Solar radiance and irradiance

The sun() function calculates beam (direct), diffuse and ground reflected solar irradiation for a given day and set of topographic and atmospheric conditions. The function relies on the GRASS module r.sun, the manual page for which contains a detailed explanation (see grassHelp("r.sun"))

Usage

sun(
  elevation,
  coeff_bh,
  coeff_dh,
  slope,
  aspect,
  hh,
  horizon_step = 90,
  albedo = 0.2,
  linke = 3,
  day = 1,
  step = 0.5,
  declination = NULL,
  solar_constant = 1367,
  distance_step = 1,
  npartitions = 1,
  beam_rad = TRUE,
  diff_rad = TRUE,
  refl_rad = TRUE,
  glob_rad = TRUE,
  insol_time = TRUE,
  lowMemory = FALSE
)

Arguments

elevation: A GRaster with values representing elevation (typically in meters).
coeff_bh: A GRaster: A raster with values of the real-sky beam radiation coefficient. Valid values are between 0 and 1.
coeff_dh: A GRaster: A raster with values of the real-sky diffuse radiation coefficient. Valid values are between 0 and 1.
slope: A GRaster: This is a raster representing topographic slope in radians. It can be generated using terrain().
aspect: A GRaster: This is a raster representing topographic aspect in degrees. It can be generated using terrain(). If generated with that function, "east orientation" must be used (i.e., argument northIs0 must be FALSE).
hh: A "stack" of GRasters: This represents height of the horizon in radians in particular directions. Horizon height can be calculated using horizonHeight(). The directions must be in "east orientation" (i.e., argument northIs0 in horzionHeight() must be FALSE). The directions must correspond with the sequence given by horizon_step (see next argument). For example, if horizon_step is 90, then hh must contain rasters representing horizon height at 0 (east), 90 (north), 180 (west), and 270 (south) aspects. #'
horizon_step: Numeric >0: Difference between angular steps in which horizon height is measured. One horizon height raster will be made per value from 0 to 360 - horizon_step degrees.
albedo: A GRaster or a numeric value: This is either a raster with values of ground albedo or a numeric value (in which case albedo is assumed to be the same everywhere). Albedo is unit-less, and the default value is 0.2.
linke: A GRaster or a numeric value: This is either a raster with values of the Linke atmospheric turbidity coefficient or a numeric value (in which case the same value is assumed for all locations). The Linke coefficient is unit-less. The default value is 3, but see also the GRASS manual page for module r.sun (grassHelp("r.sun")).
day: Positive integer between 1 to 365, inclusive: Day of year for which to calculate ir/radiation. Default is 1 (January 1st).
step: Positive integer between 0 and 24, inclusive. Time step in hours for all-day radiation sums. Decimal values are OK.
declination: Numeric or NULL (default). Declination value. If NULL, this is calculated automatically.
solar_constant: Positive numeric: The solar constant (solar energy hitting the top of the atmosphere). Default is 1367. Units are W / m^2.
distance_step: Positive numeric between 0.5 and 1.5, inclusive: Sampling distance coefficient. Default is 1.
npartitions: Positive numeric. Number of chunks in which to read input files. Default is 1.
beam_rad: Logical: If TRUE (default), generate a raster with beam irradiation with units of Wh / m^2 / day ("mode 2" of the r.sun GRASS module).
diff_rad: Logical: If TRUE (default), generate a raster representing irradiation in Wh / m^2 /day
refl_rad: Logical: If TRUE (default), generate a raster with ground-reflected irradiation with units of Wh / m^2 / day ("mode 2" of the r.sun GRASS module).
glob_rad: Logical:. If TRUE (default), generate a raster with total irradiance/irradiation with units of Wh / m^2 / day ("mode 2" of the r.sun GRASS module).
insol_time: Logical: If TRUE (default), generate a raster with total insolation time in hours ("mode 2" of the r.sun GRASS module).
lowMemory: Logical: If TRUE, use the low-memory version of the r.sun GRASS module. The default is FALSE.

Value

A raster or raster stack stack with the same extent, resolution, and coordinate reference system as elevation. Assuming all possible rasters are generated they represent:

beam_rad: Beam radiation (Watt-hours/m2/day)
diff_rad: Diffuse radiation (Watt-hours/m2/day)
refl_rad: Reflected radiation (Watt-hours/m2/day)
glob_rad: Global radiation (Watt-hours/m2/day)
insol_time: Insolation duration (hours)

Examples

if (grassStarted()) {

# Setup
library(terra)

# Example data
madElev <- fastData("madElev")

# Convert a SpatRaster to a GRaster
elev <- fast(madElev)

### Calculate input rasters

# Values below are just a guess
coeff_bh <- coeff_dh <- elev
coeff_bh[] <- 0.4
coeff_dh[] <- 0.6

slope <- terrain(elev, "slope")
aspect <- terrain(elev, "aspect", northIs0 = FALSE)

horizon_step <- 90
hh <- horizonHeight(elev, step = horizon_step, northIs0 = FALSE)

### calculate solar ir/radiance

solar <- sun(
  elevation = elev,
  coeff_bh = coeff_bh,
  coeff_dh = coeff_dh,
  slope = slope,
  aspect = aspect,
  hh = hh,
  horizon_step = horizon_step,
  albedo = 0.2,
  linke = 1.5,
  day = 1,
  step = 0.5,
  declination = NULL,
  solar_constant = 1367,
  
  distance_step = 1,
  npartitions = 1,

  beam_rad = TRUE,
  diff_rad = TRUE,
  refl_rad = TRUE,
  glob_rad = TRUE,
  insol_time = TRUE,

  lowMemory = FALSE
)

solar

}

Usage

Arguments

Value

See also

Examples