There are two clear but seemingly irreconcilable truths when it comes to issues of water management.  First: water is scarce.  Much of it isn’t drinkable because of salt or pollutants.  And that which is drinkable doesn’t always end up near the people who need to drink it – massive ice caps on the north pole do little for drought-stricken California and Australia.

The second truth, then, is that the water supply is essentially infinite.  All water, even salt water, polluted water, and inaccessible water will eventually evaporate, form into clouds, and fall as clean, fresh, drinkable rain.

So how, in light of the water cycle, can there ever be a water shortage?

Enter the concept of “residence time.”   If you don’t want to click through on the link, the quick-and-dirty (as opposed to infinite and clean) definition of residence time is the amount of time water remains in a state before moving on to the next stage of the water cycle.  And while there are certainly variances, residence times can be very long.

Take those afore-mentioned polar ice caps.  Water frozen at the earth’s poles will remain frozen for, on average, 16,000 years before it evaporates and rejoins the water cycle.  That’s an extreme example, but other water states can still be far longer than any person’s lifetime:  water remains in salt lakes up to 1,000 years but remains only (only?) a century in freshwater lakes.  Oceans, which comprise two thirds of the world’s water, have a residence time of 3,000 years.

And thus, water conservation must be both forward and backward looking.  If your drinking water supply gets contaminated, it will eventually naturally clean itself through evaporation, but this will most likely happen long after you’re dead and gone.  Keeping our water drinkable today is necessary to ensure there even will be future generations.