There are a number of possible ways to do this, 4 come in my mind right now.
1) What are your other render settings? Most important: what's the detail setting?
GI is relative to the detail setting, that's why it's named "GI
relative detail".
This means that for detail 0.5 and GI relative detail @ 2 you effectively have 0.5 x 2 = GI detail @ 1.
So with low detail settings you have also lower GI "coverage".
You can check this by looking at the number of dots in the GI prepass.
With GI 2/2 and render detail 0.5 you will have much less dots than with render detail 1.
GI quality sampling means that for every dot in the GI prepass the renderer will sample x amount of times, which is defined by this setting.
The more you sample, the more accurate the value is for each dot in the prepass.
GI surface details definitely helps, but even at low sun angles it can give unsatisfactory results + that GI surface details is horrendously slow.
Also, GI surface details is only suitable when you want more detail in fine shadows, say in vegetation.
So all in all I'd first make sure that you have enough of those GI dots in your shadow areas.
For terrain these are relatively large areas, often, so you probably don't need very high GI detail.
Upping samples is better and faster, allowing for a more accurate result.
This is all pretty heavy dependant on each scene, so without an example of the situation and knowing more about the other rendersettings its difficult to say.
2) An easier and quicker one is to increase the "strength on surface" parameter in the enviro light node.
Increase it to 1.5 or 2 and see if it is satisfactory.
This can give a bit unbalanced results which would require some post-processing.
3) Increase ambient colour from black to white in the atmosphere node.
This works best if you have a bit thicker haze in your atmosphere node.
4) always save as EXR
Of these 3 options I like to use a combination of 1, 2 and 4.
I don't want to spend too much time and memory on calculating GI, so I make sure that I'm not generating an insane number of dots for GI cache.
As a matter of fact you can increase GI to 8 or 10 and post the GI prepass as finished render, you can get it that dense
So when I think I have enough dots and enough sampling I usually increase the strength on surfaces in the enviro light a bit.
This should all be sufficient to get enough detail in the raw render which you can easily get right when you save as EXR and post-process accordingly.
Please show us what you got, so we can give a bit more accurate directions.
Cheers,
Martin