Every map of fresh water you have ever seen is a map of the exception. Rivers and lakes are the visible sliver; the actual reservoir is rock — vast basins of saturated sandstone and limestone under the Sahara, the High Plains, the Pampas and the Australian outback. This is where the water is, who is spending it faster than it returns, and what the map makes obvious that a table of numbers hides.
★ The map — great aquifer systems & freshwater lakes, coloured by depletion stress
Extents are hand-simplified for legibility; every volume, place name and stress rank on the map is a published figure. Stress classes follow Richey et al. (2015), which used NASA's GRACE gravity satellites to weigh 37 of the world's largest aquifer systems from orbit.
01 · The proportions
Earth carries about 1.338 billion km³ of water. Almost all of it is ocean. Of the 2.5% that is fresh, more than two-thirds is frozen — which is why the liquid map below is really a map of rock.
Source: USGS Water Science School, after Shiklomanov (1993). Every river on the planet, running at once, holds about 2,120 km³ — roughly a tenth of Lake Baikal.
02 · The great stores
The Nubian Sandstone Aquifer System sits beneath Egypt, Libya, Sudan and Chad — some of the driest land on Earth — and holds an estimated 150,000 km³, several times every great lake combined.
Sources: Geoscience Australia (Great Artesian Basin); UNESCO/OAS Guaraní programme; UNESCO World Heritage (Baikal); US EPA (Great Lakes); USGS SIR 2017–5040 (High Plains, 2.91 billion acre-feet in 2015 ≈ 3,590 km³). Nubian and Guaraní volumes are published estimates, not measurements — Guaraní figures in the literature range from about 30,000 to 45,000 km³.
03 · The surface exceptions
Liquid surface fresh water is a rounding error globally — and it is spectacularly concentrated in a rift in Siberia and five lakes on the US–Canada border.
The two headline shares are measured against slightly different denominators — Baikal against unfrozen fresh water, the Great Lakes against surface fresh water — so they should not simply be added. Both point the same way: surface storage is extraordinarily lumpy.
04 · The overdraft
Between 2003 and 2013 the GRACE satellites weighed the world's biggest aquifer systems from orbit by tracking tiny changes in gravity. Twenty-one were losing more water than nature returned; thirteen of those were classed as significantly distressed.
Source: Richey et al., Quantifying renewable groundwater stress with GRACE, Water Resources Research (2015); NASA JPL / UC Irvine summaries. A companion paper by the same team stressed that total aquifer volumes remain poorly known — we can measure the loss far better than the balance.
05 · One basin, measured
The High Plains aquifer is the best-surveyed big aquifer in the world, which makes it the clearest picture we have of what irrigation does to stored water over seventy years.
Source: USGS Scientific Investigations Report 2017–5040. Pre-development storage is derived here by adding the reported 273.2 million acre-foot loss back onto the 2.91 billion acre-feet remaining in 2015 — a derived figure, converted at 1 acre-foot = 1,233.5 m³.
06 · The politics of rock
Water in rock ignores the lines drawn above it. IGRAC's global inventory keeps finding more shared basins the closer anyone looks — and the great majority still have no formal agreement governing them.
Source: IGRAC & UNESCO-IHP, Transboundary Aquifers of the World, 2021 edition (scale 1:50,000,000). Europe accounts for the remaining 97. The rise from 366 to 468 reflects better mapping, not new water.
07 · What it's for
Groundwater is not a reserve held for emergencies. It is the ordinary, daily supply for a large share of humanity — which is precisely why the overdraft matters.
What to notice on the map
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