Earlier this week, I wrote about how Lake Mead, America's largest man-made reservoir, has shrunk to its lowest level ever. It's a huge deal for the 25 million people in California, Arizona, Nevada, and Mexico who depend on freshwater from this crucial system.

Now NASA's Earth Observatory has posted two satellite images that show the dramatic decline of Lake Mead between 2000 and 2015. The shrinkage is stunning — the lake has lost more than half its water and is down to just 37 percent capacity:

Quick backstory: The federal government built the Hoover Dam in the 1930s (and, later, the Glen Canyon Dam) to tame the flow of the Colorado River and store and supply water for cities and farmland around the region. This reservoir system is what made the modern-day Southwest possible.

In the decades since, the region's population has exploded, and more and more cities and farmers are drawing water from Lake Mead. (Las Vegas, which gets nearly all its water from the reservoir, had 8,000 people in 1940; it has more than 600,000 today.) On top of that, the region has been in an on-and-off drought since 2000, which has meant less snow in the mountains, less runoff in the spring and summer, and less water in Lake Mead.

At a certain point, something has to give. Lake Mead's water levels currently sit at a record low 1,074 feet above sea level. If they stay below 1,075 feet by January 1, the federal government will have to declare a shortage and start cutting back on water deliveries. As I explored here, thanks to Byzantine rules, Arizona will be the first to see cutbacks, but more could follow. (Right now forecasters think water levels will go up enough in the coming months to avoid a shortage in 2017 — but a shortage in 2018 seems increasingly likely.)

Now, this shortage doesn't have to end in apocalypse or "water wars," as some headlines have suggested. California, Nevada, and Arizona are all exploring ways to more sensibly share Lake Mead's water and avoid drastic shortages. The details are complex — see here — but it require ever more stringent conservation measures as well as reducing the water used by farmers in places like central Arizona and California's Imperial Valley.

Expect to hear much more about this issue in the future. The interstate agreements that govern water use in the Colorado River were forged back in the 1920s, one of the wettest decades on record. But climate change is expected to lead to more frequent and severe droughts in the region in the future. And coupled with population growth, everyone's going to need to learn to share. The water system that allowed the desert to bloom no longer works the way it used to.

Further reading:

Here's our full explainer on Lake Mead and the slow-motion water crunch in the Colorado River.

full explainer Our cities' water systems are becoming obsolete. What will replace them?

Watch: The Grand Canyon, filling with fog