Water Management in Horticultural Crops

4.1.USWB Class-A pan evaporimeter

• The standard USWB Class-A pan evaporimeter is the most widely used evaporimetr in the world for finding evaporation from the free water surface. It consists of a 121.5 cm diameter and 25.4 cm deep pan made of 20 gauge galvanized iron sheet with a stilling well. A vertical pointer is provided in the stilling well to show the level of water maintained in the pan. The pan is painted white and is placed on a wooden frame so that air may circulate beneath the pan.
• Daily evaporation rate is given by the fall of water level in the stilling well during 24-hour period. Measurements of the fall of water level may be made at closer intervals to know the evaporation rate during different parts of a day. Water levels in the stilling well are measured by hook gauge. Adjustments are made to the evaporation values if rain occurs during a period of measurement.
• The rainfall is measured by standard rain gauge. Evaporation loss may also be computed from the measured quantity of water added to bring the water level to the tip of the pointer in the stilling well. The amount of water added is divided by the surface areas of pan and stilling well together to find out the depth of water added which is taken as the daily evaporation rate. After measuring the fall in water level each time, water is added to the pan to bring back the water level to the original position of pointer tip level.
• As the rate of evaporation from pan evaporimeter is higher than that over a large free water surface, the pan evaporation value is multiplied by 0.7 to obtain the evaporation rate over the large free water surface (Eo). The relationship between actual evaporation and pan evaporation rates may be presented as,


Where,

Kp = Pan evaporation coefficient (a commonly used value of 0.7)
Epan = Evaporation value from pan evaporimeter