International water sharing is based on Customary International Law of International Waters which are often referred to as Opinio Juris. These are based on certain rules:
1. Riparian States
Only those nations through which the river flows shall have any legal right to use river water. Upper riparian states claim absolute territorial sovereignty on the river water and lower riparian states claim absolute integrity of the river i.e. related to quality and quantity of the river.
2. Rule of equitable utilisation
The rule of equitable utilization, based on the concept that an international drainage basin is a coherent legal and managerial unit, embodies a theory of restricted sovereignty under which each nation recognizes the right of all riparian nations to use water from a common source and the obligation to manage their uses so as not to interfere unreasonably with like uses in other riparian nations. Nations often allocate water under this.
3. No harm rule
The reasonability and equitability are measured in accordance to Helsinki Rules. In 2004 it was superseded by Berlin Rules on Water Resources
1. The geography of the basin, including in particular the extent of the drainage area in the territory of each basin State.
2. The hydrology of the basin, including in particular the contribution of water by each basin State
3. The climate affecting the basin
4. The past utilization of the waters of the basin, including in particular existing utilization
5. The economic and social needs of each basin State
6. The population dependent on the waters of the basin in each basin State
7. The comparative costs of alternative means of satisfying the economic and social needs of each basin State
8. The availability of other resources
9. The avoidance of unnecessary waste in the utilization of waters of the basin
10. The practicability of compensation to one or more of the co-basin States as a means of adjusting conflicts among uses
11. The degree to which the needs of a basin State may be satisfied, without causing substantial injury to a co-basin State