Calculation of RHmean by averaging hourly readings and calculation of ea from Tmean are discouraged because computations may be biased.

Vapor Pressure Deficit (VPD) The VPD (kPa) is computed as

where es is the saturation vapor pressure (kPa) and ea is the actual vapor pressure (kPa). Net Radiation (Rn)

The net radiant energy Rn available at the evaporating surfaces is a fraction of the extraterrestrial radiation Ra and is calculated from the radiation balance.

Part of Ra is lost by absortion or reflection when passing through the atmosphere. The total incoming shortwave solar radiation Rs is termed global radiation. A fraction of the global radiation is reflected back to the atmosphere, depending on the reflectivity characteristics of the surface, that is, the albedo a. A part of that which is absorbed by the surface is reradiated back as long wave radiation Riu. A fraction of Riu is returned as incoming long wave radiation from the atmosphere, Rid. The radiation balance therefore can be written as

where the arrow indicates when the radiation component is incoming (i) or outgoing (t).

Extraterrestrial Radiation (Ra). This radiation at the top of the atmosphere can be computed as a function of the latitude of the site and the day of the year as

Ra = 37.6 dr (&>s sin p sin S + cos p cos S sin &s), (5.26)

where Ra is the daily total extraterrestrial radiation (MJ m-2 day-1); 37.6 is a coefficient relative to the time duration of calculation and the solar constant (MJ m-2 day-1), dr is the relative distance from Earth to the Sun (); S is the solar declination (rad); p is the latitude (rad) and is negative for the Southern Hemisphere); and rns is the sunset hour angle (rad).

The sunset hour angle is a function of the latitude and the solar declination:

The relative distance from Earth to the Sun and the solar declination can be estimated as functions of the day in the year:

where J is the number of the day in the year (), with January 1st = 1. For daily values, J can be determined by

provided that, if M < 3, then J = J + 2 and in a leap year when M > 2, then J = J + 1, where M is the month number (1-12), starting in January, and D is the day in the month. For monthly values, J can be determined for the middle of the month by

Global Radiation Rs. When not measured, Rs can be estimated from sunshine hours and Ra as

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