BY, M.MANIVANNAN

BTE-06-017

CLASS A EVAPORATION PAN

SUNKEN COLORADO PAN

• Installed in the field • Filled with a known quantity of water (the surface

area of the pan is known and the water depth is measured)

• allowed to evaporate during a certain period of time (usually 24 hours).

• After 24 hours, the remaining quantity of water (i.e. water depth) is measured

• The amount of evaporation per time unit (the difference between the two measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24 hours)

• The E pan is multiplied by a crop coefficient, Kc, to obtain the ETo.

ETo = KC × E pan ETo : reference crop evapotranspiration

KC : crop coefficient E pan : pan evaporation

Month's Mean Monthly Pan Evaporation (in mm/day)

January 2.8258

February 3.5821

March 4.5968

April 5.1933

May 5.3613

June 6.74

July 4.94516

August 4.0129

September 4.2733

October 2.7581

November 1.7133

December 1.8097

Month Crop & Crop Stage Kc

January Rice ( Harvest) 0.60

February Ground nut (Initial) 0.5

March Ground nut (Development) 1.15

April Ground nut (Mid –Season) 1.15

May Ground nut (Late-Season) 0.6

June Sugarcane 0.75

July Sugarcane 0.80

August Sugarcane 0.85

September Sugarcane 0.85

October Sugarcane 0.90

November Sugarcane 0.95

December Sugarcane 1.00

Month Mean Monthly Pan

Evaporation(in mm/day)

Epan

Kc Mean Monthly Crop

Evaporation(in mm/day)

Ec

Evapotranspiration for month

( in mm)

January 2.8258 0.60 1.69548 52.56

February 3.5821 0.5 1.79105 50.15

March 4.5968 1.15 5.28632 163.86

April 5.1933 1.15 5.972295 179.168

May 5.3613 0.6 3.21678 99.72

June 6.74 0.75 5.055 151.65

July 4.94516 0.80 3.956128 122.64

August 4.0129 0.85 3.410965 105.74

September

4.2733 0.85 3.632305 108.97

October 2.7581 0.90 2.48229 76.95

November 1.7133 0.95 1.627635 48.83

December 1.8097 1.00 1.8097 51.10

∑ = 1216.34

Total Evapotranspiration in 1998= 1216.34 mm