EVAPORIMETER – Estimation of evaporation is of utmost importance in many hydrologic problems associated with planning and operation of reservoirs and irrigation systems.
In arid zones, this estimation is particularly important to conserve the scarce water resources.
However, the exact measurement of evaporation from a large body of water is indeed one of the most difficult tasks.
The amount of water evaporated from a water surface is estimated by the following methods
o Using evaporimeter data
o Empirical evaporation equations
o Analytical methods
Types of Evaporimeters
Evaporimeters are water-containing pans which are exposed to the atmosphere and the loss of water by evaporation measured in them at regular intervals. Meteorological data, such as humidity, wind movement, air and water temperatures and precipitation are also noted along with evaporation measurement.
Class A Evaporation Pan:
It is a standard pan of 1210mm diameter and 255 depth used by the US Weather Bureau and is known as Class A Land pan. The depth of water is maintained between 18cm and 20cm (Fig. 3.1).
The pan is normally made of unpainted galvanised iron sheet. Monel metal is used where corrosion is a problem. The pan is placed on a wooden platform of 15 cm height above the ground to allow free circulation of air below the pan. Evaporation measurements are made by measuring the depth of water with a hook gauge in a stilling well.
ISI Standard pan
This pan evaporimeter specified by IS 5973- also known as modified Class A Pan, consists of a pan l in diameter with 255 mm of depth. The pan is made of copper sheet of 0.9 mm thickness, tinned inside and painted white outside (Fig. 3.2).
A fixed point gauge indicates the level of water. A calibrated cylindrical measure is used to add or remove water maintaining the water level in the pan to a fixed mark.
The top of the pan is covered fully with a hexagonal wire netting of galvanized iron to protect the water in the pan from birds. Further, the presence of a wire mesh makes the water temperature more uniform during day and night.
The evaporation from this pan is found to be less by about 14% compared to that from unscreened pan. The pan is placed over a square wooden platform of 1225 mm width and 100 mm height to enable circulation of air underneath the pan.
Colorado Sunken Pan
This pan, 920 mm square and 460 mm deep is made up of unpainted galvanized iron sheet and buried into the ground within 100 mm of the top (Fig. 3.3). The chief advantage of the sunken pan is that radiation and aerodynamic characteristics are 5 to those of a lake.
(i) difficult to detect leaks,
(ii) extra care is needed to keep the surrounding area free from tall grass, dust etc.
(iii) expensive to install.
Pan Coefficient C
Evaporation pans are not exact models of large reservoirs and have the following principal drawbacks
1. They differ in the heat-storing capacity and heat transfer from the sides and bottom. The sunken pan and floating pan aim to reduce this deficiency. As a result of this factor the evaporation from a pan depends to a certain extent on its size. While a pan of 3 m diameter is known to give a value which is about the same as from a neighbouring large lake, a pan of size 0 m diameter indicates about 20% excess evaporation than that of the 3 m diameter pan.
2. The height of the rim in an evaporation pan affects the wind action over the surface. Also, it casts a shadow of variable magnitude over the water surfac
The heat-transfer characteristics of the pan material is different from that of the reservoir.
In view of the above, the evaporation observed from a pan has to be corrected to get the evaporation from a lake under similar climatic and exposure C a coefficient is introduced as
Lake evaporation = C x pan evaporation
in which c = pan coefficient. The values of C, in use for different pans are given in
It is usual to install evaporation pans in such locations where other meteorological data are also simultaneously collected. The WMO recommends the minimum net work of evaporimeter stations as below:
1. Arid zones —One station for every 30,000 km
2. Humid temperate climates one station for every 50,000 km and
3. Cold regions —One station for every 100,000 km2.
Currently India has about 200 pan-evaporimeter stations maintained by the India Meteorological Department.
A typical hydrometeorological station contains the following Ordinary rain gauge; Recording rain gauge; Stevenson Box with maximum and minimum thermometer and dry and wet bulb thermometers; wind anemometer, wind direction indicator, sunshine recorder, thermohydrograph and pan evaporimeter