r.sun GRASS GIS .. calculate sunhours for a specific coordinate pair

Question

I am wondering if I can calculate the sunhours for a specific coordinate pair with r.sun on GRASS GIS. I want to give a coordinate pair as input and calculate how many sunhours there are on a specific day.

Answer 1

You have to use r.sunhours instead. I am not very familiar with the command in the GRASS environment but a graphical interface of it can be found in Processing Tools of QGIS 3 (it uses GRASS 7). In this case, you can produce a raster for whatever specific coordinate pair (by the way, you only need latitude; not coordinate pair) on a specific day.

For instance, in following image I have a shapefile world borders where it can be observed r.sunhours command filtered in Processing Tools browser of QGIS 3.

Double click in command name for opening following dialog; where it was setting values for day 2021-01-24 (number day equal 24), g.region of world_borders, region cellsize 0.1 degrees and open output only for 'Sunshine Hours'.

After clicking in Run, it was obtained Sunshine Hours map for day 2021-01-24 where it can also be observed that hours days only depend of latitude.

For this arbitrary point in USA (-92.83933054393307 35.402592979079515), "Sunshine Hours" is 10.11789 (see following image). By using this alternative method, obtained value was 10.216379 hours. There is a difference of (10.21637-10.11789)*60 = 5.90925 minutes.