Tiffany Haney
Pan evaporation is a weather measurement system that integrates several climatic conditions, including temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. The rate of evaporation is highest during hot, sunny, windy, and dry days and lowest during cloudy, calm, and humid weather. An evaporation pan holds the water used during the process, with the observer noting the quantity of water under certain weather conditions and the change in quantity over time. Pans come in different sizes and shapes, the most common being circular and square. The most well-known pans are the Class A evaporation pan and the Sunken Colorado Pan, with the Symon's Pan being popular in India, Europe, and South Africa.
| Characteristics | Values |
|---|---|
| Purpose | To measure evaporation rate and determine the amount of water required by crops |
| Pan Type | Class A, Sunken Colorado, Symons Pan/Tank |
| Pan Shape | Circular, Square |
| Pan Material | Stainless Steel, Unpainted Galvanized Iron |
| Pan Size | Diameter: 47.5 inches (120.7 cm), Depth: 10 inches (25 cm) |
| Water Level | Maintained at a fixed mark, typically 2 inches below the top |
| Measurement Technique | Manual readings, Hook Gauge, Water Level Transmitter, Analog Output Evaporation Gauge |
| Environmental Factors | Temperature, Humidity, Rainfall, Drought Dispersion, Solar Radiation, Wind |
| Limitations | Not suitable for heavy rainfall (>30mm), errors in measurements due to local environment |
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What You'll Learn
Pan evaporation measurements
Pan evaporation is a weather measurement system that combines several climate elements: temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. Evaporation is highest on hot, windy, dry, sunny days and is reduced when clouds block the sun, and when the air is cool, calm, and humid. Pan evaporation is particularly useful for farmers and ranchers, who use the measurements to understand how much water their crops will need.
The pans themselves vary in size and shape, with circular and square pans being the most common. Pans are often automated with water level sensors and a small weather station located nearby. The Symons pan or tank is a standard instrument of the UK Met Office, and is a steel container that is 1.83m (6ft) on each side and 0.61m (2ft) deep, sunk into the ground with a rim of 7.6-10cm (3.0-3.9 inches) above ground. Its evaporation rate is lower than the Class A pan, so conversion factors must be used.
The Class A Evaporation Pan is a cylinder with a diameter of 47.5 inches (120.7cm) and a depth of 10 inches (25cm). It is placed on a carefully levelled wooden base and is often enclosed by a chain-link fence to prevent animals and insects from interfering with the water. The pan is filled with water to within 2.5 inches of the top and the evaporation rate is determined by measuring the depth of water after 24 hours. The amount of water needed to refill the pan to the initial quantity is the evaporation rate.
The Sunken Colorado Pan is square, 0.92m (3ft) on each side and 0.46m (18 inches) deep, made of unpainted galvanised iron. It is buried in the ground to a depth of 2 inches below its rim.
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$22.17
Climate elements
Pan evaporation is a measurement that integrates the effects of several climate elements: temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. Evaporation is typically highest on hot, dry, sunny, and windy days. Conversely, evaporation is significantly reduced when the sun is blocked by clouds, and when the air is cool, calm, and humid.
Pan evaporation measurements are particularly useful for farmers and ranchers, as they indicate how much water their crops will need. An evaporation pan is used to hold water during observations for determining the quantity of evaporation at a given location. Pans come in various sizes and shapes, with circular or square being the most common. The "Class A" evaporation pan and the "Sunken Colorado Pan" are well-known examples. In Europe, India, and South Africa, a Symon's Pan (or Symon's Tank) is used.
Evaporation pans are often automated with water level sensors, and a small weather station is usually located nearby. 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 placed on a carefully leveled wooden base and is sometimes enclosed by a chain-link fence to prevent animals from drinking from it.
The measurement process involves filling the pan to exactly two inches from the top at the start of the day. After 24 hours, the amount of water needed to refill the pan to the initial level is measured. If precipitation occurs during this period, it is taken into account when calculating evaporation. In cases of significant rainfall, the Class A pan may need to be emptied more than once per 24 hours to prevent overflowing.
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Evaporation pans
The "Class A" evaporation pan is a cylinder with a diameter of 46.5-47.5 inches (118.1-120.7 cm) and a depth of 10 inches (25 cm). It rests 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 to within 2-2.5 inches of the top with water, which is then exposed to represent an open body of water. The evaporation rate can be measured by manual readings or with a low-range submersible water level transmitter, data logger, or analogue output evaporation gauge. The amount of water evaporated is then replaced, and the new water level is recorded.
The Symons pan or tank is a steel container used by the UK Met Office. It is 6 feet (1.83 m) long, 2 feet (0.61 m) deep, and painted black internally. Its evaporation rate is lower than the Class A pan. The Sunken Colorado Pan is square, with a width of 3 feet (0.92 m) and a depth of 18 inches (0.46 m). It is made of unpainted galvanised iron and is buried in the ground to a depth of 2 inches (5 cm) from its rim.
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Measuring evaporation rates
Evaporation pans are used to measure the rate of evaporation in a given location. They are popular among farmers and ranchers who need to understand how much water their crops require. The pans come in various sizes and shapes, but they are typically circular or square.
The most well-known evaporation pans are the "Class A" evaporation pan and the "Sunken Colorado Pan". The Class A evaporation pan is a cylinder with a diameter of 46.5-47.5 inches (118.1-120.7 cm) and a depth of 10 inches (25 cm). It is placed on a levelled wooden base and often enclosed by a chain-link fence to prevent animals and insects from interfering. The Sunken Colorado Pan is square, with a width of 3 feet (0.92 m) and a depth of 18 inches (0.46 m). It is made of unpainted galvanized iron and is buried in the ground, with only 2 inches of its rim above the surface.
To measure evaporation, the pan is filled with water to a certain level, typically within 2-2.5 inches of the top. The water level is then monitored over a 24-hour period, and the amount of water needed to refill the pan back to the original level is measured. This amount represents the rate of evaporation for that day. If precipitation occurs during the 24-hour period, it is taken into account in the calculations.
In some cases, evaporation pans are automated with water level sensors and a small weather station located nearby. The rate of evaporation can be determined manually or with the help of instruments such as a water level transmitter, data logger, and evaporation gauge.
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Drawbacks of evaporation pans
Evaporation pans are a weather measurement system that combines several climatic conditions, including rainfall, humidity, solar radiation, wind, temperature, and drought dispersion. While they are popular among farmers, they have certain drawbacks.
Firstly, evaporation pans are not exact models of large reservoirs. Their heat storage capacity and heat transfer characteristics differ from those of reservoirs. The heat transfer characteristics of the pan material are also different from those of a reservoir, requiring corrections to the evaporation measurements to match the conditions of a large lake under identical climatic and exposure conditions. The height of the rim in an evaporation pan also affects wind action over the water surface and casts a shadow of varying size on the water surface, impacting evaporation rates.
Secondly, the size of the evaporation pan can impact the rate of evaporation. A larger pan provides a greater surface area for evaporation, potentially resulting in faster evaporation rates. However, if the energy source is fixed, as with an electric burner, a pan much wider than the burner may result in uneven heating and a lower evaporation rate.
Additionally, evaporation pans may not always account for local environmental factors. Changes in the local environment, such as increasing tree density near the pans, can elevate surface friction and slow local wind runs, reducing pan evaporation. This phenomenon, known as the evaporation paradox, highlights the need to compensate for local terrain moisture levels in the data analysis.
Furthermore, certain types of evaporation pans, such as the Class A evaporation pan, have limitations during heavy rainfall events. On days with rainfall exceeding 30 mm, the Class A pan may overflow, and the daily evaporation rates may be spuriously higher due to the influence of heavy rainfall. Therefore, it is essential to empty the pan more than once per 24 hours during heavy rainfall to obtain accurate measurements.
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