Animation by Jöns Mellgren and Anna Mantzaris, special to ProPublica.

Seventy five years ago, American scientists discovered a secret about how the planet’s water works. The phenomenon involves the Earth’s lakes, rivers and streams, and their connection to the treasured stashes of water that can be found in the deep pools that have been sequestered between layers of underground rock for centuries.

A scientist pumped water from an underground well, but found the ground was constantly being refilled. Over time, the scientist concluded, the well was drawing most of its water directly from a stream miles away. Experiments proved that the waters were often intermingled parts of the same system — and that if water was removed from an aquifer underground, it would be replenished by some other part of that same water system.

Today, the dynamic is accepted as a basic principle of hydrogeology — the discipline of science that delves into the movement of water inside the Earth. Yet its fundamental implications for years have been largely ignored. Government officials in some western states have been treating water supplies as if they always come from, in effect, two separate bank accounts — one from rivers and lakes above ground and another from below. And so as America’s west has waged its battle against water scarcity, some of its officials have been miscalculating to some degree just how much water is actually available. If states in the West keep managing water this way, we risk a water crisis even worse than we fear.

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