Understanding Evaporation from Ponds

Evaporation is the process where water changes from a liquid state to a gaseous state (water vapor) and rises into the atmosphere. Measuring evaporation directly from a large body of water like a pond can be difficult. Therefore, scientists often use a standard instrument called an evaporation pan (like a Class A evaporation pan) to estimate evaporation. However, the evaporation rate from a pan is typically higher than that from a larger body of water because the pan's sides are exposed to sunlight and wind, and heat transfer through the pan bottom also occurs. To account for this difference, a pan coefficient is used.

What is a Pan Coefficient?

A pan coefficient is a multiplier used to convert pan evaporation measurements into estimated actual evaporation from a nearby lake, reservoir, or pond. The value of the pan coefficient is usually less than 1.0 and varies depending on the type of pan used, its surroundings, and the climate.

Formula for Calculating Pond Evaporation

The relationship between pan evaporation and evaporation from a pond (or other larger water body) is given by the formula:

\text{Pond Evaporation} = \text{Pan Evaporation} \times \text{Pan Coefficient}

Step-by-Step Calculation of Pond Evaporation

We are given the following information:

• Pan Evaporation = 45 mm
• Pan Coefficient = 0.70

We need to calculate the evaporation from the pond. Using the formula:

\text{Pond Evaporation} = \text{Pan Evaporation} \times \text{Pan Coefficient}

Substitute the given values into the formula:

\text{Pond Evaporation} = 45 \, \text{mm} \times 0.70

Now, perform the multiplication:

\text{Pond Evaporation} = 31.5 \, \text{mm}

So, the estimated evaporation from the pond is 31.5 mm.

Final Result

Based on the calculation, the evaporation from the pond is 31.5 mm.

Revision Table: Evaporation Concepts

Term	Definition	Relevance to Evaporation
Evaporation	Process of liquid water changing to water vapor.	Fundamental process being measured.
Pan Evaporation	Measurement of water depth reduction in an evaporation pan over a specific period.	A standard method for estimating evaporation rates.
Evaporation Pan	A meteorological instrument (e.g., Class A pan) used to measure evaporation.	The device used to get pan evaporation data.
Pan Coefficient	A factor used to convert pan evaporation to evaporation from a natural water body.	Corrects for the difference between pan and natural water body evaporation.
Pond Evaporation	Estimated actual evaporation from a pond.	The value we are trying to calculate.

Additional Information: Factors Affecting Evaporation

Evaporation rates from ponds and evaporation pans are influenced by several factors. Understanding these factors helps in interpreting the measurements and using the pan coefficient correctly.

• Temperature: Higher water and air temperatures increase evaporation.
• Humidity: Lower humidity leads to higher evaporation rates as the air can hold more water vapor.
• Wind Speed: Wind removes saturated air from above the water surface, allowing more evaporation to occur.
• Solar Radiation: Sunlight provides the energy needed for the phase change from liquid to vapor.
• Atmospheric Pressure: Lower pressure can slightly increase evaporation.
• Water Body Size: Larger water bodies tend to have lower evaporation rates per unit area compared to small pans due to heat storage, boundary layer effects, and less perimeter influence.
• Water Quality: Dissolved substances can slightly lower vapor pressure and reduce evaporation.

The pan coefficient helps bridge the gap between the easily measured pan evaporation and the more complex actual evaporation from a natural water body by implicitly accounting for some of these differences.