Evaporation and its Methods of Measurement

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What is Evaporation and How it Occurs?

Before rainfall reaches the outlet of a basin as runoff, certain demands of the catchment such as interception, depression storage and infiltration have to be met. Besides these, evaporation and transpiration processes transfer water to the atmosphere as water vapour. Evaporation from water bodies and the soil mass together with transpiration from vegetation is called evapotranspiration (ET). That portion of Precipitation which is not available as surface runoff is termed as “loss”.

Evaporation

Evaporation is the process in which a liquid changes to the gaseous state as the free surface, below its boiling point, through the transfer of energy. Evaporation is a cooling process- the latent heat of vapourisation (~585 cal/g of evaporated water) must be provided by the water body. Rate of evaporation depends on

• Vapour pressures at the water surface and the air above
• Wind speed - Incident solar radiation
• Atmospheric pressure - Quality of water
• Air and water temperatures
• Size of the water body

Vapour pressure – Rate of evaporation is proportional to the difference between the saturation vapour pressure (SVP) at the water temperature and the actual vapour pressure in the air () This equation is called Dalton’s Law of Evaporation. Evaporation occurs till . If condensation takes place. Temperature – Rate of evaporation increases with an increase in water temperature. Although there is an increase in the rate of evaporation with increase in air temperature, a high correlation does not exist between. For the same mean monthly temperature, evaporation from a lake may be different in different months. Wind – Wind helps to remove the evaporated water vapour from the zone of evaporation, thereby creating greater scope for evaporation. Rate of evaporation increases with increase in wind velocity up to some limit (critical wind speed) and thereafter any further increase in wind velocity does not have any effect on the evaporation rates. This critical wind speed value is a function of the size of the water surface (large water bodies – high wind speeds) Atmospheric Pressure – Other factors remaining the same, a decrease in atmospheric pressure (as in high altitude areas) increases the evaporation rate Soluble salts – When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence it causes reduction in the rate of evaporation. The percentage reduction in the evaporation rate approximately corresponds to the percentage increase in specific gravity Under identical conditions evaporation from sea water is about 2-3% less than that from fresh water

Heat storage in water bodies

Deep water bodies have more heat storage capacity than shallow water bodies. A deep lake stores radiation energy received in summer and releases it in winter resulting in less evaporation in summer and more evaporation in winter when compared to a shallow lake exposed to similar situations. The effect of heat storage is to change the seasonal evaporation rates and the annual evaporation remains more or less unaltered.

Estimation / Measurement of 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

Figure: USGS Class A Evaporation Pan

(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

Figure: ISI Evaporation 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

Figure: Colorado Sunken Pan

(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 Table: Values of Pan Coefficients

Sl. No.	Types of Pan	Average Value	Range
1	Class A Land Pan	0.70	0.60 – 0.80
2	ISI Pan (Modified Class A)	0.80	0.65 – 1.10
3	Colorado Sunken Pan	0.78	0.75 – 0.86
4	USGS Floating Pan	0.80	0.70 – 0.82

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

(1) Meyer’s Formula

(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
3. Mass Transfer Method

(1) Water Budget Method

can only be measured. can only be estimated. If the unit of time is kept very large, estimates of evaporation will be more accurate. It is the simplest of all the methods, but the least reliable.

(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

Figure: Energy Balance in a water body

This is the energy balance in a period of 1 day. All energy terms are in calories/ sq.mm/day. If time periods are short, can be neglected as they are negligibly small All terms except can either be measured or evaluated indirectly is estimated using Bowen’s ratio

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.