Tukuk-OSOf all water on Earth, roughly 97% is salty ocean. Of the ~2.5% fresh, about two-thirds is locked in ice sheets and glaciers and much of the rest lies underground, so river and lake water — the visible, refillable, life-supporting part that most ecosystems and our taps depend on — is well under 1% of one percent of the total. A glass of salt water out of every litre of Earth’s water, with a teaspoon of the fresh portion drinkable, is the honest proportion.
What moves that teaspoon is the water cycle, and its reservoirs differ wildly in how long they hold: a water molecule spends about a week in the atmosphere as cloud and rain, months to years in rivers and soil, centuries in a deep lake or a temperate groundwater pool, and tens of thousands of years in an ice sheet or a fossil aquifer. Groundwater is the quiet savings account: India and the US Great Plains and much of the Middle East irrigate largely on aquifers, some being pumped faster than rain refills them — Ogallala in the US, the North China Plain, the Arabian fossil water being spent within decades.
The human pressure is now measurable: the UN counts roughly two billion people living with water stress, and agriculture withdraws about seven in ten freshwater litres. But “water shortage” is usually management, not physics: the cycle returns the same water it always has — clean, cold, and in the right place is an engineering, pollution and pricing problem, not a shrinking-ocean problem. Reuse at Singapore scale proves it.