✿ Evaporation



Figure 3.1 : Evaporation Process

What is mean by Evaporation?

Evaporation or vaporization is the process by which water changes from liquid state to vaporous state. This change in state requires an exchange of approximately 2.5 kJ energy for each gram of water evaporated. Two main factors influencing the evaporation from an open water surface are the supply of energy to provide latent heat of vaporization and the ability to transport the vapour away from the evaporating surface. Solar radiation is the main source of heat energy. The ability to transport the vapour away from the evaporating surface depends on the wind velocity over the surface and the specific humidity gradient in the air above it.

Estimating Evaporation from Free Water Surfaces

Rates of evaporation vary depending on meteorological factors and the nature of the evaporating surface and the quality of evaporating water. Meteorological factors affecting evaporation are solar radiation, differences in vapour pressure between a water surface and the overlying air, relative humidity, temperature, wind, atmospheric pressure, etc. For large bodies of water, because of the extensive surfaces involved, direct measurements of evaporation are not possible. As a consequence, a variety of techniques have been derived for determining or estimating evaporation. Six methods are available for estimating evaporation from free water surface.

1. Water Budget Method
  • The water budget method for the estimation of evaporation uses the water budget equation of the lake or reservoir. Evaporation from the lake or reservoir is estimated by knowing all the other terms in the water budget equation (i.e., precipitation, net surface and ground water inflow, transpiration and change in storage). The water budget method for determining evaporation is a very simple procedure, but it seldom produces reliable results. This is because some other terms in the water budget equation are also difficult to measure (eg seepage).
2. Energy Budget Method
  • This is an application of conservation of energy. Evaporation of water takes energy. How much evaporation has occurred can be estimated by how much energy is consumed. How much energy is consumed can be estimated by using the energy budget equation. The energy equation accounts for incoming and outgoing energy balanced by the amount of energy stored in the system and consumed in evaporation. Evaporation requires heat to vaporize the liquid in form of latent heat of vaporization, which is the amount of heat absorbed by a unit mass of a substance, without change in temperature, while passing from liquid to the vapour state. The latent heat of vaporization varies with temperature which is in calories per gram and in joules per kilogram are respectively wherein T is in °C.
  •  Lhv = 597.3-0.564 T and Lhv = 2.501 x 10^6 - 2370 T
  • Example;   
              T   = 1.1
              Lhv  = 597.3-0.564 T                     = 596.6796     

                           Lhv    = 2.501 x 10^6 - 2370 T     = 2498393


3. Mass Transfer (Aerodynamic) Method

  • Besides the supply of heat energy, the second factor controlling the evaporation rate is the ability to transport the vapour away from the evaporating surface. The transport rate is governed by the wind velocity over the surface and the specific humidity gradient in the air above it. This method estimates the rate of evaporation based on the turbulent transfer of water vapour from an evaporating surface to the atmosphere. The rate of transfer is mainly a function of the vapour pressure gradient. Most commonly used equation of this type is Dalton Model, which is:
               E = fw (es - ea) 
  • Where E = rate of evaporation; and fw = wind function which is dependent on the wind velocity, atmospheric pressure and other factors; several empirical equations are available for fw. Most commonly used equation is in the form:
            fw = a(1 + bu )
  • Where a and b are empirical constants, and u = wind velocity at some fixed height from the water surface. es, ea = the saturation vapour pressure at water surface temperature (Ts) and the vapour pressure at air temperature (Ta), respectively. Since ea = Rh×es, so  



4. Combination of Mass Transfer and Energy Budget Method
  • Evaporation may be computed by the aerodynamic method when energy supply is not limiting and by the energy balance method when vapour transport is not limiting. But, normally, both of these factors are limiting, so as a combination of the two is needed. The most widely used is the Penman's Equation. Through a simultaneous solution of an aerodynamic equation and an energy budget equation, Penman derived the following equation for daily evaporation E:

  • where Ea is the pan evaporation or evaporation calculated from aerodynamic methods; Er is evaporation rate computed from the rate of net radiation Rn ( Er = 0.0353 Rn mm/day and Rn in W/m 2); γ = psychrometric constant (~66.8 Pa/°C); ∆ is the slope of the saturation vapour pressure vs. temperature curve at the air temperature Ta ie:

∆/( ∆ + γ) and γ/( ∆ + γ) are weighting factors they sum to unity. 

  • The combination method of calculating evaporation from meteorological data is the most accurate method when all the required data are available and the assumptions are satisfied. The chief assumptions of the energy balance are that steady state energy flow prevails and that changes in heat storage over time in the water body are not significant. This assumption limits the application of the method to daily time intervals or longer, and to situations not involving large heat storage capacity, such as large lakes. The chief assumption of the aerodynamic method is associated with the wind function. Thus the combination is well suited for application to small areas with detailed climatological data. The required data include net radiation, air temperature, humidity, wind speed, and air pressure. When some of these data are unavailable simpler evaporation equations requiring fewer variables must be used. Usually, instrumentation for energy budget and mass transfer methods is quite expensive to install and maintain, as a result, the water budget method and evaporation pans are more common.  

5. Empirical
  • The most widely used method of finding reservoir evaporation is by means of evaporation pans. Pans are basically water filled containers. Evaporation is found by observing how much water is lost over time. There are different designs for these pans, e.g. the US class A pan, ISI standard pan, Colorado sunken pan and the Russian GGI pan. Likewise they may be sunken, floating, and surface type. Burying the pan tends to eliminate objectionable boundary effects, such as radiation on the side walls and heat exchange between the atmosphere and the pan itself, but creates observational problems. Sunken pans collect more trash; they are difficult to install, clean, and repair; leaks are not easily detected; and height of vegetation adjacent to the pan is quite critical. The evaporation from a pan floating in a lake more nearly approximates evaporation from the lake than that from an on shore installation, but even so, the boundary effects are appreciable.
Figure 3.2 : US Class A Pan

Figure 3.3 : Colorado Sunken Pan


Figure 3.4 : ISI Evaporation Pan

  • Observational difficulties are prevalent with floating pans (splashing frequently renders the data unreliable), and installation and operational expense is excessive. Pans exposed above ground experience greater evaporation than sunken pans, primarily because of the radiant energy intercepted by the side walls, and heat exchange through the pan produces unrealistic effects which must be taken into account. Insulating the pan can minimize both deficiencies. The principal advantages of surface exposure are economy and ease of installation, operation, and maintenance.
  • ISI is a surface exposure type pan. The top of the pan is covered fully with a hexagonal wire netting of galvanized iron to protect the water in the pan from bird.  Further, the presence of a wire mesh makes the water temperature more uniform during day and night. The evaporation from this pan is less by 14% compared to that from unscreened pan.
  • Evaporation pans are not exact models of large reservoirs as they differ in heat storage capacity and heat transfer from sides and bottom, and height of the rim affects wind action and cast shadow of variable magnitude. Due to these, normally, the pan will overestimate E. In some circumstances the pan will underestimate E because a lake will conserve heat longer. Actual evaporation from large bodies of water is the evaporation measured by pans multiplied by a factor of 0.70 - 0.75 (pan coefficient), but this factor varies by season and location. So
       Lake/Reservoir evaporation = Pan coefficient × Pan evaporation

  • Evaporation from a water surface is a continuous process. Its estimation is of utmost importance in many hydrologic problems associated with planning and operation of reservoirs and irrigation systems. Under Indian conditions evaporation loss from a water body is about 1.60 m in a year and still higher in arid regions. Thus the quantity of water lost by evaporation in a year is very large, which is an economic loss to the valuable investment made in developing reservoirs and dams.
  • In arid zones where water is scarce, the importance of conservation of water through reduction of evaporation is obvious. This can be achieved by reducing surface area of water bodies, by mechanical covers, and by applying a thin chemical (monocular) film on the water surface. Monocular layer (cetyl alcohol) is the effective and feasible method, which inhibit evaporation (20-50%) by preventing the water molecules to escape past them. It is colorless, odorless and nontoxic, and pervious to oxygen and carbon dioxide, so the water quality is not affected by its presence. However wind, oxidation and birds may disturb the layer requiring regular replenishments.

















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