Tillie Doyle
Pan coefficients are used to estimate the reference evapotranspiration (ETo) in a given area, which is essential for irrigation practices and water resource management. The most common method for calculating ETo is the FAO-56 Penman-Monteith (PM) equation, which requires meteorological data. However, in regions with poor or missing meteorological observations, alternative methods such as the FAO-24 Pan method are used, which relies on pan coefficients (Kpan). To find the pan coefficient, water is allowed to evaporate from a pan over a certain period, typically 24 hours, and the amount of evaporation per time unit is calculated. This value, known as pan evaporation (Epan), is then multiplied by the pan coefficient, Kpan, to obtain the ETo. The Kpan value depends on factors such as daily wind run, average relative humidity, and fetch distance, and it can vary based on the type of pan used, with Class A and Sunken Colorado pans being the most common.
| Characteristics | Values |
|---|---|
| What is it used for? | To estimate plant water use and water demands, and to determine crop water requirements. |
| How is it calculated? | The amount of evaporation per time unit (the difference between two measured water depths) is calculated. This is the pan evaporation: E pan (in mm/24 hours). The E pan is multiplied by a pan coefficient, K pan, to obtain the ETo. |
| What is the pan evaporation method? | A measurement that combines the effects of several climate elements: temperature, humidity, rainfall, drought dispersion, solar radiation, and wind. |
| What is the most common theoretical method for calculating ETo? | The modified Penman method. |
| What is the most common pan used? | The "Class A" evaporation pan. |
| What is the pan coefficient determined by? | The daily wind run (WR), the average relative humidity (RH), and the fetch distance (FD) of the pan. |
| What is the best Kpan model? | The study referenced states that the best Kpan model was Cuenca (1989) with an agreement index value of 0.927. |
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What You'll Learn
The FAO-24 Pan method
To apply the FAO-24 Pan method, the pan evaporation (Epan) is calculated by measuring the amount of evaporation per unit of time from a water surface in the pan. This Epan value is then multiplied by the appropriate Kpan to obtain the ETo. The colour, size, position, and type of the pan can influence the measured results, so these factors should be considered when selecting the pan coefficient. Additionally, the ground cover, surroundings, wind, and humidity conditions can also impact the results.
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The Blaney-Criddle method
This method relies solely on temperature data, making it useful in areas where climatic data is limited to air temperature. While it provides a rough estimate, the Blaney-Criddle equation is not very accurate, especially under extreme climatic conditions. For instance, in windy, dry, and sunny areas, evapotranspiration may be underestimated by up to 60%, while in calm, humid, and cloudy areas, it may be overestimated by up to 40%.
The Blaney-Criddle formula, developed by H. F. Blaney and W. D. Criddle in 1950, is expressed as:
U = K * F
Where:
- U represents the consumptive use over a specific period (in inches)
- K is an empirical coefficient for the given period, typically the growing season
- F denotes the sum of monthly consumptive use factors
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Pan evaporation
There are various types of evaporation pans, including the Class A evaporation pan, the Sunken Colorado Pan, and the Symons Pan. The Class A evaporation pan is a standard National Weather Service pan for measuring water evaporation. It is a cylinder with a diameter of 46.5-47.5 inches (117.6-120.7 cm) and a depth of 10 inches (25 cm). The pan is placed on a levelled wooden base and is often enclosed by a chain-link fence to prevent interference from animals and insects. The pan is filled with water to within 2-2.5 inches of the top, and the evaporation rate is determined by measuring the depth of water after a certain period, typically 24 hours. If it rains during this period, the amount of rainfall is also measured, and if necessary, water is added or removed from the pan to maintain the initial water level. The evaporation rate is then calculated by measuring the amount of water needed to refill the pan to its original level.
The Sunken Colorado Pan is square, with a side length of 0.92 m (3 ft) and a depth of 0.46 m (18 inches). It is made of unpainted galvanized iron and is buried in the ground, with only about 5 cm (2 inches) of its rim above the surface. The Symons Pan, also known as the Symons Tank, is used in India, Europe, and South Africa. It is a steel container with a side length of 1.83 m (6 ft) and a depth of 0.61 m (2 ft). It is painted black internally and is sunk into the ground with an above-ground rim of 7.6-10 cm (3.0-3.9 inches). The Symons Pan has a lower evaporation rate than the Class A pan, and conversion factors must be used when comparing the two.
The accuracy of pan evaporation measurements can be affected by various factors, such as heavy rainfall, local terrain moisture levels, and changes in the local environment, such as increasing tree density. To improve the accuracy of measurements, research has been conducted on installation practices, and various formulas and models have been developed to account for different climatic conditions and surrounding factors. For example, the Food and Agriculture Organization (FAO) of the United Nations recommends the FAO-56 Penman-Monteith (PM) equation as the standard method for calculating reference evapotranspiration (ETo). However, this method requires a significant amount of meteorological data, which may not always be available. As a result, simpler methods such as the FAO-24 Pan method, which only requires pan coefficients (Kpan), are also commonly used. It is important to accurately determine these pan coefficients to ensure reliable results when using the FAO-24 Pan method.
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Class A and Sunken Colorado pans
The pan coefficient is used to convert pan evaporation values into lake evaporation values. It accounts for differences in heat transfer between pans and natural water bodies. The type of pan, its environment, and the climate all influence the pan coefficient.
The Class A evaporation pan is the standard pan used by the US Weather Bureau. It has a diameter of 1210 mm and a depth of 255 mm. The depth of water in the pan is maintained between 18 cm and 20 cm. The pan is usually made of unpainted galvanised iron sheet, although Monel metal is used when corrosion is an issue. The pan is placed on a wooden platform 15 cm above the ground to allow air circulation.
The Sunken Colorado Pan has a square side of 920 mm and a depth of 460 mm. It is made of unpainted galvanised iron sheet and is buried 50 mm below the ground. The advantage of the Sunken Colorado Pan is that its radiation and aerodynamic characteristics are similar to those of a lake. However, it can be difficult to detect leaks.
Values of Kp for Class A and Sunken Colorado pans under various plant covers and environmental and climatic conditions are presented in FAO-24 (Doorenbos and Pruitt, 1977) and FAO-56 (Allen et al., 1998). When observed conditions are outside the range listed in the tables, estimates of Kp values may be inaccurate. Frevert et al. (1983), Cuenca (1989), Snyder (1992), Allen et al. (1998), Raghuwanshi and Wallender (1998) and Grismer et al. (2002) developed regression models to determine Kp based on data from Class A pans.
The M5 model tree has been used to determine Class A and Sunken Colorado pan coefficients, providing more accurate estimates of Kp than other methods.
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The Penman-Monteith equation
While the Penman-Monteith equation is generally accurate, errors when compared to direct measurements or other techniques can range from -9 to 40%. This equation also involves a number of parameters with different units, which can be a source of confusion for first-time users. Therefore, it is important to carefully consider the units when applying the equation. Additionally, the standard Penman-Monteith equation can be modified for use with an hourly time step, as done by the American Society of Civil Engineers.
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Frequently asked questions
A pan coefficient, also known as Kpan or Kp, is a value used to calculate the reference evapotranspiration (ETo) from pan evaporation (Epan). ETo is a standard method for determining crop water requirements and is used in agriculture and hydro-meteorological studies.
Pan evaporation measurements help farmers understand how much water their crops will need. The pan coefficient is a critical component of this process, as it accounts for the effects of various climatic factors on evaporation rates, such as temperature, humidity, rainfall, drought dispersion, solar radiation, and wind.
The pan coefficient is calculated by multiplying the pan evaporation rate (Epan) by the pan coefficient value (Kpan or Kp). The pan evaporation rate is determined by measuring the amount of water evaporated from a pan over a specific period, typically 24 hours.
There are several methods to determine the pan coefficient. One common method is the FAO-24 Pan method, which is simple and only requires pan coefficients. Other methods include the Blaney-Criddle method, which uses temperature data, and various regression models, such as the M5 regression tree model, that utilise data mining techniques to predict pan coefficients.
The accuracy of the pan coefficient can be influenced by several factors, including the local environment, moisture levels, and the specific type of pan used (e.g., Class A pan or Sunken Colorado pan). Additionally, the accuracy of pan coefficient models can vary depending on the location and climatic conditions, so calibration for different climates may be necessary.
