HYDROLOGY
Assist. Prof.Dr AGNE
KARLİKANOVAİTE BALIKÇI
agne.karlikanovaite@okan.edu.tr
EVAPORATION
Mechanism of Evaporation
The transition of water from liquid to gas (water vapor) is called evaporation.
90% of the precipitation returns to the atmosphere by evaporation. When molecules
between air and water gain sufficient kinetic energy, they begin to move. Molecules
passing from water to air and from air to water are found near the water surface.
If the number of molecules passing from the water to the air is more, it is accepted
that there is evaporation. The mechanism of evaporation can be seen in the figure
below
ew: Vapor pressure at the water surface
ea: vapor pressure of air above water
The difference between the two vapor
pressures (ew , ea) can be positive, zero, or
negative, and
• if ew > ea evaporation is occurring;
• if ew < ea water is condensing on the
surface and
• if ew = ea neither condensation or
evaporation is occurring.
Evaporation
Evaporation depends on various factors:
•
The amount of evaporation is proportional to the difference between the
vapor pressure at the water surface (i.e., the saturated vapor pressure ew of
the air at the water surface temperature) and the vapor pressure ea of ​the air
above the water (Dalton's law).
E : rate of evaporation (mm/day)
c : coefficient depend on wind velocity, atmospheric pressure and other factors
•
•
•
The movement of air (wind) increases evaporation. If the air is still, ea
increases and becomes equal to ew as time passes, and in this case
evaporation stops.
Energy. The heat required for the evaporation of 1 gram of water varies
between 539-597 calories depending on the temperature of the environment.
Salts dissolved in water reduce evaporation. This reduction is about 1%.
Evaporation
5.It may be possible to reduce evaporation by forming a very thin
(monomolecular) chemical film on the water surface.
6. Water depth also has an effect on the seasonal evaporation amount. Deep
water bodies adapt later to changes in air temperature. For this reason,
evaporation in deep water is less in summer than in shallow water, and more
in winter.
7. As the air pressure decreases, evaporation increases.
Evaporation from water surface
• Daily evaporation amount from water surfaces
varies between 1-10 mm.
• Determining the amount of evaporation is
especially important in reservoirs.
• 800 million m³ of water evaporates annually
from the reservoir of the Keban dam.
Water balance method
𝑋−𝑌= ᴧS
(P+X)-(E+Y+F) =ᴧ S
• Applying the continuity equation to a mass of water such as a lake
or a reservoir:
𝑬 = 𝑷 + 𝑿 − 𝒀 − 𝑭 − ∆𝑺
•
•
•
•
•
•
E = The amount of evaporation
P = The amount of precipitation,
X = inflow,
Y = outflow,
F = infiltration,
∆S = variation of the volume of water.
This method can be used only in estimating the evaporation rate for a long time
interval (such as a month or a year),and the expected error is usually not less than
10%.
Example
It has been determined that the annual precipitation amount on a lake with
an average surface area of ​200 km² is 70 cm, the annual average inflow of
rivers entering the lake is 1.20 m³ / sec, and the average outflow of the lake
is 1.27 m³ / sec. At the end of this year, the water level in the lake rised 9
cm. Calculate the annual evaporation height from the lake surface, since it
is known that infiltration losses are negligible.
P= 70 cm
X= 1.20 m³/sn
Y= 1.27 m³/sn
ΔS= +9 cm.
F= 0
E=?
---------------------------------------------------------------------------------------------------------------------------------------------------------------Solution:
Inflow to the lake: X=
1.2𝑚³
∗365𝑑𝑎𝑦𝑠∗86400𝑠/𝑑𝑎𝑦𝑠
𝑠
200𝑘𝑚2∗106
1.27𝑚³
∗365𝑑𝑎𝑦𝑠∗86400𝑠/𝑑𝑎𝑦
𝑠
200𝑘𝑚²∗106
Outflow of the lake: Y=
=0.19m=19cm
=0.2m=20cm
𝑬=𝑷+𝑿−𝒀−𝑭−∆𝑺=70cm+19cm-20cm-0cm-9cm=60cm
Energy balance method
•
Applies the principle of energy conservation to a mass of water
𝐻x−𝐻y = ∆𝐻
Hi-(H0+HC+He)=∆𝐻
𝑯𝒆 = 𝑯𝒊 − 𝑯𝟎 − 𝑯𝑪 − ∆𝑯
Hi = heat that enters the mass( sum of the heat from the sun and the heat of inflow)
H0 = the sum of the heat of outflow and the reflected heat
HC = the energy loss to the atmosphere from the water surface by the conduction
He = the energy used in evaporation,
∆H = the heat to change the temperature of the mass of water.
The difference of the heat from sun and the reflected heat, Hi-Ho, is measured by pyranometers, actiometers and
radiometers. Hc can not be measured but it is related to He.
𝐻𝑐=𝑅𝐻𝑒
He = L E
R=Bowen ratio
L= heat of evaporation (590 cal/cm³- at normal atmospheric pressure)
Energy balance method
• R = Bowen ratio
𝑅 = 6 × 10
;1
𝑇𝑤 − 𝑇𝑎
. 𝑝0
𝑒𝑤 − 𝑒𝑎
P0 = Atmopheric pressure (kg/cm2 ),
TW , Ta = water and air temperature (0 C),
ew , ea = vapor pressure in water and air (kg/cm2 ),
Bowen ratio is usually beween 0.2 – 0.3.
Energy balance method
• E indicates the evaporation height in cm.
𝐻𝑖 − 𝐻0 − ∆𝐻
𝐸=
𝐿 1+𝑅
• The time interval in the application of this equation is one week or
longer.
• Where does the formula given above comes from?:EXPLANATION:
Hc=RLE
He=LE
LE=Hi –Ho -RHe-ᴧH
LE+RLE=Hİ-H0-ᴧH
E(L+RL)=Hİ-H0-ᴧH
𝑯𝒊;𝑯𝒐;ᴧ𝑯
E=
𝑳(𝟏:𝑹)
Example
The total heat energy coming to a lake with a surface area of ​20 km² is 600cal / cm²
in a day. 15% of this heat is reflected. Calculate the daily evaporation height and
daily volume of the lake, since the temperatures brought and carried by the streams
entering and leaving the lake are neglected and the temperature of the water in the
lake does not change during that day.
--------------------------------------------------------------------------------------------------------------• Hi=600cal/cm²
• Ho=0.15*600=90cal/cm²(15% of this heat is reflected )
• ᴧH=0 (accepted)
• L=590cal/cm³
• R=0.25
Daily evaporation height in the lake:
600;90;0
=0,69cm
590(1:0,25)
E=
Daily evaporation volume:
0,0069m*20km²*106 =138000m³.
Example
The reservoir surface area of ​a dam is 32 km². The heat energy coming to the
reservoir in a day is 500 cal/ cm². The albedo of the water surface is 10%. Calculate
the daily evaporation volume in the lake. (The temperatures brought and carried by
the streams entering and leaving the lake will be neglected and it will be assumed
that the temperature in the lake does not change during that day.)
------------------------------------------------------------------------------------------------------------• Hi=500 kal/cm²
• Albedo=the percentage of reflection of heat reaching the earth=%10
• H0=500*0.1=50cal/cm²
• ∆H=0
• R=0.2-0.3
• L=590cal/cm³
Daily evaporation height:
500;50;0
E=
=0.61cm
590(1:0.25)
Daily evaporation volume:
0.00061m*32*106m²=19520m³
Measurement of evaporation
•
•
•
•
Evaporation pans are used to measure the water surface.
Annual pan coefficient = 0.70
In Turkey evaporation measurements are performed by DMI and DSİ
It is recommended to install an evaporation pan at least every
5000km²
• Recording gages that recod evaporation from wet filter paper on a
rotating chart are also used. There give results similar to those of
evaporation pans.
A class evaporation pan at Samsun 10 regional metereology directorate station
Example
Evaporation height measured from a dam reservoir surface, pan coefficients and
reservoir surface areas per month are given in the table. What is the volume of water
evaporating from the dam reservoir during summer months?
Month
E (mm)
May
June
July
August
September
130
150
160
145
100
Pan
Surface area
coefficient
(km²)
0,63
2,63
0,66
2,53
0,68
2,42
0,7
2,35
0,71
2,3
𝐸H=𝐶×𝐸L
C:pan coefficient
EL=value measured in a pan
May = EH = 0.63×0.130×2.63×106 = 215 397 m³
June = EH = 0.66×0.150×2.53×106 = 250 470 m³
July = EH = 0.68×0.160×2.42×106 = 263 296 m³
August= EH= 0.70×0.145×2.35×106 = 238 525 m³
September= EH = 0.71×0.100×2.30×106 = 163 300 m³
Answer: Total volume = 1 130 988 m³
Evaporation from soil and snow
•
•
•
•
When sufficient water exists in upper soil (equal to a field capacity),
evaporation from soil will be equal to evaporation from water.
The amount of evaporation from the soil is limited by the amount of water
available of the soil. When the wilting point is reached, evaporation stops.
Sublimation = Evaporation from the snow and ice surface.
Evaporation from snow may be as high as 5mm in windy days, but usually
it is 5-30mm a month, about 0.2-0.25 of the evaporation from water in
similar conditions.
Transpiration and interception
• Plants take water from the soil by their roots, they use it
in processes for survival
and then return it to
atmosphere through their leaves. This is called
TRANSPIRATION.
• Transpiration loss is 0.1-7mm a day for various plants.
• Transpiration can be measured by phytometers, in which
a certain plant is grown. Evaporation from the soil is
prevented, and transpiration is estimated from the
change in the weight of the container.
• INTERCEPTION-it is precipitation that is intercepted by
plants and cannot reach the ground.
Evapotranspiration losses
• Evapotranspiration (ET) = total losses in a region by
transpiration, soil evaporation and evaporation from water
and snow surfaces
• Potential Evapotranspiration = losses that occur when
sufficient water is available in the soil.
• Actual Evapotranspiration = limited by the existing water in
the soil , it stops when the soil moisture is at the wilting point.
Annual precipitation in Turkey - 65 cm.
0.37×65 = 24 cm → Streamflow
0.63 × 65 = 41 cm → Evapotranspiration
Example
The surface area of ​a lake is 150 hectares. Meteorological measurements made in
April are given below. Find the volume of water evaporating from the lake surface
during this month.
• Inflow = 0.55 m3 /s,
• Outflow = 0.45 m3 /s,
• Infiltration rate= 1500 m3 /month,
• Precipitation = 46 mm/month,
• Increasement in lake level= 70 mm/month
--------------------------------------------------------------------------------------------------------------
Solution:
1 ha = 104 m² → 150 ha = 150×104 m²
X = Inflow = 0.55×86400×30 = 1425600 m³,
Y = Outflow = 0.45×86400×30 = 1166400 m³,
F = Infiltration = 1500×30 = 45000 m³
P = Precipitation = 46×10̄³×150×104 = 69000 m³
ᴧS= 70×10;3×150×104 =105000𝑚3
E=P+X-Y-F-ᴧS
E=69000 𝑚3 +1425600 𝑚3 -1166400 𝑚3 -45000 𝑚3 -105000 𝑚3 =178200𝑚3