Because of São Paulo's water woes I've seen several blog posts and comments about Brazil having 12% or one-eighth of Earth's freshwater. The posts generally refer to the irony of Sao Paulo's situation given all the freshwater Brazil has.
Here's the first paragraph from an article in the New York Times:
Endowed with the Amazon and other mighty rivers, an array of huge dams and one-eighth of the world’s fresh water, Brazil is sometimes called the “Saudi Arabia of water,” so rich in the coveted resource that some liken it to living above a sea of oil.
Brazilian river basins hold some 12 percent of the world’s fresh water — a resource essential to sustain biodiversity, food production and power generation in the country. Brazil’s rivers link many of the nation’s distant geographies, proving that activities in one region can have significant benefits — or consequences — for places downstream.
I'll be the first to admit that the mighty Amazon River has about 18-20% of all Earth's river discharge to the oceans. No argument there. Number 2 is the Congo, less than one-fifth of the Amazon's discharge. And the mighty Mississippi is under one-tenth.
But 12% of Earth's freshwater in Brazil? No way.
So Where is Earth's Freshwater?
Most of Earth's freshwater - about 70% - is tied up in Earth's glaciers and permanent icecaps. So you're saying, 'Wait a minute. they're talking about liquid freshwater.'. Okay, let's consider liquid water. Turns out most of Earth's liquid freshwater is groundwater. Check this out from the USGS freshwater page:
Let's say that you want to neglect groundwater, as many so-called experts are wont to do these days. Under that assumption, where is most of Earth's liquid freshwater? It's in freshwater lakes, which have about 40 times the amount of water in rivers or about 3 times the amount of freshwater contained in rivers, marshes and swamps, and in the soil.
Still with me?
So most of Earth's liquid freshwater, excluding groundwater, is contained in freshwater lakes by a factor of over 3. Think of the huge freshwater lakes (by volume) - the North American Great Lakes, Lake Baikal (holds 20% of all lake volume all by itself), Lake Tanganyika, Lake Victoria, etc. Do you see a large freshwater lake in Brazil (no), or even in South America (yes - two)? Largest artificial lakes? Not one is even close to natural lakes in terms of volume.
Since most of Earth's liquid freshwater is contained in large lakes, and since Brazil has no large lakes, how can Brazil have 12% of Earth's freshwater? Answer: it can't and it doesn't.
Origin of the 12% Figure
So what is the source of the '12% figure'. It's based on renewable freshwater flows. In fact, this recent post by Sandra Postel gets it (pretty much) correct:
Unlike Los Angeles, São Paulo is not a desert city. Whereas LA averages just 15 inches (381 millimeters) of rain a year, São Paulo averages 57.3 inches (1455 mm). With more than 12% of the world’s renewable freshwater, Brazil is sometimes called the “Saudi Arabia of water.”
See Chapter 3 of the FAO report for more information on how these country numbers were calculated. Note that the volume unit used - cubic kilometer - equals 811,000 acre-feet.
Of Earth's total renewable water resources of 43 750 km3/year, Brazil has 5,418 km3/year - 12.4% or 12%, about 1/8 - of internal renewable water resources. Groundwater recharge is not explicitly counted because it is assumed that it eventually shows up a baseflow to the streams, so it's already being counted as streamflow (see the 'Internal Resources: Overlap' column). That's not entirely true, since some groundwater may be lost to evapotranspiration.
Note that these figures are expressed as flows, not stocks. That's typical for people interested in renewability or sustainability. It neglects the huge stocks of groundwater (or even surface water - think lakes) that may be available for use. In fact, Brazil sits atop about 62% (surface area, not volume) of the Guarani Aquifer System, which may contain as much as 30,000 cubic kilometers of water. The blue area on the Brazil map above shows the aquifer's extent. It's definitely a storage-dominated system.
My Ten Cents
Brazil has 12% of Earth's renewable surface freshwater flows - that's the correct way to say it. For the ultra-fussy, put liquid' in front of 'surface'. The ice types will say, 'Hey - glaciers flow!'
“I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind..." - Lord Kelvin (William Thomson)