Split the underground cable into 2 circuits

The underground cable coming from the house under the terrace is only a 5x1.5mm² cable
Just checked. The electrician really thought that through...

Then you should take 10A for a 10m circuit and 6A for the 50m circuit, but that is already calculated generously and for the 50m circuit almost the absolute limit!!!

What kind of protection you need to put in there will surely be known by my electrician.

For me, it is only important to know what that means for me.
If we assume 6A, what kind of consumers can I then still connect to the line?
As I said, lamps and a decorative pump would be enough for me.
So maybe 50W.

And what would be the maximum consumer I am allowed to connect?
Simply calculate? (230V * 6A = 1380W)
At 1380W, the line or the fuse would burn out, right?
At least I can see how far I am from a meltdown.
If I only plug in 100W consumers, I am on the safe side.

If you assume 6A, what kind of consumers can I still connect to the line?

Consumers up to 6A, what else? Therefore, your 1380W is also mathematically correct (although actually 240V comes from the cord), but due to the circumstances already described here, I would assume a maximum of 1000W, rather less. So for a bit of lighting and a decorative pump, definitely sufficient. If garden sockets are also connected to it, you have to keep that in mind when you eventually look for a socket for the hedge trimmer or the garden shredder...

OK, thanks to you.
It helped me a lot!

oh wow, 100m is quite a statement
If you want 16A, you need a 10mm² underground cable

If you're already doing a voltage drop calculation, please do it properly. With alternating current, you have to consider both the outgoing and return conductors. So instead of 100m, it would actually be 200m. If it were three-phase current, then 100m would be correct.

There should definitely be only ONE fuse provided for the cable. Never operate multiple fuses in parallel on one cable. The only exception is three-phase consumers.