Campbell Scientific VisualWeather Software User Manual

Page 61

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Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs

where the atmospheric pressure is obtained from the following equation:

P = 101.3

293- 0.0065Z

293

5.26


⎣⎢


⎦⎥

(18)

where, Z = Elevation in meters. In the above calculation the temperature
is assumed to be 20°C; hence,

T (Kelvin) = T (°C) + 273 =293. Note that by taking the value of
temperature to be 20 °C at all altitudes the temperature dependence of

γ

has been ignored.

At sea level Z= 0; therefore, P = 101.3 kPa. At a higher altitude the
pressure P drops below this value, since the air is less dense. As the
altitude increases, P decreases and so does

γ.

VisualWeather computes the atmospheric pressure, P, based on the user-
entered elevation.

Note that the latent heat of vaporization of water,

λ is also a function of

temperature. However, its variation with temperature is small enough to
be neglected. For the FAO 56 PM equation the value of

λ is assumed to

be constant,

λ = 2.45 MJ/kg.

5. Calculation of e°(T

hr

) (saturation vapor pressure at air temperature

T

hr

).

The saturation vapor pressure is related to air temperature, T

e°(T

hr

) = 0.6108 e

(17.27T) / (T + 237.3)

(19)

Note that the right-hand side of this equation forms a part of the
numerator in equation (3) above.

6. Calculation of e

a

(average hourly actual vapor pressure).

e

a

= e°(T

hr

)*RH

hr

/100 (20)

RH

hr

is the average hourly relative humidity (%)

Finally these individual results are combined to calculate hourly values of
ET

o

by using equation (2).

Denominator

=

Δ + γ (1 + 0.34 u

2

)

First term = 0.408

Δ (Rn-G) / (Δ + γ (1 + 0.34 u

2

))

Second term =

γ (37/(T

hr

+ 273)) u

2

(e°(T

hr

)-e

a

)) / (

Δ + γ (1 + 0.34 u

2

))

ET

o

(mm/hour) = First term + Second term

A-7