Rad_net: Daily net radiation

In VEZY/DynACof: A Process-Based Model to Study Growth, Yield and Ecosystem Services of Coffee Agroforestry Systems

Description

Compute the daily net radiation of the system using incident radiation, air temperature, wind speed, relative humidity and the albedo. A clear description of this methodology can be found in Allen et al. (1998) or in An et al. (2017).

Usage

Rad_net(
  DOY,
  RAD,
  Tmax,
  Tmin,
  VPD,
  Rh = NULL,
  Latitude,
  Elevation,
  albedo,
  sigma = Constants()$sigma,
  Gsc = Constants()$Gsc
)

Arguments

DOY	Ordinal day, which is the day of year from 1st January (day)
RAD	Incident daily total radiation (MJ m-2 d-1)
Tmax	Maximum daily air temperature (celsius degree)
Tmin	Minimum daily air temperature (celsius degree)
VPD	Mean daily Vapor Pressure Deficit (hPa), only needed if Rh is missing
Rh	Average daily relative humidity (%)
Latitude	Latitude (deg)
Elevation	Elevation (m)
albedo	Shortwave surface albedo (-)
sigma	Stefan-Boltzmann constant (W m-2 K-4), default to Constants()$sigma.
Gsc	Solar constant (W m-2), default to Constants()$Gsc (= 1367).

Details

The daily net radiation is computed using the surface albedo. This method is only a simple estimation. Several parameters (ac, bc, a1 and b1) are taken from Evett et al. (2011). The net radiation is computed as:

Rn= (1-albedo)*RAD-(ac*(RAD/Rso)+bc)*(a1+b1*ea^0.5)*sigma*((Tmax^4+Tmin^4)/2)

And is derived from the equation :

Rn= (1-albedo)*RAD-Rln

where Rln is the net upward longwave radiation flux, α is the albedo, R_{so} the daily total clear sky solar irradiance, computed as follow:

R_{so}= (0.75+0.00002*Elevation)*Rsa

where R_{sa} is the daily extra-terrestrial radiation, computed using Rad_ext(). The actual vapor pressure ea can be computed using either VPD or the relative humidity and the maximum and minimum daily temperature. If both are provided, Rh will be used.

Value

Rn, the daily net radiation (MJ m-2 d-1)

References

An, N., S. Hemmati, and Y.-J. Cui, Assessment of the methods for determining net radiation at different time-scales of meteorological variables. Journal of Rock Mechanics and Geotechnical Engineering, 2017. 9(2): p. 239-246.

Examples

# Daily net radiation on january 1st at latitude 9 N :
Rad_net(DOY= 1,RAD= 5,Tmax= 16,Tmin= 10,VPD=1.05,
        Latitude=9,Elevation=1000,albedo=0.146)

VEZY/DynACof documentation built on Feb. 3, 2021, 8:52 p.m.