altitudeelevationmodisslopesolar radiation
I used DSR (Downward Shortwave Radiation) data from MCD18A1 product and GHI (Global Horizontal Irradiation) data from SOLARGIS to create solar maps:
Using this elevation map:
I see that the solar radiation seems to decrease with the elevation, however I don't understand why this happens and if this happens because of elevation or slope. Can someone explain me the reason why the radiation decreases?
When generating shaded reliefs it's important to transform the elevation models from geographic lat-long to a metres based projection, Albers, Lambert and Mercator work well, else you end up with horrible shadows and gross east-west stretching deformaties. Transform the reliefs back to decimal degrees afterwards if necessary. Also instead of accepting the default 45° sun angle put it higher in the sky, about 70°, and an upper left aspect, about 315°.
I assembled the Geobase elevation models into larger tiles which are somewhat easier to use, see http://www.maphew.com/Projects/Canada_50k_Digital_Elevation_Model. The storage resolution (pixel dimensions) is 16 metre cells, not to be confused with nominal resolution which ranges from 10m to 30m+ depending on original data source and latitude.
update: here is a sample generated from 105G08 freshly downloaded from Geobase and using the above parameters in Spatial Analyst's Hillshade. Screen clip captured from Arcmap at "zoom to raster resolution" scale.
I do not know much about the background of r.sun and SAGA algorithms. However, cannot be this a problem with interpretation of the units or interpretation of input data?
In case of r.sun this should be daily sum per sq meter. Attaching the screenshot of typical daily values near Krakow from Solargis database, in June approx. 5 kWh/m2/day is just fine.
In case of SAGA units - I do not know. Just a guess - the values might correspond to instant energy. During the clear-sky summer day easily reach around 800 W, even up to 1000 W (=1 kW), presented as instant value.
In both cases, the variability of data in your area is too high, not realistic (at least I do not see any terrain or other features which should cause the shading effects and be responsible for such results).
Best Answer
The main reason here are the prevailing cloud formations. The volcano terrain force the air mass to rise. Rising also means lower air-pressure, which triggers adiabatic cooling, which may trigger condensation (if some other physical conditions of the environment are met). For such typical synoptic situation see image below.
Therefore you have more often clouds (thus slightly less solar irradiation) in the centre of the island than on the coast. Of course, some longer period of time needs to be summarized.
In Solargis,