The first paragraph says it all "modeling projections". When you can model the climate 30 days from now than I will believe your projections for 30 years from now.
Does man have an impact on local water suppplies? Yes. What do you want to do about? Restrict economic growth. Guess what has the greatest influence on reducing population (excluding migration)? Economic Growth.
As you pointed out water (matter) dosen't get destroyed, it just gets used and reused.