Measurement of changes in volume of water in an evaporation pan is a standard te
ID: 113927 • Letter: M
Question
Measurement of changes in volume of water in an evaporation pan is a standard technique for estimating potential evapotranspiration. United States Class A evaporation pans are cylindrical with the following dimensions: depth = 10.0 in and diameter = 47.5 in. This pan is in a corn field in southern Minnesota. Initially, the pan contains 10.0 U.S. gallons of water. After 24 hours in an open field, the volume of water left in the pan is determined to be 9.25 gallons. Calculate the pan evaporation rate (mm/hr).
Explanation / Answer
Type of pan: Class A evaporation pan
Water depth in pan before 24 hours = 10 gallons (37854.1 mm)
Water depth in pan after 24 hours = 9.25 gallons (35015.06 mm)
Rainfall (during 24 hours) = 0 mm
K pan = 0.75
Formula: ETo = K pan × E pan
where:
ETo : reference crop evapotranspiration
K pan : pan coefficient
E pan : pan evaporation
Calculation:
E pan = 37854.1 - 35015.06 = 2839.04 mm/day
ETo = 0.75 × 2839.04 =2129.28 mm/day
To convert the answer from mm per day into mm per hour, 2129.28 / 24 = 88.72 mm/hr
Therefore, pan evaporation rate is 88.72 mm/hr.