r.sun - computes solar rays incidence angle raster maps for given time and latitude, and amount of direct solar energy raster maps for given day and latitude from elevation, slope and aspect raster files. The shading effect of surrounding terrain is incorporated.

SYNOPSIS

r.sun

r.sun help

r.sun [-s]

elevin = name [zmult] aspin = name slopein = name [incidout = name] [energyout = name] latitude dej [lum_time]

DESCRIPTION

This program computes solar rays incidence angle raster map incidout for given day dej, time lum_time and latitude latitude and amount of direct solar energy energyout for a given day dej and latitude from elevation elevin, slope slopein and aspect aspin raster files. If elevations in the raster elevation map elevin are in different units than the mapset coordinate system, a multiplier zmult must be used. For instance, if elevations are in centimeters and x, y coordinates in meters, you should use zmult = 100. Specified day dej is the number where January 1 is day no.1 and December 31 is 365 (366). Time lum_time must be a local time in decimal system, e.g. 7.5 (i.e. 7h 30m). Latitude must be also in decimal system and has positive values for northern hemisphere and negative for southern one. Incidence angle is angle between normal vector of given surface and solar ray vector. Output incidence angles are in degrees. Amount of direct solar energy for given day is computed integrating the incidence angle between sunrise and sunset times. Ouput is in kW per squared meter. The incidence angle and amount of direct solar energy can be computed without shading influence of surrounding terrain by default, they can be computed incorporating this influnce using the flag -s. In hilly areas this can lead to very different results! A declination is computed internally using Cooper's approximation for each day and energy input using solar constant 1370 kW per squared meter. It is possible to compute an amount of direct solar energy for some time interval during the year (e.g. a vegetation period). This can be done using a shell script. Elevation, aspect and slope input values should not be reclassified into coarser categories. This could lead to incorrect results.

OPTIONS The user can run this program either interactively or non- interactively. The program will be run non-interactively if the user specifies program arguments and flag settings on the command line using the form:

r.sun [-s] elevin = name [zmult = val] aspin = name slopein = name [incidout = name] [energyout = name] latitude = val dej = val [lum_time = val]

Alternately, the user can simply type r.sun on the command line without program arguments. In this case, the user will be prompted for parameter values using the standard GRASS parser interface.

Flag:

[-s] Incorporates shading effect of terrain (default not)

Parameters:

elevin=name Use the existing raster file with elevationsname as input.

zmult=val Set a multiplier for elevations to val.

aspin=name Use the existing raster file with aspectname as input.

slopein=name Use the existing raster file with slopename as input.

incidout=name Output solar rays incidence angle values to raster file named name.

energyout=name Output direct solar energy values to raster file named name.

latitude=val Set the value of latitude of given region to val.

dej=val Set the serial number of day to val. lum_time=val Set the decimal value of time to val.

NOTES

Solar energy is important input parameter in different models concerning landscape, vegetation, evapotranspiration, snowmelt or remote sensing. Solar rays incidence angle can be effectively used in radiometric corrections in hilly terrain where very precise investigations are performed. Incidence angle multiplied by solar constant (here is used 1370 kW per squared meter) gives irradiance which can be computed using r.mapcalc.

SEE ALSO

s.surf.tps, r.slope.aspect

AUTHOR

Original version of the program : Jaroslav Hofierka and Maros Zlocha, Comenius University, Bratislava, Slovakia,

Modified program (adapted for GRASS): Jaroslav Hofierka, Comenius University, Slovakia

REFERENCES

Mitasova, H. and Hofierka, J. (1993): Interpolation by Regularized Spline with Tension: II. Aplication to Terrain Modeling and Surface Geometry Analysis. Mathemtical Geology (in press).

Krcho, J. (1977, 1990) ...

Jenco, M. (1992): Distribution of direct solar radiation on georelief and its modelling by means of complex digital model of terrain. Geograficky casopis 44, 1992, pp.342-355.(in Slovak)

NOTICE

This program is part of the alpha section of the GRASS distribution. Unlike the code in the main section of GRASS, the alpha code has not yet been fully tested for one release cycle.