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 CLASSE A EVAPORATION PAN


GENERAL

The US class A pan was recommended by WMO and IAHS as a reference instrument. Its performance has been studied under a range of climatic conditions within wide limits of latitude and elevation.

Several types of automatic evaporation pan are in use. The water level in the pan is kept automatically constant, and the amount of water added to or removed from the pan is recorded.
 
 
 

MEASUREMENT SPECIFICATIONS

The water level in the pan must be measured accurately before and after water is added. The water level may be determined by means of a hook gauge consisting of a movable scale and vernier fitted with a hook enclosed in a steel-water chamber in the pan.
The depth of the water in the pan should be maintained between 17,5cm and 20 cm
 
 

CONDITIONS OF USE

The class A evaporation pan is mounted above the ground on a wood floor. The site of the pan should be reasonably level and free from obstruction. At sites where normal climate and soil do not permit the maintenance of soil cover, the ground cover should be maintained as near as possible to the natural cover common in the area. Obstructions such as trees, buildings, shrubs, or instrument shelters should not be closer than four times the height of the object above the pan. The minimum size of the plot should be 15 by 20 metres.

A wire-mesh screen of standard design should be installed over the pan to prevent animals from drinking the water.

Daily evaporation is computed as the difference in water level in the pan on successive days, corrected for any precipitation during the period. The amount of evaporation that has occurred between two observations of water level in the pan is determined by : E = P ? ?d where P is the depth of precipitation during the period between the two measurements, and ? is the depth of water added (+) to or removed (-) from the pan.
 
 

TECHNICAL SPECIFICATIONS

It is generally made of stainless steel, circular shape with a diameter of 121,9cm and a depth of 25,4cm.

The pan should be equipped with a hook gauge evaporimeter enclosed in a steel-water chamber for accurate reading of the water variations in the pan.

The class A evaporation pan should be installed on a duckboard at about 15 cm above the ground level.



 COLORADO EVAPORATION PAN

GENERAL

It has the shape of a parallelepiped, the section is a square of 0.914 m length, the depth is of 0.426 m. It is buried in the soil in such a way that the upper edges have to be 10 cm above the ground level. The water level in the pan is maintained closely to the ground level.

The advantage is that the buried pans are less sensitive to the temperature variations and to side effect due to solar radiation.
 

MEASUREMENT SPECIFICATIONS

The water level in the pan must be measured accurately before and after water is added. The water level may be determined by means of a hook gauge consisting of a movable scale and vernier fitted with a hook enclose in a steel-water chamber in the pan.
 
 

CONDITIONS OF USE

The site of the pan should be reasonably level and free from obstruction. At sites where normal climate and soil do not permit the maintenance of soil cover, the ground cover should be maintained as near as possible to the natural cover common in the area. Obstructions such as trees, buildings, shrubs, or instrument shelters should not be closer than four times the height of the object above the pan. The minimum size of the plot should be 15 by 20 metres.

A wire-mesh screen of standard design should be installed over the pan to prevent animals from drinking the water.

Daily evaporation is computed as the difference in water level in the pan on successive days, corrected for any precipitation during the period. The amount of evaporation that has occurred between two observations of water level in the pan is determined by : E = P ? ?d where P is the depth of precipitation during the period between the two measurements, and ? is the depth of water added (+) to or removed (-) from the pan.
 

TECHNICAL SPECIFICATIONS

It is generally made of galvanized iron or sheet steel painted in grey, square shape of 0.914 m length, the depth is of 0.426 m. The depth of the water in the pan should be maintained closed to the ground level.

The pan should be equipped with a hook gauge evaporimeter enclosed in a steel-water chamber for accurate reading of the water variations in the pan.