## DESCRIPTION

i.sunhours creates a sunshine hours map from any map, it considers a perfect clear day. This method follows Iqbal (1983) as found in the AHAS manual (Parodi, 2000).

The latitude input map can be created with the i.latlong module, or with r.mapcalc's `y()` function in a latitude-longitude location (possibly reprojected with r.proj.

## EXAMPLE

    g.region rast=elevation -p
    r.mapcalc "doy_180 = 180"
    i.sunhours dayofyear=doy_180 latitude=latitude output=photoperiod_doy_180

## REFERENCES

Iqbal, M., 1983. An Introduction to Solar Radiation. Iqbal, M., Editorial: Academic Press. Toronto, Canada.

Parodi, G., 2000. AVHRR Hydrological Analysis System. Algorithms and Theory, Version 1.0. WRES - ITC, The Netherlands.

## SEE ALSO

i.evapo.tsa, i.latlong

## AUTHOR

Yann Chemin, GRASS Development Team

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