Evaporation Homework 

REVIEW QUESTIONS   
a. Explain briefly the evaporation process.  
Evaporation is the process by which a liquid is converted into gaseous form. Is the spontaneous
escape of high energy molecules from liquid surface into vapor state. It happens before the
boiling point. Is a surface phenomenon. The molecule will gain so much kinetic energy that
break the force of attraction and scape as gas.  
 
b. Discuss the factors that affect the evaporation from a water body.  
The factors which affect evaporation are quality of water and solar radiation.

Atmospheric pressure: At higher altitude, there will be less pressure that means that evaporation
will be higher.  
Force of attraction: The more of force of attraction result in less evaporation. 
Temperature: Heat increase the Kinect energy on the molecules of water. So, with more
temperature molecule can gain the kinetic energy to overcome the force of attraction between
area. 
Surface area: More the surface area means that there is going to be more evaporation.  
Wind: If the wind is blowing the rate of evaporation will be faster. Because the air blown above
the surface of the liquid takes away the skipping molecules and space is made available for the
other molecules. It means that the wind speed increases the result in the greater amount of the
water vapor skipping as a result the rate of evaporation increases.  
 
c. Mention and describe commonly used evaporimeters.  
USWB Class A Evaporation Pan: It have a diameter of 1210 mm and depth of 255 mm. The
depth of water is maintained between 18 and 20 cm. The pan is made of unpainted GI sheet and
it is placed on a wooden platform of height 15 cm above ground level to allow free air circulation
below the pan. Evaporation is measured by measuring the depth of water in a stilling well with a
hook gauge.
ISI Standard Pan: Is specified by IS:5973 and know as the modified Class A Pan. It has pan
diameter of 1220 mm and depth 255 mm, copper sheet 0.9 mm thick, tinned inside and painted
white outside. It is placed on a square wooden platform of width 1225 mm and height 100 mm
above ground level to allow free air circulation below the pan. At fixed point gauge indicates the
level of water. Evaporation from this pan is about 14% lower as compared to that from an
unscreened pan.
Colorado Sunken Pan: It has a 920 mm square pan made of unpainted GI sheet. 460 mm deep
and buried into the ground within 100 mm of the top. The main advantage is its aerodynamics
and radiation characteristics are like that of a lake. Its disadvantage is that is difficult to detect
leaks, expensive to install and extra care is needed to keep the surrounding area free from tall
grass and dust.
Buried and above soil surface: The pan evaporimeter above soil surface has the largest pan
correction coefficient.
USGS Floating Pan
d. Explain the water budget, energy and mass transfer methods for estimating
evaporation from a lake.  
Water budget is a basic equation that says that the change in storage is equal to the inflows minus
the outflows. Evaporation is part of the outflows so you can clear the evaporation to one side so
you can get the estimation.

Energy method is a theoretical method that measure parameters such as radiation and fraction of
radiation reflected. This method is widely used for estimating the amount of evaporation from a
large body of water such as lakes and reservoirs. When vapor transport is not limiting, use the
energy balance method.

Mass transfer method or aerodynamical looks at how the wind can move the moisture away from
the surface. When energy supply is not limiting, use the aerodynamic method.

e. Discuss the importance of evaporation control of reservoirs and possible methods of

achieving the same.  
The main importance of preventing evaporation from reservoir is that evaporation can cause a
big lose in the amount of water that a reservoir can hold. That is a huge disadvantage is places
that suffer from drought. One way to fight against evaporation is using the method that is being
use in Los Angeles, they put 96 million balls in the reservoir fill with water, that reduce the
amount of radiation that hit the water. There can be other proposals like cover the reservoir from
side to side with a tent.
 
f. Explain briefly what evapotranspiration is.
Evapotranspiration if the sum of evaporation and transpiration from the earths land surface to
atmosphere. Is the combination of evaporation from the soil surface and transpiration from the
vegetation. The same factors governing open water evaporation also govern evapotranspiration,
namely energy supply and vapor transport.
 
g. Describe the factors affecting evapotranspiration process.  
Weather parameters: The principals weather parameter that affects evapotranspiration are
radiation, air temperature, humidity and wind speed. Several procedures have been developed to
assess the evaporation rate from these parameters.

Crop factors: The crop type, variety and development stage should be considered when assessing
the evapotranspiration from crops grown in large, well-managed fields. Differences in resistance
to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting
characteristics result in different ET levels in different types of crops under identical
environmental conditions.

Management and environmental conditions: Factors such as soil salinity, poor land fertility,
limited application of fertilizers, the presence of hard or impenetrable soil horizons, the absence
of control of diseases and pests and poor soil management may limit the crop development and
reduce the evapotranspiration. Other factors to be considered when assessing ET are ground
cover, plant density and the soil water content. The effect of soil water content on ET is
conditioned primarily by the magnitude of the water deficit and the type of soil. On the other
 hand, too much water will result in waterlogging which might damage the root and limit root
water uptake by inhibiting respiration.
  
PROBLEMS  
1. Calculate the evaporation rate from an open water source, if the net radiation is 300
W/m2 and the air temperature is 30° C. Assume value of zero for sensible heat, ground heat flux,
heat stored in water body and advected energy. The density of water at 30° C = 996 kg/m3.
[Hint: Calculate latent heat of vaporization L by the formula: L = 2501 - 2.37 T (kJ/kg), where T
= temperature in °C.]  
2. A class A pan was set up adjacent to a lake. The depth of water in the pan at the
beginning of a certain week was 195 mm. In that week there was a rainfall of 45 mm and 15 mm
of water was removed from the pan to keep the water level within the specified depth range. If
the depth of the water in the pan at the end of the week was 190 mm calculate the pan
evaporation. Using a suitable pan coefficient estimate the lake evaporation in that week. 
3. A reservoir has an average area of 50 km2 over a year. The normal annual rainfall at the
place is 120 cm and the class A pan evaporation is 240 cm. Assuming the land flooded by the
reservoir has a runoff coefficient of 0.4, estimate the net annual increase or decrease in the
streamflow as a result of the reservoir.  
4. At a reservoir in the neighborhood of Delhi, the following climatic data were observed.
Estimate the mean monthly and annual evaporation from the reservoir using Meyer’s formula.
[The table has an error on the last column.  It is supposed to be “Wind velocity at 8 m above GL
(km/h)”].  

 
5. A reservoir had an average surface area of 20 km2 during June 1982. In that month the
mean rate of inflow = 10 m3/s, outflow = 15 m3/s, monthly rainfall = 10 cm and change in
storage = 16 million m3. Assuming the seepage losses to be 1.8 cm, estimate the evaporation in
that month.  
The following problems are from the Ground and Surface Water Hydrology Book from Larry
Mays.  Any additional information mentioned in the problems, go check chapter 7 of the
mentioned book. 
6. Solve Example 7.3.1 for an average net radiation of 92.5 W/m2.  Compare the resulting
evaporation rate with that in Example 7.3.1.  
7. Solve Example 7.3.2 for a roughness height zo = 0.04 cm.  Compare the resulting
evaporation rate with that in Example 7.3.2.  
8. Solve Example 7.3.3 for an average net radiation of 92.5 W/m2.  Compare the resulting
evaporation rate with that in Example 7.3.3.  
9. Solve Example 7.3.4 for an average net radiation of 92.5 W/m2.  Compare the resulting
evaporation rate with that in Example 7.3.4.  
10. At a certain location during the winter, the average air temperature is 10° C and a net
radiation is 40 W/m2 and during the summer the net radiation is 200 W/m2 and the temperature
is 25° C.  Compute the evaporation rates using Priestley-Taylor method.  
11. The average weather conditions are net radiation = 40 W/m2; air temperature = 28.5° C;
relative humidity = 55%; and wind speed = 2.7 m/s at a height of 2m.   Calculate the open water
evaporation rate in mm/day using the energy method, the aerodynamic method, the combination
method, and the Priestley-Taylor method.  Assume standard atmospheric pressure is 101 kPa and
zo = 0.03 cm.  
12. A 600-ha farmland receives annual rainfall of 2500 mm. There is a river flowing through
the farmland with an inflow rate of 5 m3/s and an outflow rate of 4 m3/s.  The annual water
storage in the farmland increases by 2.5x10^6 m3.  Based on the hydrologic budget equation,
determine the annual evaporation amount (in mm). [Note: 1 ha = 10,000 m2].