the rules of international law concerning shared freshwater apply to any and all “parts” of an international watercourse that may be located in a given country.
Thus they would apply, for example, to: headwaters or tributaries in State A of a stream that flows into State B;
a groundwater basin that straddles the border between States A and B and is fed by surface water in State A;
or a groundwater basin wholly located in State A that feeds a tributary of a stream flowing into State B.