Even when the water in the radiator has not dried,but merely low,it was refilled with about 50 cl of water, There was no sign of overheating but after goes on a trip of one kilometer, the water in the radiator started boiling. What does this portend?
1/2 liter is not much. But is your radiator using just water or the correct coolant mix?
The “boiling over” after such a short amount of driving may actually be signs of a blown head gasket, what you are seeing is combustion gasses in your coolant.
Bottom line: have your car diagnosed by a competent mechanic.