17.3. Evaporimeters (Evaporation Pans)

Evaporimeters are instruments used for direct measurement of evaporation. These are standardized pans filled with water and exposed to the atmosphere.

Evaporimeters are tools designed specifically to measure the rate of evaporation by collecting water in a standardized pan. The pans are filled with water and placed in an open atmosphere, allowing sunlight, air, and wind to facilitate evaporation. By analyzing how quickly the water level decreases over time, we can accurately determine the evaporation rate from the surface.

Think of an evaporimeter as a small, open swimming pool. When the sun shines on the pool and the wind blows over it, water will slowly disappear. By measuring how much water we lose from the 'pool' at different times, we can understand how much is evaporating. This is similar to how evaporimeters provide important data for scientists.

Most commonly used pan evaporimeter. Specifications: Circular pan made of galvanized iron or stainless steel, Diameter: 1207 mm, Depth: 255 mm, Filled to 180 mm depth, Placed on a wooden platform 15 cm above ground. Measurement: Water loss is measured daily with a hook gauge or point gauge. Limitations: Evaporation from the pan is usually more than from a large water body. Hence, a Pan Coefficient (Kₚ) is applied (typically 0.7–0.8).

The Class A evaporation pan is the most widely used type of evaporimeter. It has specific measurements that allow for standardized data collection. It is made from materials like galvanized iron or stainless steel and has a circular shape with a diameter of about 1207 mm. The pan is typically filled to a depth of 180 mm and is elevated on a wooden platform to ensure proper exposure to elements. To measure evaporation, water loss is tracked daily using a simple measuring device like a hook gauge. However, evaporation measured in this pan tends to be higher than that from natural water bodies, so scientists use a correction factor known as the Pan Coefficient (Kₚ), commonly ranging between 0.7 to 0.8, to adjust the data.

Imagine trying to measure how much water evaporates from a swimming pool using a small bucket instead. Naturally, the bucket will lose more water quickly than the pool because its surface area is smaller. The Pan Coefficient is like a scorecard we use to balance this difference and get a better understanding of evaporation from the larger water body.

Developed by the Indian Meteorological Department. Rectangular in shape. Made of copper or galvanized iron. Enclosed in a screen to reduce wind effect. Suitable for Indian climatic conditions.

The ISI Standard Pan Evaporimeter was specifically designed to meet the needs of the Indian environment. Unlike the Class A pan, this one is rectangular and made from materials like copper or galvanized iron. To improve accuracy, especially in windy areas, this pan is enclosed in a screen that minimizes the wind's impact on evaporation rates. This design adjustment makes it a practical choice for precise measurements in various climatic conditions across India.

Think about using a water bottle with a straw to measure how fast you drink a beverage in a windy area. If there’s wind, you might get a lot of air instead of liquid. The ISI Standard Pan, with its screen, works like a protective cover around the water bottle, ensuring accurate measurements despite the external conditions.

Floats on the water surface of a reservoir or lake. Mimics actual evaporation conditions better than land-based pans. Requires anchoring to prevent drifting.

The Floating Pan Evaporimeter is designed to float on the surface of large bodies of water, such as lakes or reservoirs. This positioning allows it to mimic real-world evaporation conditions more effectively than pans placed on land. To ensure the evaporimeter stays in one spot and doesn’t drift away, it requires anchoring. By floating, it can capture more accurate data about evaporation that occurs in its immediate environment.

Picture a raft floating in a lake. As the sun shines and the wind blows on it, the raft experiences the same conditions as the lake itself. This is like the floating pan, which measures evaporation under conditions similar to those found in natural water bodies. Without an anchor, the raft would drift away, much like how an unanchored floating pan might get misplaced.

Sunken into the ground with rim at ground level. Reduces error due to wind and temperature difference from surrounding ground. Used in long-term hydrological studies.

The Colorado Sunken Pan is designed to sit flush with the ground, with its rim level with the surrounding terrain. This design feature helps minimize the effects of wind and temperature differences that can skew evaporation readings. By being partially buried, it is particularly useful in long-term studies of hydrology, providing consistent data over extended periods without the variability caused by airflow above the pan.

If you imagine a pool that is level with the ground around it, it would experience less wind blowing over it than a raised pool. The Colorado Sunken Pan operates similarly, as its unique design helps maintain stable conditions, leading to more reliable findings over time.

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