What is used is not water, but fresh water flow. In a given unit of time, in a certain region, a certain amount of water that will flow through it, due to river

What is used is not water, but fresh water flow. In a given unit of time, in a certain region, a certain amount of water that will flow through it, due to rivers, rain or water stored in a dam. You can use some or all of that flow in that time period. If that flow is not used, then it is gone. If you use part of that flow on one thing, usually you can't use it for something else. The last tradeoff is what is often concerning.