What about per-species? Sounds like humans are getting a full 20% of the water, all other species (plant and animal) get the rest. And I feel like you're arguing that humans should get a bigger slice?
What should the right % be? Presumably not 0. Agriculture after all produces food for same humans. So it doesn't seem like closing agriculture is the right option.
So perhaps choosing agriculture that consumes less water? That seems reasonable - but there are diminishing returns in that direction. Sure rice seems unnecessary, but what about almonds? I'm no farmer but does it mean they should all grow the most efficient crop? A monoculture?
Ultimately is the real problem just too many humans in a place with little water? Should humans then supplement that water via say desalination? Sure its more expensive but maybe that helps reduce population growth?
