some tanks or pans, the level of the water is also recorded continuously by means of a float in the stilling well. The float operates a recorder.

Question 18

Methods of Estimating Evaporation

Answer 18

Water Budget or Storage Equation Approach - This method con­sists of drawing up a balance sheet of all the water entering and leaving a particular catchment or drainage basin. If rainfall is measured over the whole area on a regular and systematic basis then a close approximation to the amount of water arriving from the atmosphere may be made.

Energy budget method - This method, like the water budget approach, involves solving an equation which lists all the sources and sinks of thermal energy and leaves evaporation as the only unknown. It involves a great deal of instrumentation and is still under active development. It cannot readily be used without much data that is not normally available and so it is a specialist approach.

Empirical formulae - Many attempts have been made to produce satisfactory formulae for the estimation of evaporation. These are usually for evaporation from an open water surface, as indeed, are the more general methods to follow. The reason for this is simple. Evaporation if it is to take place, presupposes a supply of water. Whatever the meteorological conditions may be, if there is no water present, then there can be no evaporation. Accordingly, estimating methods using meteorological data work on the assump­tion that abundant water is available, i.e. a free water surface. The results obtained therefore are not necessarily a measure of actual, but of potential evaporation.

Question 19

Transpiration

Answer 19

Growing vegetation of all kinds needs water to sustain life, though different species have very different needs. Only a small fraction of the water needed by a plant is retained in the plant structure. Most of it passes through the roots to the stem or trunk and is transpired into the atmosphere through the leafy part of the plant.

Transpiration is the process of water loss from plants through stomata.

Question 20

Evapotranspiration

Answer 20

Is the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere. In field conditions it is practically impossible to differentiate be­tween evaporation and transpiration if the ground is covered with vegetation. The two processes (evaporation and transpiration) are commonly linked together and referred to as evapo-transpiration.

Question 21

Evaporation Control

Answer 21

Evaporation losses can be greatly significant at any location. Consequently, the concept of evaporation reduction is receiving widespread attention. Evaporation losses from soils can be controlled by employing various types of mulch or by chemical alteration. They may be reduced from open waters by:

-storing water in covered reservoirs

-making increased use of underground storage

-controlling aquatic growths

-building storage reservoirs with minimal surface area

-use of chemicals

-conveying in closed conduits rather than open channels

Question 22

Evapo-transpiration Control

Answer 22

Water conservation through transpiration reduction is being seriously studied, and certain preventative practices are presently in use. Methods of control include the use of chemicals to inhibit water consumption (analogous to the use of films to control surface evaporation except that chemicals are applied in the root zone). harvesting of plants, improved irrigation practices, and actual removal of certain vegetative types.