The work discusses the principles of evaporation as one of the processes in the hydrologic cycle and the different methods to estimate the evaporation rate. Four different methods namely the energy balance, aerodynamics, combination method and Priestley-Taylor method are included in the discussion. In the end there are some examples with solutions.
Evaporation is the process that liquid water enters the atmosphere as water vapour and replenishes the humidity of the air. It is also considered as the pathway in which the water travels from its liquid state back into the hydrologic cycle as an atmospheric water vapor. Water bodies such as oceans, seas, lakes and rivers provide nearly 90 % of the moisture in the atmosphere through evaporation, as some study reveals.
TABLE OF CONTENTS
1.0 Introduction
2.0 Evaporation
3.0 Principles of Evapotranspiration
4.0 Estimation Methods of Evaporation
4.1 Energy Balance Method
4.2 Aerodynamic Method
4.3 Combination Method
4.4 Priestley-Taylor Method
5.0 Sample Problems
References:
1.0 Introduction
Evaporation is the process that liquid water enters the atmosphere as water vapour and replenishes the humidity of the air.1 It is also considered as the pathway in which the water travels from its liquid state back into the hydrologic cycle as an atmospheric water vapor. Water bodies such as oceans, seas lakes and rivers provide nearly 90% of the moisture in the atmosphere through evaporation, as some study reveals.2
The ebook discusses the principles of evaporation as one of the processes in the hydrologic cycle and the different methods to estimate the evaporation rate. Four (4) different methods namely; the energy balance, aerodynamics, combination method and priestley-taylor method are included in the discussion.
2.0 Evaporation
Evaporation occurs when water is heated, thus the evaporation process needs the heat from the sun. Evaporation also helps form the clouds, and then the clouds will release the moisture in the form of rain or snow. When precipitation falls, it is again waiting to be evaporated, and that is how the water cycle starts all over again.3 In a simple point of view, evaporation is the process by which liquid turns into gas.4
Two factors influencing evaporation:5
- Supply of energy to provide the latent heat of vaporization (Solar radiation)
- Wind velocity and specific humidity,
Meteorological factors affecting evaporation:6
1) Solar radiation
2) Wind
3) Relative humidity
4) Temperature
3.0 Principles of Evapotranspiration
Vegetation needs water, and its water demand differs on its kind of specie. The water required by a certain vegetation to grow passes through its roots and to the stem and eventually transpired into the atmosphere. If the land surface is covered with vegetation, the evaporation and transpiration simultaneously occurs which is known as the process of evapotranspiration.7
Evapotranspiration is the collective process of evaporation from the land surface and the transpiration from vegetation.8 Aside from the two factors influencing evapotranspiration as mentioned previously, evapotranspiration is also influenced by another factor which is the supply of moisture at the evaporatove surface.9 In general, the term evapotranspiration is the totality of evaporation and transpiration.
The Potential Evapotranspiration (PET) is the amount of evaporation calculated from a well vegetated surface.10 “Potential evapotranspiration is the evaporation from an extended surface of a short green crop which fully shades the ground, exerts little or negligible resistance to the flow of water, and is always well supplied with water.”11
Potential evapotranspiration (PE), assumes no control of water, is defined as a measure of the ability of the atmosphere to remove water from the surface by the processes of both the evaporation and transpiration.12
4.0 Estimation Methods of Evaporation
The following are the different methods used in the estimation of evaporation rate.13
4.1 Energy Balance Method
Abbildung in dieser Leseprobe nicht enthalten
(Eq.1)
If the sensible heat flux Hs and the ground heat flux, G are both zero, Equation 2 can be used.
Abbildung in dieser Leseprobe nicht enthalten
(Eq.2)
where:
Er = evaporation rate
Rn = net radiation
Iv = latent heat of vaporization
Iv = 2500 – 2.36T
T = Temperature
Pw = density of water
The energy balance method (latent heat flux) is calculated as the residual term in the general energy balance equation.14
4.2 Aerodynamic Method
15 The first factor controlling the evaporation is the heat energy which was the main concern of the Energy balance method, but the second controlling factor is the ability to transport vapor. Aerodynamic method is concerned on this factor which is governed by the humidity gradient and wind speed.16
Abbildung in dieser Leseprobe nicht enthalten
)(Eq. 3)
Abbildung in dieser Leseprobe nicht enthalten
(Eq. 4)
where:
Ea = evaporation rate
B = vapor transfer coefficient
ea = vapor pressure
eas = vapor pressure at the surface
z0 = roughness height
u2 = wind speed
4.3 Combination Method
17 Evaporation rate can also be computed by combining the two previous methods of Energy Balance and Aerodynamic methods.
Abbildung in dieser Leseprobe nicht enthalten
(Eq. 5)
where:
Abbildung in dieser Leseprobe nicht enthalten
(Eq. 6)
Abbildung in dieser Leseprobe nicht enthalten
(Eq. 7)
Δ = gradient of saturated vapor pressure
g = psychrometric constant
Er = Evaporation rate from Energy balance method
Ea = Evaporation rate from Aerodynamics method
Cp = specific heat at constant pressure
Kh = heat diffusivity
Kw = vapor diffusivity
Kh/Kw = commonly taken equivalent to 1
4.4 Priestley-Taylor Method
Abbildung in dieser Leseprobe nicht enthalten
(Eq. 7)
where:
Abbildung in dieser Leseprobe nicht enthalten
18 The Priestley-Taylor equation is useful for conditions where weather inputs for the such as relative humidity and wind speed are not available.19
The Priestley-Taylor coefficient, a, is the fraction of surface moisture available for evaporation. Several authors had presented different methods for the determination of a such as Holtslag and Van Ulden (1983), Steiner et al. (1991) and German (2000).20
5.0 Sample Problems
Problem 1:
Assuming there is no sensible heat in an open water surface, determine the evaporation rate (Er) using the following data:
Net Radiation – 190 W/m[2]
Air temperature - 20[0]C
Use the energey Balance Method.
Solution:
Abbildung in dieser Leseprobe nicht enthalten
The density of water should be taken from Table 1 with respect to its temperature. Using the 20[0]C air temperature, the density of water is 998 kg/m[3]. In case the air temperature is not included in the table, you may use interpolation.
[...]
1 Kara Rogers, Evaporation, Encyclopaedia Britannica, https://www.britannica.com/science/evaporation
2 Howard Perlman, The Water Cycle: Evaporation, U.S. Geological Survey's (USGS) Water Science School, 2016, https://water.usgs.gov/edu/watercycleevaporation.html
3 Evaporation, National Geographic, https://www.nationalgeographic.org/encyclopedia/evaporation/
4 eSchoolToday (2017), https://www.eschooltoday.com/water-cycle/what-is-evaporation-of-water.html
5 Ven Te Chow, David Maidment, Larry Mays, Applied Hydrology, International Edition (1988), McGraw-Hill Book Company
6 E.M. Wilson, Engineering Hydrology, 4th Edition, 1990, Macmillan International Higher Education
7 E.M. Wilson, Engineering Hydrology, 4th Edition, 1990, Macmillan International Higher Education, p 35
8 Ven Te Chow, David Maidment, Larry Mays, Applied Hydrology, International Edition (1988), McGraw-Hill Book Company, p 80
9 Ibid, p 81
10 Ibid
11 M B Kirkman, Potential of Soil and Plat Water Relations, Academic Press, 2005, https://www.sciencedirect.com/science/article/pii/B9780124097513500268
12 Pidwirny, M. (2006). "Actual and Potential Evapotranspiration". Fundamentals of Physic Geography, 2nd Edition. http://www.physicalgeography.net/fundamentals/8j.html
13 Ven Te Chow, David Maidment, Larry Mays, Applied Hydrology, International Edition (1988), McGraw-Hill Book Company, p 83
14 A. Ershadi, M.F. McCabe , J.P. Evans, J.P. Walker, Evaluation of energy balance, combination, and complementary schemes for estimation of evaporation, Proceedings of IAHS Lead Symposia held during IUGG2011 in Melbourne, Australia, July 2011) (IAHS Publ. 3XX, 2011), http://users.monash.edu.au/~jpwalker/papers/iugg11.pdf
15 Ven Te Chow, David Maidment, Larry Mays, Applied Hydrology, International Edition (1988), McGraw-Hill Book Company, p 84
16 Ibid
17 Ibid, 86
18 Ibid, 89
19 Priestley-Taylor Equation, 2009,CRA-CIN http://bioma.jrc.ec.europa.eu/components/componentstools/evapotranspiration/help/Priestley-Taylor.html
20 Priestley-Taylor Coefficient, 2009,CRA-CIN http://bioma.jrc.ec.europa.eu/components/componentstools/evapotranspiration/help/Priestley-Taylor_coefficient.html
-
-
-
-
Téléchargez vos propres textes! Gagnez de l'argent et un iPhone X. -
-
Téléchargez vos propres textes! Gagnez de l'argent et un iPhone X. -
-
Téléchargez vos propres textes! Gagnez de l'argent et un iPhone X. -
-
Téléchargez vos propres textes! Gagnez de l'argent et un iPhone X. -
-
Téléchargez vos propres textes! Gagnez de l'argent et un iPhone X.