How can metro Phoenix erase an acute housing shortage without draining its aquifers, now that the groundwater on which much of this growth once relied is spoken for? It’s a thorny policy question to weed through. State law requires subdivisions in metro Phoenix to prove they have enough water to sustain themselves, before they build. And for years, that happened in one of two ways: A designated water provider — one that has enough renewable water to serve current and some future customers for 100 years — could agree to serve the new development. Or a housing subdivision could earn a certificate from the state to pump groundwater, then join a district that would replenish most of what they pump on their behalf. But the state cut off the latter practice once it was clear that we had begun bumping up against our groundwater supply’s limits.