Consumptive water rights for each country (as well as any other political jurisdiction) should be based upon a calculation of an amount of water precipitating over that county’s watersheds in an average year, minus the estimated evaporation and transpiration. In other words, countries would be able to consume a volume of water equal to the total volume that falls within their territory minus that portion which naturally evaporates or transpires into the air. Said another way, every country could consume a volume of water equal to its naturally occurring runoff. Water which is not consumed (i.e., injected into the atmosphere by any means) but used for any off-stream or even in-stream purposes, must be equivalent to the quality of the water that was withdrawn.
Countries which do not consume all of their allotted water should have the right to sell their unused water to downstream countries which have already exceeded their natural allotment of water, but they should not be allowed to artificially reduce or cutoff natural flows to raise prices or otherwise threaten downstream users.
Perhaps the rate of the consumptive increase of water (including water withholdings to fill reservoirs) in countries which share watersheds with downstream neighbors should be limited to a maximum annual increase of 10%. This would help reduce the potential for strife induced by dramatic reductions in water flow to downstream countries within relatively short periods of time.
Evaporation from artificially increased water areas (such as artificial reservoirs created behind dams) should be treated as consumptive use by the state or political jurisdiction in which these artificial water surface areas are located. A certain amount of water from the Colorado River, for example, is allocated to each state in its watershed. Therefore, each state should count the volume of water that is evaporating from artificial reservoirs along the Colorado River towards its allowable quota for consumptive use. (Idea from ML Rudee 1-1-01 Letter to the Editor, LA Times.)
After this fundamental principle of water rights is understood and implemented, countries can then negotiate with each other about whether or how much they are willing to trade, sell or negotiate their water resources with their neighbors.