16.3. Measurement of Evaporation

Evaporation can be measured using direct or indirect methods.

Class A Evaporation Pan

• Standard instrument used in hydrological studies.
• Made of galvanized iron (diameter = 1207 mm, depth = 255 mm).
• Installed in an open area above ground surface.
• Daily evaporation is measured by change in water level.

Limitations:

• Evaporation from a pan is generally more than from a natural water body due to greater exposure and heating.

The Class A Evaporation Pan is a widely recognized tool used to directly measure evaporation rates. It is constructed from galvanized iron, ensuring durability, with specific dimensions designed to standardize measurements across different studies. To utilize this pan, it is set up in a location that allows for unobstructed exposure to environmental conditions. The evaporation rate is determined by measuring changes in the water level within the pan on a daily basis. However, it's important to note that evaporation rates recorded by the pan may be higher than those from natural water bodies, primarily due to increased heat absorption and exposure in the open pan.

Think of the Class A Evaporation Pan as a weather gauge that tells us how much water is 'escaping' into the atmosphere. Similar to how a sponge that is left out in the sun dries out faster than one in a shaded area, the pan is more exposed to heat and wind, which speeds up the evaporation process compared to a natural lake surrounded by trees that provides some shade.

ISI Standard Pan (Modified Class A Pan)

• Used in India; similar to Class A pan but placed within a square enclosure of bricks with grass.

The ISI Standard Pan serves a similar purpose as the Class A pan, but it features modifications that enhance its usability in specific geographical contexts, such as India. It is enclosed within a protective square brick structure and surrounded by grass, which helps to reduce the impact of wind and potential disturbances that could affect evaporation readings. This adaptation improves the accuracy of evaporation measurements by creating conditions that more closely mimic natural environments.

Imagine the ISI Standard Pan as a cozy jar with a lid that keeps warm soup from cooling too quickly. Just like the jar provides insulation, the brick and grass enclosure shields the evaporation pan from extreme weather effects, giving a more controlled environment for measuring how much water is evaporating.

Floating Pan

• Placed directly on the water surface of a reservoir.
• More accurate representation of actual conditions.

The Floating Pan is a specialized measurement tool that is designed to float on the surface of a reservoir's water. This positioning allows it to measure evaporation in conditions that directly reflect the localized environment surrounding the water body. Because it is not affected by the heating and wind exposure that may occur with standard pans, the Floating Pan provides a more accurate measurement of evaporation rates, which is crucial for effective water resource management.

Consider the Floating Pan like a boat bobbing on the water's surface. Just as a boat moves with the waves and reflects the water's conditions, the Floating Pan measures the evaporation as if it were part of the natural reservoir, providing insights that align more closely with real-life circumstances.

Lysimeter

• Measures evaporation from soil and plants (evapotranspiration).
• Used for agricultural and watershed hydrology studies.

A Lysimeter is an instrument used to measure the amount of water lost from soil and plants through evaporation and transpiration (the process of water movement through a plant and its evaporation from aerial parts). It is especially useful in agricultural and hydrology studies, providing valuable information for understanding and managing water usage in cropping systems and watershed areas. By isolating a specific volume of soil and surrounding it with a controlled environment, researchers can accurately assess how much water is lost over a given period.

Think of a Lysimeter as a science experiment in your backyard where you can see how much water a plant drinks each day. Just as you might measure how much lemonade evaporates from a cup left out on a hot day, the Lysimeter collects data on how much moisture plants use from the soil through their leaves, helping farmers know how much water they need to provide for healthy crop growth.

Atmometer

• Also called an evaporimeter.
• Measures evaporation from a wet porous surface (porous cup or disc).

The Atmometer, or evaporimeter, is an instrument designed to measure the rate of evaporation from a saturated porous surface. This device functions by allowing water to evaporate through a material like a sponge or disc that simulates the effect of soil or plant leaves. The rate at which water is lost from this porous material provides insights into the surrounding environmental conditions, such as temperature and humidity, making it useful for both agricultural and meteorological applications.

Imagine the Atmometer as a sponge sitting on a windowsill on a warm day. Just like the sponge soaks up water and eventually dries out, the Atmometer helps us track how quickly water evaporates into the air from wet surfaces. By understanding this evaporation rate, we can tell how different weather conditions might affect local water availability.

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