Evaporation

Evaporation refers to the process in which a liquid changes to the gaseous state at the free surface, below the boiling point through the transfer of heat energy. In hydrologic cycle, evaporation mainly occurs from ocean, but it can also occurs from landmass and during rainfall event. Thus, evaporation from water bodies and soil masses together with transpiration from vegetation is commonly termed as evapotranspiration.

physics of evaporation

Evaporation is a phase change that occurs when heat is induced in a water system and temperature starts to rise. Water molecules which is composed of light and heavy molecules is being bonded by Intermolecular Forces of Attraction (IMF). Similarly, the atmosphere in the immediate neighbourhood of the water surface contains water molecules within the water vapor in motion and some may penetrate the surface. The net escape of water molecules from the liquid state to the gaseous state is what constitutes the evaporation

factors affecting evaporation

Evaporation is a continuous process which is dependent on two factors, namely:

Physical Factors
Meteorological Factors

PHYSICAL FACTORS

METEOROLOGICAL FACTORS

FACTORS AFFECTING EVAPOTRANSPIRATION

meteorological factors

The methods for calculating evapotranspiration using meteorological data require various climatological and physical parameters.
Meteorological factors include the following:

Solar radiation
Air Temperature
Air Humidity
Wind Speed

atmospheric parameters

Atmospheric Parameter consists of the following:

Atmospheric Pressure
Latent Heat of Vaporiation
Psychometric Constant
Air Humidity
Vapor Pressure
Saturation Vapor Pressure
Dewpoint Temperature
Relative Humidity

sample problems

measurements of different factors for evaporation

Estimation of evaporation is of utmost importance in many hydrologic problems associated with planning and operations of reservoirs and irrigation systems. However, the exact measurement of evaporation in large bodies of water will be too cumbersome to approximate

Evaporation pans are water containing pans that are exposed to the atmosphere and the loss of water by evaporation is measured in regular intervals. Meteorological data such as humidity, wind movement, air, and water temperatures and precipitation are also noted together with evaporation measurement
Shown in the left figure are the most common types of evaporation pans used and the measuring principles for obtaining pan evaporation.

In order to calculate the pan coefficient, the following equation and irrigation & drainage paper are used, as shown in the right figure.

sample problems

methods for estimating evaporation from open water

Various number of empirical equations are used to estimate evaporation in the bodies of water using the available meteorological data. The most commonly used formulas for estimating evaporation are the Energy Balanced Method and Aerodynamic Method. The formulas of the said empirical methods are provided in the right figure.

sample problems

ENERGY BALANCE METHOD

AERODYNAMIC METHOD

assessment tasks:

PROBLEM SET # 3

Evaporation Problem Set_BARTOLOME.pdf

QUIZ # 3

Evaporation.pdf

VIDEO PRESENTATION LINK: https://youtu.be/mv3OqBNLPoU

REFLECTION:

After watching the video presentation regarding evaporation, I have grasped several learnings and knowledge on how evaporation cyclically initiates hydrologic cycle processes. For example, evaporation is controlled by various factors, which include net radiation, wind speed, atmospheric humidity, air temperature, etc. Thus, it is an endothermic process. Studying evaporation is significantly important in understanding hydrology due to the correlation between evaporation losses and the water budget equation, which constitutes all of the major processes in a hydrologic cycle. Also, I learned that there is no standard potential equation to exactly compute for evaporation due to the complexity of data and its corresponding factors. That includes radiation, humidity, heat flux, wind velocity, and such. Luckily there are certain empirical methods that can be used to approximate evaporation data from open water such as energy budget and aerodynamic method.

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