If a place gets less than 40cm of rain per year, adding water collection for the entire roof of a 100 sq meters building will only result in 40 cubic meters of

If a place gets less than 40cm of rain per year, adding water collection for the entire roof of a 100 sq meters building will only result in 40 cubic meters of water, or 40000 liters. At 100 liters per day usage (lower than a typical European), collecting rainwater gives you 40 days of usage, and in reality much less due to evaporation and other inefficiencies. What is needed most are laws preventing, or at least disincentivizing, taking more out than what is coming in. Of course this is really hard: water systems don’t follow neat political boundaries, so one entity drilling deeper or damming a river also drains the water of its neighbors.