17.3.1. Class A Evaporation Pan

• 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

The Class A Evaporation Pan is a widely utilized instrument for measuring evaporation. It is circular in shape and constructed from either galvanized iron or stainless steel, which makes it durable and suitable for outdoor conditions. Its diameter is 1207 mm, and it has a depth of 255 mm, ensuring a sufficient water surface area for evaporation measurements. The pan is typically filled with water to a depth of 180 mm for accurate readings and is mounted on a wooden platform that elevates it 15 cm above the ground. This elevation helps to minimize disturbances from the ground and allows for a more accurate evaporation measurement.

Think of the Class A Evaporation Pan as a large cooking pot on a stove, where the water inside the pot represents the water body you’re measuring evaporation from. Just like you need a certain amount of water for cooking, the pan needs to be filled to a specific level to measure the evaporation accurately. The height of the stove (or the wooden platform in this case) allows for effective heating, just as the platform permits accurate evaporation readings without interference from ground moisture.

• Measurement:
  • Water loss is measured daily with a hook gauge or point gauge

To ascertain how much water has evaporated from the Class A Evaporation Pan, a measurement is taken daily using specific tools such as a hook gauge or point gauge. This involves checking the water level in the pan after a set period (usually a day), allowing for the calculation of the evaporation rate based on how much water has been lost. Regular daily measurements provide a consistent dataset to analyze evaporation trends over time.

Consider yourself filling a bathtub with water and then checking the water level every day to see how much water has evaporated due to the heat. The gauges used with the Class A Evaporation Pan are like the measuring stick you use to check the height of the water level in your bathtub. Every morning, just as you would record any difference in your bathtub, the evaporation pan records any changes, providing valuable information about evaporation rates.

• 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)

While the Class A Evaporation Pan is an effective tool for measuring evaporation, it does have some limitations. Notably, it tends to record higher evaporation rates compared to larger water bodies like lakes or ponds. This discrepancy occurs because the pan's dimensions and environment differ from natural bodies of water. To reconcile this difference, a Pan Coefficient (Kₚ) is applied, which typically ranges from 0.7 to 0.8. This coefficient helps calibrate the pan's readings so that they more accurately reflect actual evaporation rates from larger water bodies.

Imagine trying to measure how much ice cream melts in a small bowl compared to a large tub on a hot day. The small bowl might show rapid melting because it has less volume to hold the ice cream. Similarly, the evaporation pan shows more evaporation because of its smaller size in comparison to a lake. We use the Pan Coefficient like adjusting our observations: if you notice the small bowl melts faster, you factor that into your expectations for the larger tub, so you measure accordingly.

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