Evaporation is the process by which water changes

from a liquid to a gas or vapor. Evaporation is the

primary pathway that water moves from the liquid
state back into the water cycle as atmospheric water
vapor. Studies have shown that the oceans, seas,
lakes, and rivers provide nearly 90 percent of the
moisture in the atmosphere via evaporation, with the
remaining 10 percent being contributed by
plant transpiration.
A very small amount of water vapor enters the
atmosphere through sublimation, the process by
which water changes from a solid (ice or snow) to a
gas, bypassing the liquid phase. This often happens
in the Rocky Mountains as dry and warm Chinook
winds blow in from the Pacific in late winter and
early spring. When a Chinook takes effect local
temperatures rise dramatically in a matter of hours.
When the dry air hits the snow, it changes the snow
directly into water vapor, bypassing the liquid phase.
Sublimation is a common way for snow to disappear
quickly in arid climates.
Physics of evaporation-Its broadest definition,
evaporation is the process by which molecules in a liquid
(water) spontaneously become gaseous (water vapors) and
escape the liquid state or changes in pressure, more so
than temperature, strongly influence how quickly
liquids turn to gas, while evaporites are the resultant
mineral precipitates accumulating in and around an
increasingly saline residual brine that has reached a state of
supersaturation with respect to a particular mineral salt or
salts. Water molecules in the liquid phase are in continuous
motion and so will collide. As they collide, they transfer
energy to each other in varying degrees, based on how they
collide. Evaporation, then, is a simple matter of solution
kinetics in this milieu of molecular motion and is a response
to varying degrees of heat absorption at the molecular scale.
Factors That Affect the Evaporation
 Temperature: As the temperature increases,
the rate of evaporation also increases.
Temperature and rate of evaporation are
proportional to each other.
 Surface area: As the surface area increases,
the rate of evaporation increases. The surface
area and rate of evaporation are proportional to
each other.
 Humidity: The rate of evaporation decreases
with an increase in humidity. Humidity and the
rate of evaporation are inversely proportional
to each other.
 Wind speed: Increase in wind speed results in
increased evaporation. Wind speed and rate of
evaporation are proportional to each other.
 Depth of Water:If the depth of water is more,
evaporation will be less, but less depth of water
causes more evaporation.
Measurements of different factors for evaporation
This is done by the following methods
 Using evaporimeters
 Using empirical equations
 By analytical methods

Types of Evaporators
Evaporimeter
These are pans containing water which are exposed to the atmosphere. Loss of water by
evaporation from these pans are measured at regular intervals (daily). Meteorological data
such as humidity, wind velocity, air and water temperatures, and precipitation are also
measured and noted along with evaporation.
(1) USWB Class A Evaporation Pan
 A pan of diameter 1210mm and depth 255mm
 Depth of water is maintained between 18 and 20cm
 The pan is made of unpainted GI sheet
 The pan is placed on a wooden platform of height 15cm 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

(2) ISI Standard Pan

 Specified by IS:5973 and known as the modified Class A Pan
 A pan of diameter 1220mm and depth 255mm
 The pan is made of copper sheet 0.9mm thick, tinned inside and painted white
outside
 The pan is placed on a square wooden platform of width 1225mm and height 100mm
above ground level to allow free air circulation below the pan
 A fixed point gauge indicates the level of water
  Water is added to or removed from the pan to maintain the water level at a fixed mark
using a calibrated cylindrical measure
 The top of the pan is covered with a hexagonal wire net of GI to protect water in the
pan from birds
 Presence of the wire mesh makes the temperature of water more uniform during the
day and night
 Evaporation from this pan is about 14% lower as compared to that from an
unscreened pan

(3) Colorado Sunken Pan

 920mm square pan made of unpainted GI sheet, 460mm deep, and buried into the
ground within 100mm of the top
 Main advantage of this pan – its aerodynamic and radiation characteristics are similar
to that of a lake
 Disadvantages – difficult to detect leaks, expensive to install, extra care is needed to
keep the surrounding area free from tall grass, dust etc
(4) USGS Floating Pan
 A square pan of 900mm sides and 450mm deep
 Supported by drum floats in the middle of a raft of size 4.25m x 4.87m, it is set afloat
in a lake with a view to simulate the characteristics of a large body of water
 Water level in the pan is maintained at the same level as that in the lake, leaving a rim
of 75mm
 Diagonal baffles are provided in the pan to reduce surging in the pan due to wave
action
 Disadvantages – High cost of installation and maintenance, difficulty in making
measurements
Pan Coefficient
Evaporation pans are not exact models of large reservoirs. Their major drawbacks are the
following: – They differ from reservoirs in the heat storage capacity and heat transfer
characteristics from the sides and the bottom (sunken and floating pans aim to minimise this
problem). Hence evaporation from a pan depends to some extent on its size (Evaporation from
a pan of about 3m dia is almost the same as that from a large lake whereas that from a pan of
about 1m dia is about 20% in excess of this). – The height of the rim in an evaporation pan
affects wind action over the water surface in the pan. Also it casts a shadow of varying size on
the water surface. – The heat transfer characteristics of the pan material is different form that
of a reservoir. Hence evaporation measured from a pan has to be corrected to get the
evaporation from a large lake under identical climatic and exposure conditions.
Lake Evaporation = Pan Coefficient x Pan Evaporation

Evaporation pans are normally located at stations where other hydro-meteorological data are
collected
Evaporation Stations
WMO recommends the following values of minimum density of evaporimeters
 Arid Zones – 1 station for every 30,000 sq.km
 Humid Temperate Zones - 1 station for every 50,000 sq.km
 Cold regions - 1 station for every 1,00,000 sq.km
A typical hydro-meteorological station has the following:
 Recording rain gauge and non-recording rain gauge
 Stevenson box with maximum, minimum, wet, and dry bulb thermometers
 Wind anemometer and wind vane
 Pan evaporimeter
 Sunshine Recorder etc
EMPIRICAL EQUATIONS
Most of the available empirical equations for estimating lake evaporation are a Dalton type
equation of the general form
(2) Rohwer’s Formula
Accounts for the effect of pressure in addition to the wind speed effect

Wind Velocity
In the lower part of the atmosphere, up to a height of about 500m above the ground level,
wind velocity follows the one-seventh power law as

Analytical Methods of Evaporation Estimation

(1) Water Budget Method
(2) Energy Budget Method

 It involves application of the law of conservation of energy

 Energy available for evaporation is determined by considering the incoming energy,
outgoing energy, and the energy stored in the water body over a known time interval
 Estimation of evaporation from a lake by this method has been found to give
satisfactory results, with errors of the order of 5%, when applied to periods less than a
week.
Comparison of Methods
 Analytical methods can provide good results. However, they involve parameters that are
difficult to assess.
 Empirical equations can at best give approximate values of the correct order of
magnitude.
 In view of the above, pan measurements find wide acceptance in practice.

Available methods/procedures for estimating evaporation from

open water
2.1 Pan factors
Evaporation pans have been used to estimate evaporation rates for many years (see
Hounam (1973) for a review). These pans can be of varying dimensions but the most
common is the US Class A pan. Pan evaporation is simply the depth of water evaporated
from the pan during a day. Some authors have used pan coefficients to relate pan
evaporation to observed open water evaporation. There are numerous coefficients
reported in the literature but the shortfall of this technique is that coefficients are specific
to the pan type, its location and the nature of the water body and so require calibration
for individual applications. For larger water bodies pan coefficients may also vary in
time to account for heat storage effects. While interpolated daily pan evaporation
estimates are available for all of Australia from the SILO database, the uncertainty
involved in developing coefficients, particularly given the lack of suitable datasets for
calibration, makes this approach unattractive.
2.2 Mass balance
Mass balance techniques calculate evaporation by looking at the differences between
storage volume and inflows and outflows for specific water bodies. Some authors report
errors of just 5% (e.g. Lapworth, 1965) using such methods. While simple in principle
such a method requires detailed and accurate measurements of surface and subsurface
flows which are very rarely available. Any errors in estimating components of the mass
balance results in a direct error to the evaporation estimate (see Gangopaghaya et al.,
1966). The range and number of water bodies in the MDB and the lack of basic mass
balance data excludes this method.

2.3 Energy budget

In this method the evaporation from a water body is estimated as the difference between
energy inputs and outputs measured at a site. The energy loss through evaporation
represents a major component of the energy balance in a typical water body. This method
can be accurate if suitable measurements are available (Anderson, 1954; Stewart and
Rouse, 1976), but its problem is that specialised equipment is required for each water
body if accurate budgets are to be constructed. As with the mass balance approach, any
errors in energy balance components are passed through to evaporation estimates
directly. The site-specific nature of this method also excludes it from this project.

2.4 Bulk transfer (aerodynamic method)

Evaporation rate ( E ) can also be estimated using the application of bulk transfer
formulae. A simple version of such a bulk transfer equation is shown in Equation 1
(Dalton, 1802): )( * s −= eeCUE Equation 1
where C is a mass transfer coefficient, U is wind speed and )( * ees − is the difference
between saturated vapour pressure at the temperature of the water surface and the vapour
pressure at a specified height in the air above the water surface. The mass transfer
coefficient is similar in concept to a drag coefficient implicitly incorporating transfer
across the viscous skin layer at the water surface and through the turbulent flow above
it. Numerous studies have shown that the coefficient changes at wind speeds
corresponding to the onset of capillary wave formation on the water surface. As well,
the coefficient depends on the stability of the atmosphere (Liu et al., 1979). The
coefficient may also vary depending on fetch across the water surface and vegetation of
the surrounding land. This method requires measurements of wind speed, vapour
pressure, and air and water surface temperature, as well as estimates or measurements of
water temperature. While not all of these variables are readily available for the MDB,
techniques exist by which to estimate them. Bulk transfer techniques are best suited to
larger water bodies with fetches of at least several hundred metres. Hence these
techniques have limited applicability to the size and range of water bodies for which
evaporation rate needs to be modelled in the MDB.
2.5 Combination methods
The so-called ‘combination methods’ combine the mass transfer and energy budget
principles in a single equation. Two of the most commonly known combination methods
are the Penman equation (Penman, 1948) and the Penman-Monteith equation (Monteith,
1965). The combination equations require inputs of net radiation, air temperature,
vapour pressure and wind speed. Air temperature and vapour pressure data have been
interpolated for all locations in Australia, as has solar radiation which can be used to
estimate net radiation (see below). This data – in combination with wind speed data from
widespread Bureau of Meteorology (BoM) gauges – makes the application of
combination models at any location in the MDB possible. When applied to open water
evaporation, the Penman-Monteith approach allows adjustment to the amount of energy
available for evaporation based on changes in heat storage within the water body. Such
an adjustment can be obtained if the temperature of the water body is known or can be
estimated. Useful models by which to determine water temperature have been developed
and most are based on the concept of an equilibrium temperature (e.g. de Bruin, 1982;
Edinger et al., 1968; Keijman and Koopmans, 1973). Such models utilise the same
meteorological driving data as the Penman-Monteith model and indeed the loss of heat
through evaporation is an important part of the energy calculation used to calculate
temperature. The equilibrium temperature is defined as the surface temperature at which
the net rate of heat exchange would be zero. Shallow water bodies may be in temperature
equilibrium with their meteorological forcing, but deeper water bodies may store
sufficient heat in the water column that they are not in thermal equilibrium and the
surface temperature is greater than or less than the equilibrium temperature. A
modification of the equilibrium temperature method allows for this factor to be taken
into account in the determination of surface temperature and hence evaporation rate.

2.6 Summary
For the purposes of open water evaporation estimation in the MDB the Penman-
Monteith model, with an inclusion for water body heat storage, is considered to be most
suitable. The key factors making this technique most appropriate are that: • calculations
are based on readily available data sources • the model has limited empirical basis and
therefore it is more readily applicable to a variety of water bodies • the model takes into
account heat storage within water bodies. While bulk transfer methods could also have
been applied to the MDB we have chosen to use the PenmanMonteith model for
consistency across the range of terrestrial and aquatic systems being modelled in the
broader Murray-Darling Basin Sustainable Yields Project. The key assumption of the
Penman-Monteith model with adjustments for heat storage is that the water body is well
mixed and that no thermal stratification develops. The remainder of this report will focus
on its implementation, testing and application.

Allierose Jane Aguilar