GP literally said start by reserving water for residential and sustainability use and only sell the rest on the market (i.e. the portion currently used for commercial activity such as power and agriculture). If you demand extreme pedantry, you can expand the reserved quantity to whatever arbitrary domestic carveout usage you desire as profit-seeking, commercial water depletion is >80% of water usage, so any non-commercial carveout is largely irrelevant to depletion.