NAME

r.sun - computes solar illumination (incidence) angle raster maps for given time and latitude and solar irradiance (direct solar radiation) raster maps for given day and latitude. They are computed from elevation, slope and aspect raster maps. Sunrise, sunset times, declination for given day are displayed along with solar azimuth and zenith angle for specified local time. The shadowing effect of the topography is optionally incorporated.

SYNOPSIS

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

DESCRIPTION

This program computes solar illumination angle raster map incidout for given day dej, time lum_time and latitude latitude and solar irradiance raster map energyout for a given day dej and latitude latitude from elevation elevin, slope slopein and aspect aspin raster files. Elevation, slope as well as aspect must be floating point raster maps. Null data in cells must be valid data (zeroes). Please, run r.support to create/reset null file for your input raster maps.
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 (NOT a zone time, e.g. GMT, CET) in a decimal system, e.g. 7.5 (i.e. 7h 30m A.M.), 16.1 is 4h 6m P.M.. The terrestrial latitude must be also in decimal system with positive values for northern hemisphere and negative for southern one. The program uses the Linke's atmosphere turbidity coefficient. The default linke=2.5 is near the annual average for rural areas in Central Europe. The absolute clear atmosphere has linke = 1.0. The illumination angle is the incidence angle between horizon and solar ray vector. Output illumination angle is in degrees. The irradiance for given day is direct solar radiation computed integrating the illumination angles between sunrise and sunset times for given day. A time step is one hour. The output units are in Wh per squared meter per given day (Wh/(m*m)/day). The illumination angle and irradiance can be computed without shadowing influence of relief by default, they can be computed with this influence using the flag -s.

In mountainous and even hilly areas this can lead to very different results! Incorporating the shadowing effect of relief can dramatically slow down the speed of computing especially when the Sun is near above the horizon. The shadowed areas written in ouput maps as zero values. The solar declination is computed internally using Cooper's approximation for each day and irradiance using the solar constant = 1370 W per squared meter. The sunrise and sunset times, solar declination, solar zenith angle (vertical angle) and solar azimuth angle (horizontal angle) are displayed as on- screen information. There is a possibility to compute irradiance for some time interval within the year (e.g. for specified vegetation period). This can be done using a shell script loop. 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] [linke=value]

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. Currently, user may specify either illumination angle map ncidout or irradiance energy map nergyout during one command run.

Flag:

-s
Incorporates shadowing effect of terrain (default not)

Parameters:

elevin=name
Use the existing FCELL raster file with elevationsname as input.
zmult=val
Set a multiplier for elevations to val.
aspin=name
Use the existing FCELL raster file with aspectname as input.
slopein=name
Use the existing FCELL raster file with slopename as input.
incidout=name
Output solar rays illumination angle values to FCELL raster file named name.
energyout=name
Output direct solar irradiance (direct radiation) values to FCELL 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.
linke=val
Set the Linke's atmosphere tubidity coefficient to val.

NOTES

Solar energy is an important input parameter in different models concerning energy industry, landscape, vegetation, evapotranspiration, snowmelt or remote sensing. Solar rays illumination angle can be effectively used in radiometric and topographic corrections in mountainous and hilly terrain where very accurate investigations should be performed. For details, see the literature. Average monthly values of the Linke's turbidity coefficient for a mild climate:
     Month Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec  annual
mountains  1.5  1.6  1.8  1.9  2.0  2.3  2.3  2.3  2.1  1.8  1.6  1.5  1.90  
rural      2.1  2.2  2.5  2.9  3.2  3.4  3.5  3.3  2.9  2.6  2.3  2.2  2.75  
city       3.1  3.2  3.5  4.0  4.2  4.3  4.4  4.3  4.0  3.6  3.3  3.1  3.75  
industrial 4.1  4.3  4.7  5.3  5.5  5.7  5.8  5.7  5.3  4.9  4.5  4.2  5.00
It should by noted that r.sun currently uses a lot of memory, some improvements are under way. As the latitude latitude and local time lum_time are defined as a constant value, the user should take into account extent of the area/region.

SEE ALSO

s.surf.rst, 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, J. Gresaka 22, 08501 Bardejov, Slovakia,
© 1999, Jaroslav Hofierka
hofi@geomodel.sk

REFERENCES

Hofierka, J. (1997): Direct solar radiation modelling within an open GIS environment. Proceedings of the JEC- GI'97 Conference in Vienna, pp. 575-584.

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

Kittler, R., Mikler, J. (1986): Basis of the utilization of solar radiation. VEDA Bratislava, p. 150.(in Slovak)