Campbell CS105MD Dokumentacja Pobierz pdf (Strona 60)

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

Lz = Longitude of the center of the station's time zone (west of the

Greenwich line as 0

) around the earth in clockwise direction (degrees

west of Greenwich)

Lm = Longitude of the measurement site (west of the Greenwich line as

)

= seasonal correction for the solar time (hour) =

0.1645 sin (2b) -0.1255 cos(b) -0.025 sin(b) (14)

b = 2π (J-81)/364 where J is day of the year between

1 (January 1) and 365 or 366 (December 31) (15)

3. Calculation of G (soil heat flux).

The equation to calculate soil heat flux changes depending upon the ET

method used, and whether it is night or day. The following table describes the

equation to use for each method and daytime/nighttime.

Nighttime:

Standardized Short Grass 0.1Rn

Standardized Tall Grass 0.4Rn

FAO56 0.1Rn

Daytime:

Standardized Short Grass 0.5Rn

Standardized Tall Grass 0.5Rn

FAO56 0.2Rn

4. Calculation of γ (psychrometric constant).

γ = c

P/ελ (16)

where, c

= specific heat capacity at constant pressure = 1.013 x 10

-3

MJ/kg/°C)

ε = ratio of molecular weight of water to molecular weight of dry

air = 0.622

λ = latent heat of vaporization of water = 2.45 MJ/kg

P = atmospheric pressure (kPa)

Substituting values of constants, γ becomes

γ = 0.000665 P (17)

A-6

1 2 ... 55 56 57 58 59 60 61 62 63 64 65 ... 77 78

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Campbell CS105MD Dokumentacja Strona 60