Types of evaporimeters with sketches

An evaporimeter (evaporation pan) is an open container used to measure the evaporation from a free water surface. The depth of water lost per day gives the pan evaporation, which is converted to lake evaporation by a pan coefficient:

$$E_{lake} = C_p \times E_{pan}$$

1. Class A Land Pan (US Weather Bureau)

• Circular, 120.7 cm diameter and 25 cm deep, made of unpainted galvanised iron.
• Placed on a wooden platform 15 cm above ground; water is kept 5 cm below the rim.
• Evaporation is found from the daily change in water level (hook gauge in a stilling well).
• $C_p = 0.70$ (range 0.60 to 0.80).

      ____________  <- water 5 cm below rim
     |            |
     |   water    |  25 cm
     |____________|
   ====wooden frame====  15 cm
   ///////////////////  ground
          120.7 cm dia

2. ISI (Bureau of Indian Standards) Pan

• A modified Class A pan: cylindrical, 122 cm diameter and 25.5 cm deep, made of 0.9 mm copper sheet, tinned inside and painted white outside.
• A wire-mesh cover is fitted to reduce interference by birds and animals.
• $C_p = 0.80$.

3. Colorado Sunken Pan

• Square, 92 cm side and 46 cm deep, buried in the ground with the rim 10 cm above ground.
• The water temperature is close to that of the soil; $C_p = 0.78$.

4. Floating Pan

• 90 cm square and 45 cm deep, floated on a raft in a lake; $C_p = 0.80$.
• It gives the best simulation of lake conditions, but is costly and hard to observe.

Pans overestimate evaporation because of the heat exchange through the pan walls, hence the coefficient.