Evaporation Pan

The circular pan constructed of monel metal  is 10 inches deep, 47 ½ inches in diameter (inside diameter).

The pan should be centered on supports resting on leveled ground free from flooding.. The top of the support should be ½ inch above the dirt leaving leaving an air space between the bottom of the pan and the ground  to simplify inspecting the pan for leaks.

Maintenance

Inspect the pan for leaks when reading the pan daily and report any  leaks to the NWS for  repair or replacement. Clean the pan as often as necessary to keep it free from sediment, scum, algae, and oil films. The albedo or the amount of solar radiation that is reflected back from the pan is affected by how  the pan is maintained. A dirty pan will have higher evaporation rates than a clean pan. Oil films greatly reduce evaporation.  Empty the pan by siphoning the water out. During months when freezing conditions are likely, empty, clean, and store the pan   upside down. A small amount of copper sulfate may be added to the water to discourage algae growth.

Measuring the Amount of Evaporation

Evaporation is measured either using a point gage or or hook gage and a stilling well.

Using the point gage, evaporation is measured by determining the amount of water required to bring the water level in the stilling well exactly to the tip of the pointed rod. Use the transparent measuring tube to add or remove water from the evaporation pan. When water must be added, fill the measuring tube to the zero mark (the top mark on the tube), then pour (slowly) exactly enough water into the evaporation pan (not in the stilling well) to bring the water level to the tip of the fixed point. Next, read the level of water remaining in the measuring tube. If this reading is closest to the 12 mark, for example, 0.12 inches of water has evaporated (or else evaporation has exceeded precipitation by 0.12 inches).

If precipitation has occurred since the previous observation, the water level may be above the tip of the fixed point. In this case, remove water by filling the measuring tube up to the zero level with water from the evaporation pan as many times as necessary to bring the water level to the fixed point. Keep track of the number of times the tube is filled. Each filling represents 0.15 inches of water. When enough water has been removed to bring the water level below the fixed point, fill the measuring tube with water  to the zero level, and pour enough water back into the pan to bring the level to the tip of the fixed point. Deduct this amount from the total removed.

For recording purposes, water added is positive and water removed is negative. For example, if 0.15 inches of water must be added, record this as +0.15. If rain has fallen and the measuring tube must be filled three times to bring the level below the fixed point, 0.45 inch is deducted. If 0.06 inch of water is then added to bring the level back to the fixed point, enter the sum of –0.45 and +0.06, or –0.39, on the recording form ( Form B-92).

To measure the amount of evaporation using the hook gage, place the hook gage in the stilling well and lower the hook below the water surface. Raise the hook gage until the tip of the hook reaches the water level. Read the value of the hook gage. The evaporation is equal to the difference in the hook gage since the previous reading. When the water level fall to 3 inches below the pan lip, fill the pan after taking an evaporation pan reading and record a new hook gage reading.

Filling the pan every day using the point gage system of measuring pan evaporation results in more consistent measurements. When the water level drops daily using the hook gage method, the amount of evaporation is affected  not only by the climatic conditions but by the depth  the water is below the pan rim

B-92 form with the columns for evaporation pan readings.