The water-rich country that's water-poor
Brazil has more renewable freshwater than any country on Earth.
Rank every country by the total volume of water its rivers and rainfall renew each year, and the leaderboard reads Brazil, Russian Federation, Canada — big, wet, lightly populated giants. It's a real measurement. It's also nearly useless as a guide to whether anyone is going thirsty, and here's why.
Whose water?
A total counts cubic metres. It doesn't count the people those cubic metres have to go around. So divide the resource by the population, and the ranking you get answers a completely different question: not "which country has the most water" but "which country has the most water per person." Do that and the top of the list empties out and refills with names you didn't expect — Iceland, Guyana, Suriname, small countries with enormous rainfall and almost nobody to share it with.
The giants slide. India holds one of the larger renewable supplies on the planet — 1,446 bn m³ in total, but 1,014 m³ per person — 9th in the world by volume, 125th per head. China is the same story: 2,813 bn m³ in total, but 1,992 m³ per person — 5th in the world by volume, 104th per head. A vast total, divided among a vast population, lands each of them down near the line where hydrologists start using the word stress. The water is real. There just isn't much of it each, once you count the "each."
Where's the line?
Hydrologists mark that line at 1,700 cubic metres per person per year — below it, a country is said to face water stress; below 1,000, scarcity; below 500, absolute scarcity. They're rules of thumb, not laws of physics, but they turn the per-capita number into something you can read at a glance. Out of 180 countries with data, 74 sit below the stress line, 55 below scarcity, and 28 below absolute scarcity. None of those 74 countries would show up as remarkable on the total-volume list. The denominator is the whole story.
So which ranking is right?
Both. That's the point worth slowing down for. The total is a true fact about a country's endowment; the per-capita figure is a true fact about how far that endowment stretches. Neither is a lie, and neither is the "real" one — they answer different questions, and a country's water security depends on the answer to the second far more than the first. The mistake isn't using the total. The mistake is reading a total and thinking it told you about scarcity.
You can see both lists side by side: total renewable freshwater, where the giants lead, and renewable freshwater per capita, where they don't. Read one after the other and the lesson sticks: whenever a ranking of countries surprises you, your first question shouldn't be how can that be true. It should be what is this number actually counting. With water, it's almost always the denominator.
And the trap reaches further than a ranking. The same mistake — taking a big natural number for security — shows up in how long people live: the wettest countries don't live the longest, because what turns up in a lifespan is clean water and the wealth to deliver it, not the water in the ground.