Evaporation and Transpiration:

Evaporation and Transpiration:

Evaporation
Evaporation is the process whereby water molecules move from a liquid Phase to a gas phase in response to energy absorbed by the water molecules. The rate of evaporation is governed by three things:
1. The amount of energy.
2. The difference in concentration of water molecules contained in the adjoining air mass.
3. The maintenance of a pressure differential.

Factors affecting evaporation:
1. Temperature;
2. Humidity;
3. Wind;
4. Solar Radiation.
Evapotranspiration: if the ground covered with vegetation, it is impossible to diffrenciate between evaporation and transpiration. The two processes are linged together and referred to Evapotranspiration.

Evapotranspiration= evaporation+ transpiration.

Potential Evapotranspiration (PE): The Evapotranspiration when water supply is unlimited.








Potential Evapotranspiration (PE):
1. Estimation of PE from Eo
PE =0.6* Eo [Nov.-Feb.]
PE =0.7* Eo [March-Apr.]
PE =0.8* Eo [May-Aug.]
PE =0.7* Eo [Sep.-Oct.]
Eo : actual evaporation
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2. Thornth waits formula:

J=?(tn/5)1.514
Where:
J=heat index;
tn=av. Monthly temp.
n=1,2,3,….,12
حيث انه بعد حساب قيمة J من المعادلة السابقة يتم استخراج (PE) لأي شهر ذو معدل درجة حرارة شهرية (t) باستعمال المخطط الخاص.

PEx t(c?)
135 26.5
…. ….
…. …..
…. ….
195 38







PE= PEx*(D*T/360) (mm)
Where:
D=number of days in the month;
T=av. Number of hours between sunrise and sunset in month.
Steps of solution:
1. Compute J;
2. Draw straight line from J through point of convergence;
3. Read of the PEx value corresponding to mean temp.
4. Compute PE.
3. Evapotranspiration (ET) (Lysimeter)
• Evaporation + Transpiration
• Transpiration : Loss of water through small openings (stomata) of the leaves
• A lysimeter is a measuring device which can be used to measure the amount of evapotranspiration released from an area
• A lysimeter is a tank of soil in which vegetation is planted that resembles the surrounding ground cover
• The amount of evapotranspiration from the lysimeter is measured by means of a water balance of all inputs and outputs
• The precipitation on the lysimeter, the drainage through its bottom, and the changes in the soil water content within the lysimeter are all measured
• The amount of evapotranspiration is the amount necessary to complete the water balance

4. Blaney-Criddle method.



Direct Measurments of Eo:
Pan Evaporation:
هي اوعية تحتوي على الماء وتكون معرضة مباشرة للجو حيث يقاس Eo في فترات منتظمة مع الاخذ بنظر الاعتبار تأثير العوامل الجوية كالرطوبة وحركة الرياح ودرجة الحرارة بالنسبة للماء والهواء.
1. American (Class A) Pan (circular):








• Evaporation can be measured using a standard evaporation pan
called a Class A pan
• Pan evaporation integrates the effects of several climate elements:
Temperature, humidity, solar radiation, and wind
• A Class A pan is cylindrical with a diameter of 12 ft (122cm) and a depth of 10 inches (25.4 cm)
• The pan rests on a carefully leveled base and is often enclosed by a fence to prevent animals drinking from it
• Evaporation is measured daily as the depth of water (in inches) that evaporates from the pan
The measurement Pans Starts with 8 inches filled daily as soon the level goes down below 7 inches .Difference in levels= evaporation(precipitation addition should be considered)

2. British Pan (square):









Measurement
• Pan evaporation values are higher than the actual lake evaporation
• Eo = Eactual= Epan * K
• K = Adjustment factor
• K ranges from 0.64 to 0.81 (for British Pan)
• Average Value = 0.7 (for Class A Pan the U.S.)













Exampl1:
Determine the evaporation from a free water surface, using
the Pennman eq. Nomogram for the following cases:

U2 n/D h % Temp. (c?) Month Locality
1.2 0.5 0.5 18 July Amsterdam, 52? N
2.5 0.3 0.8 4 Jan. Seattle, 47? N

Solution:
1. [(941-892)/20] =y1/8 y1=19.6
RA=19.6+892
=911.6
Eo=-2.67+4.3+1.3+1
=3.93 mm/day
2. [(49)/20]=y2/13 y2=31.85
RA=31.85+892
=923.85
Eo=-1.13+2.38+0.4+0.42
=2.05 mm/day

Example 2:
Use the Nomogram for the solution of Pennman eqs. To
predict the daily PE from a field crop at Latitude=40? N in
April, under the following conditions:
Mean temp. =20 c?;
Mean relative humidity=70%;
Sky cover=60% cloud;
Mean u2=2.5 m/s ;
Ratio of potential evapo-transpiration to potential evaporation=0.7
Solution:
n/D=40%
RA=847
Eo=-2.45+3.58+0.49+1
=2.62 mm/day
PE=0.7*2.62
=1.83
Exampl3: Determine the evaporation from a free water surface, using
the Pennman eq. Nomogram for the following cases:

U2 n/D h % Temp. (c?) Month Locality
0.5 0.75 0.65 15 May Equator
0.5 0.75 0.65 15 Feb. 83? S
Solution: