ET pan

Evaporation pans are giving the evaporation of an open water surface. The water evaporated can be converted into evapotranspiration. The following FAO document  

IRRIGATION WATER MANAGEMENT
Training manual no. 3

Part I Principles of irrigation water needs
Part II Determination of irrigation water needs

a manual prepared jointly by C. Browser
International Institute
for Land Reclamation and Improvement
and M. Heibloem
FAO Land and Water
Development Division gives the following conversion table in annex 1.

Class A pan

Case A: Pan placed in short green cropped area

Case B¹: Pan placed in fallow area

RH mean %

low
< 40

medium 40-70

high > 70

low
< 40

medium 40-70

high
> 70

Wind
km/day

Windward side distance of green crop
m

Windward side distance of dry fallow
m

Light

1

.55

.65

.75

1

.7

.8

.85

< 175

10

.65

.75

.85

10

.6

.7

.8

100

.7

.8

.85

100

.55

.65

.75

1000

.75

.85

.85

1000

.5

.6

.7

Moderate

1

.5

.6

.65

1

.65

.75

.8

175-425

10

.6

.7

.75

10

.55

.65

.7

100

.65

.75

.8

100

.5

.6

.65

1000

.7

.8

.8

1000

.45

.55

.6

Strong

1

.45

.5

.6

1

.6

.65

.7

425-700

10

.55

.6

.65

10

.5

.55

.65

100

.6

.65

.7

100

.45

.5

.6

1000

.65

.7

.75

1000

.4

.45

.55

Very strong

1

.4

.45

.5

1

.5

.6

.65

> 700

10

.45

.55

.6

10

.45

.5

.55

100

.5

.6

.65

100

.4

.45

.5

1000

.55

.6

.65

1000

.35

.4

.45