Tiffany Haney
Pan evaporation is a weather measurement system that integrates the effects of several climate elements, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind velocity. The rate of evaporation is determined by measuring the depth of water in an evaporation pan at regular intervals, typically daily. These pans come in various shapes and sizes, with the Class A evaporation pan being commonly used in the United States and other countries. The pan is usually placed on a wooden platform and exposed to the atmosphere to simulate an open body of water. By measuring the amount of water loss in the pan, farmers and ranchers can understand how much water their crops require, and scientists can study the impact of global warming on evaporation rates.
| Characteristics | Values |
|---|---|
| Purpose | To determine the rate of evaporation based on weather factors |
| Container | Pans of varying sizes and shapes, including circular, square, and cylinder |
| Common Types | Class A evaporation pan, Sunken Colorado Pan, Symon's Pan/Tank |
| Dimensions | Class A pan: 46.5-47.5 inches in diameter and 10 inches in depth; Sunken Colorado Pan: 3 ft wide and 18 inches deep; Symon's Pan: 6 ft wide and 2 ft deep with a 3-3.9 inch above-ground rim |
| Placement | On a leveled wooden base or platform, often enclosed by a fence to prevent interference |
| Measurement Technique | Measuring water depth before and after a set time period (usually 24 hours) or using instruments like a hook gauge, water level transmitter, or data logger |
| Environmental Factors | Temperature, humidity, rainfall, drought dispersion, solar radiation, and wind velocity |
| Maintenance | Regular refilling, especially in heavy rainfall conditions, and protection from interference by animals and insects |
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What You'll Learn
Pan evaporation measurement systems
Pan evaporation is a measurement that combines the effects of several climate elements: temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. The rate of evaporation is highest on hot, windy, dry, and sunny days. It is greatly reduced when clouds block the sun and when the air is cool, calm, and humid.
There are several types of evaporation pans used to measure pan evaporation. The most well-known are the "Class A" evaporation pan, the "Sunken Colorado Pan", and the "Symon's Pan" (also known as the "Symon's Tank"). The Class A evaporation pan is a standard National Weather Service pan for measuring water evaporation. 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 typically placed on a carefully levelled wooden base and is often enclosed by a chain-link fence to prevent animals from drinking from it. The pan is filled with water to within 2.5 inches of the top and exposed to the atmosphere to represent an open body of water. The evaporation rate can then be measured manually or with an analog output evaporation gauge, a low-range submersible water level transmitter, or a data logger.
The Sunken Colorado Pan is square, with each side measuring 0.92 metres (3 feet) and a depth of 0.46 metres (18 inches). It is made of unpainted galvanised iron and is buried in the ground so that only 5 centimetres (2 inches) of the rim is showing. The Symons Pan, used in Europe, India, and South Africa, is a steel container with sides measuring 1.83 metres (6 feet) and a depth of 0.61 metres (2 feet). It is painted black internally and sunk into the ground with an above-ground rim of 7.6-10 centimetres (3.0-3.9 inches). Its evaporation rate is lower than the Class A pan, and conversion factors must be used when comparing measurements.
To measure evaporation, the depth of water evaporated from the pan is measured at regular intervals (often daily). Meteorological data such as humidity, wind velocity, air and water temperatures, and precipitation are also measured and noted. Diagonal baffles in the pan can help to reduce surging due to wave action. Evaporation pans can also be automated with water level sensors, and a small weather station may be located nearby. Additionally, tools such as the 255-212 Min-Max Thermometer can be used to measure the minimum and maximum temperatures of water in an evaporation pan. This device features a digital thermometer mounted on a non-magnetic base and includes a built-in solar radiation shield to protect against direct sunlight.
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Evaporation pan types
Evaporation pans are normally placed at stations where other hydro-meteorological data are collected. They are used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans come in varying sizes and shapes, the most commonly used being circular or square.
The best-known pan is the "Class A" evaporation pan, which is a standard National Weather Service pan type for measurement of water evaporation. It is a cylinder with a diameter of 46.5–47.5 inches (118–121 cm) and a depth of 10 inches (25 cm). The pan is placed on a carefully levelled wooden base and is often enclosed by a chain-link fence to prevent animals from drinking from it. The rate of evaporation is determined daily by recording the depth of water. The initial quantity of water is set at exactly two inches; at the end of the day, the water is then refilled. The amount of water it takes to fill the pan back to two inches is the rate of evaporation.
Another type of evaporation pan is the "Sunken Colorado Pan", which is square, 0.92 m (3 ft) on a side, and 0.46 m (18 inches) deep, made of unpainted galvanized iron. As the name suggests, it is buried in the ground to within about 5 cm (2 inches) of its rim. Evaporation from a Sunken Colorado Pan can be compared with a Class A pan using conversion constants. The pan coefficient, on an annual basis, is about 0.8.
In Europe, India and South Africa, a Symon's Pan (or Symon's Tank) is used. It is a steel container 1.83 m (6 ft) on a side and 0.61 m (2 ft) deep, sunk into the ground with an above-ground rim of 7.6–10 centimetres (3.0–3.9 in) and is painted black internally. Its evaporation rate is lower than the Class A pan and conversion factors must be used.
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How to calculate evaporation rate
Pan evaporation is a method of measuring the rate of water loss due to evaporation from a pan. This method is used to estimate the evaporation rate from a large body of water, such as a lake. The rate of evaporation is influenced by several factors, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. Evaporation is typically highest on hot, dry, sunny, and windy days, and lowest when the sun is blocked by clouds and the air is calm and humid.
To calculate the evaporation rate using a pan, the pan is first prepared and positioned. The pan is usually placed on a wooden platform raised a few centimetres above the ground to allow for air circulation. The pan is then filled with water to a fixed level, often around 2 inches (5 cm) from the top. In some cases, the pan may be covered with a wire mesh to prevent interference from birds and other animals.
Measurements are typically taken at regular intervals, commonly once every 24 hours. After the specified time period has passed, the amount of water that needs to be added to return the pan to the initial water level is measured. This amount represents the water lost due to evaporation during that time period. It is important to note that if precipitation occurs during the measurement period, it is taken into account by removing the excess water from the pan before refilling it to the initial level.
The evaporation rate can be calculated by dividing the amount of water evaporated (in millimetres) by the number of days since the last measurement. This provides a daily evaporation rate that can be used to estimate water loss over longer periods. Additionally, there are formulas and conversion factors available to compare evaporation rates between different types of pans, such as the Class A pan and the Sunken Colorado Pan, and to estimate evaporation rates for larger bodies of water like lakes.
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Environmental factors affecting evaporation
Environmental factors play a significant role in the evaporation process measured through pan evaporation methods. Pan evaporation combines the effects of several climate elements, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind.
Temperature is a critical factor, with higher temperatures generally leading to increased evaporation rates. Warmer global temperatures have been linked to changes in hydrological factors such as precipitation, evaporation, and water infiltration. However, it is important to note that evaporation from pans can be influenced by their size; larger pans may have evaporation rates similar to large lakes, while smaller pans may have higher evaporation rates.
Humidity is another key factor influencing evaporation. An increase in humidity in the environment surrounding the evaporation pan can lead to reduced evaporation rates. This relationship was observed in the Haihe River Basin, China, where a significant increase in relative humidity contributed to a decline in pan evaporation (Epan). Additionally, changes in the local environment, such as increased tree density near the pans, can elevate surface friction and slow local wind runs, further reducing evaporation.
Solar radiation and sunshine duration also impact evaporation rates. Evaporation is typically greater on sunny days, as solar radiation provides the energy required for liquid water to convert into water vapour. A study analyzing trends in the Haihe River Basin found that a significant reduction in sunshine duration was the primary cause of declining Epan values.
Wind speed and direction are additional factors influencing evaporation. Evaporation is often higher on windy days, as wind promotes the removal of water vapour from the evaporating surface. However, the height of the pan's rim can affect wind action over the water surface, and the shadow cast by the rim can also impact evaporation rates.
It is worth noting that while pan evaporation provides valuable insights, it does not perfectly replicate large reservoirs or natural bodies of water. Therefore, conversion factors and equations are used to estimate evaporation from lakes and other water bodies based on pan evaporation measurements.
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Uses of pan evaporation data
Pan evaporation is a weather measurement system that combines several climate elements, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind velocity. The data collected from pan evaporation measurements have several uses, which are described in detail below:
Agriculture
Farmers and ranchers use pan evaporation data to understand how much water their crops require. This information is crucial for effective irrigation practices and can help improve crop yields.
Climate Research
Scientists use pan evaporation data to study the effects of global warming and the resulting changes in the hydrological cycle. By comparing long-term trends in pan evaporation rates, researchers can better understand the impact of climate change on water resources and develop strategies to mitigate its effects.
Meteorology
Meteorological organizations, such as the National Weather Service in the United States, use pan evaporation data to standardize measurements and predict weather patterns. This information aids in forecasting and understanding local and regional climate conditions.
Hydrology
Hydrologists use pan evaporation data to estimate evaporation rates from larger water bodies, such as lakes and reservoirs. By applying conversion factors and pan coefficients, they can determine the amount of water lost through evaporation in different hydro-meteorological conditions.
Environmental Monitoring
Pan evaporation data can be used to monitor environmental changes and local terrain moisture levels. By comparing pan evaporation rates over time, scientists can identify trends and anomalies that may be indicative of ecological shifts or the impact of human activities on the environment.
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