Ella Walter
Pan evaporation is a measurement that combines the effects of several climate elements, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. It is used to estimate evaporation from lakes, with the most commonly used pans being the Class A evaporation pan, the Sunken Colorado Pan, and the Symons Pan. The process involves filling the pan with water and measuring the depth of water that evaporates over a 24-hour period, taking into account any precipitation that may occur. This measurement enables farmers and ranchers to understand how much water their crops require. While pan evaporation provides valuable insights, it is important to acknowledge that evaporation from a natural body of water, such as a lake, is typically lower due to differences in light penetration and the absence of metal sides that can heat up in the sun.
| Characteristics | Values |
|---|---|
| Purpose | To estimate the evaporation from lakes |
| Considerations | Light penetration in natural bodies of water decreases as depth increases, so textbooks suggest multiplying the pan evaporation rate by 0.75 |
| Climate Elements | Temperature, humidity, rainfall, drought dispersion, solar radiation, and wind |
| Pan Types | Class A, Sunken Colorado, Symon's Pan/Tank |
| Class A Pan Specifications | Cylinder with a diameter of 47.5 inches (120.7 cm) and a depth of 10 inches (25 cm) |
| Sunken Colorado Pan Specifications | Square, 0.92 m (3 ft) on each side and 0.46 m (18 inches) deep, made of unpainted galvanized iron |
| Symon's Pan/Tank Specifications | Steel container, 1.83 m (6 ft) on each side and 0.61 m (2 ft) deep, painted black internally |
| Measurement Frequency | Daily or monthly |
| Calculation Methods | Penman's formula, one-dimensional eddy diffusion model, energy budget evaluation, Bowen-ratio method, pan conversion method |
| Factors Affecting Accuracy | Evaporation rate, solar irradiance, pan geometry, latitude, elevation, solar declination, cloud cover |
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What You'll Learn
Using a Class A evaporation pan
Pan evaporation is a measurement that combines the effects of several climate elements: temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. It is used to estimate the evaporation from lakes. The most commonly used evaporation pans are circular or square, with the "Class A" evaporation pan being the most well-known.
In the United States, the National Weather Service has standardized its measurements on the Class A evaporation pan. It is a cylinder with a diameter of 47.5 inches (120.7 cm) and a depth of 10 inches (25 cm). The pan is placed on a carefully leveled wooden base and is often surrounded by a chain-link fence to prevent animals from drinking from it.
To measure evaporation using a Class A pan, the pan is filled to exactly two inches (5 cm) from the top at the start of the 24-hour measurement period. After 24 hours, the amount of water needed to refill the pan to the exact same level is measured. If there is precipitation during the 24-hour period, it is taken into account in the calculations. In some cases, precipitation may exceed evaporation, and water must be dipped out of the pan. It is important to note that evaporation cannot be measured if the water surface in the pan is frozen.
The Class A evaporation pan has a few limitations. For example, during daily rainfall events of >55mm, the pan is likely to overflow. Additionally, heavy or intense rainfall can cause spuriously high daily evaporation totals without obvious overflow. Therefore, it is important to consider these factors when using a Class A pan for evaporation measurements.
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Multiplying pan evaporation by 0.75
Pan evaporation is a measurement that combines the effects of several climate elements, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. It is used to estimate the evaporation from lakes, as there is a correlation between lake evaporation and pan evaporation.
However, evaporation from a natural body of water like a lake is usually at a lower rate than that of a pan. This is because a lake does not have metal sides that heat up in the sun, and light penetration in a lake decreases as depth increases, unlike in a pan. To account for this discrepancy, most textbooks suggest multiplying the pan evaporation rate by 0.75.
The formula for calculating pan evaporation varies depending on the type of pan used. The most commonly used pans are circular or square, with the "Class A" evaporation pan and the "Sunken Colorado Pan" being the best known. In Europe, India, and South Africa, a Symon's Pan (or Symon's Tank) is used.
The National Weather Service in the United States has standardized its measurements on the Class A evaporation pan, a cylinder with a diameter of 47.5 inches (120.7 cm) and a depth of 10 inches (25 cm). The pan is typically placed on a carefully leveled wooden base and is often enclosed by a chain-link fence to prevent animals from drinking from it.
To measure evaporation using a Class A pan, the pan is filled to exactly two inches (5 cm) from the top at the start of the 24-hour measurement period. After 24 hours, the amount of water needed to refill the pan to the exact same level is measured. If precipitation occurs during this period, it is taken into account in the calculations.
By multiplying the pan evaporation rate by 0.75, we can correct for the difference in evaporation rates between the pan and a natural body of water like a lake. This adjustment factor helps to account for the unique characteristics of lakes, such as their larger surface area and the absence of metal sides that can affect the rate of evaporation.
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Estimating monthly mean lake evaporation
To estimate monthly mean lake evaporation, several methods can be employed:
- The energy balance method: This approach considers the lake's energy balance and yields an evaporation rate.
- The Penman method: This is a simplified formula that does not require wind speed data, making it useful when reliable wind speed data is unavailable. It uses the Penman equation to estimate potential evaporation from open water.
- The Complementary Relationship Lake Evaporation model (CRLE): This model applies monthly averaged values of air temperature, air humidity, and sunshine duration to estimate evaporation rates.
- Eddy covariance measurements: This technique involves using instruments to measure cumulative evaporation from different locations near the lake, capturing the sensitivity and variability of pan estimates.
- Numerical models and innovative experimental designs: These tools help diagnose processes and accurately forecast evaporation, improving our understanding of spatial and temporal variations in evaporative water loss.
- Alluvial aquifer storage and recovery (ASR): This practice involves storing excess water in alluvium near a river, offering a potential option for water managers facing increasing population and climate change challenges.
It is important to note that errors in estimating monthly mean evaporation can occur, as seen in the estimates for Copenhagen with an error rate of around 0.3 mm day-1. Therefore, it is essential to continuously improve estimation methods and incorporate new knowledge to enhance the accuracy of reservoir evaporation forecasts.
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Converting pan evaporation to lake evaporation
Pan evaporation is a measurement that combines the effects of several climate elements: temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. An evaporation pan is used to hold water during observations to determine the quantity of evaporation at a given location. The most commonly used pans are the "Class A" evaporation pan and the "Sunken Colorado Pan". In Europe, India, and South Africa, a Symon's Pan (or Symon's Tank) is used.
Pan evaporation is used to estimate the evaporation from lakes. There is a correlation between lake evaporation and pan evaporation. Evaporation from a natural body of water is usually at a lower rate because the body of water does not have metal sides that get hot in the sun, and while light penetration in a pan is essentially uniform, light penetration in natural bodies of water will decrease as depth increases. Most textbooks suggest multiplying the pan evaporation by 0.75 to correct for this.
The pan evaporation method can be used to estimate monthly total lake evaporation with a standard error of less than 10%. This method takes into account the different water surface temperatures of the lake and the pan, as well as the corresponding saturation vapour pressures. The coefficient of 1.50 is established from Lake Hefner data with an estimated standard error of 5%.
To convert evapotranspiration to evaporation, one can divide the reference evapotranspiration (ETo) by the pan coefficient (Kp). This coefficient may vary between 0.55 and 0.80 and can be estimated using meteorological data.
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Accounting for different water surface temperatures
When calculating evaporation from a lake, it is important to consider the different water surface temperatures. Evaporation is one of the primary processes of water loss from lakes and is influenced by various factors such as temperature, humidity, rainfall, drought dispersion, solar radiation, and wind.
To account for different water surface temperatures, a one-dimensional eddy diffusion model of the lake's water temperature distribution can be used. This model evaluates daily evaporation values as a residual of the energy budget in the lake. By calibrating the model with measured values from different years, it can be used to estimate evaporation rates for other lakes with similar conditions.
Additionally, monthly mean lake evaporation can be estimated using a simplified version of Penman's formula:
E0 = f(T, z, Rs, u, Td)
Where:
- E0 is the monthly mean lake evaporation
- T is the daily mean temperature (average of extremes)
- Z is the elevation
- Rs is the solar irradiance of the lake's surface
- U is the windspeed at 2 meters
- Td is the dewpoint temperature
This formula takes into account the temperature of the lake's surface and can provide reasonably accurate estimates of lake evaporation.
Furthermore, lake evaporation can be estimated from nearby pan observations using a U.S. Class A pan. This method considers the different water surface temperatures of the lake and the pan by accounting for their corresponding saturation vapour pressures. The formula for this conversion method involves afternoon average and maximum vapour pressure values, with a coefficient established from lake data. This approach can provide monthly total lake evaporation estimates with a standard error of less than 10%.
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Frequently asked questions
Pan evaporation is a measurement that combines the effects of several climate elements, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind.
Pan evaporation data is primarily used to estimate lake evaporation. The ratio of lake evaporation rate to pan evaporation rate is about 0.77 for a US Class-A pan. This ratio varies with several factors, including the evaporation rate, solar irradiance, and geometry of the pan.
The pan-evaporation formula allows for the geometry of a US Class-A pan evaporimeter and the wetness and albedo of its surroundings. The formula includes an estimate of solar irradiance, which may be obtained from extra-terrestrial radiation and cloudiness derived from rainfall figures.
