Fresh Water Distribution: Surface and Groundwater

Fresh water is water with very low salt content; about 2.5 percent of all water on Earth is fresh, and of that, roughly two-thirds is locked in glaciers and ice caps, nearly all the rest is groundwater, and only about 1 percent is surface water in lakes, rivers, soil and the atmosphere. In brief: the oceans hold about 97 percent of the water on Earth. Most of the fresh water that people can use easily comes from rivers, lakes and shallow groundwater, which together are only a tiny fraction of the total (roughly 0.03 percent). Water is constantly cycled between the oceans, atmosphere and land by evaporation, condensation, precipitation and runoff.

Fresh water is unevenly distributed, and understanding where it is found is the basis of water management. This guide describes water on Earth, surface water and groundwater, compares the stores in a table, works through a calculation of the volumes, outlines how knowledge of the water cycle developed and corrects common mistakes.

How is water distributed on Earth?

Earth has about 1.386 billion km³ of water. Of this, about 97.5 percent is salt water, mostly in the oceans, and about 2.5 percent (roughly 35 million km³) is fresh. Most fresh water is frozen: about 68.7 percent is in glaciers, ice caps and permanent snow (Antarctica and Greenland hold most), and about 30.1 percent is groundwater. Only about 1.2 percent is in surface water and other forms, such as lakes, soil moisture, wetlands, rivers, permafrost, plants and the atmosphere. (Figures are rounded estimates, which differ slightly between sources.) The water moves continuously through the hydrological (water) cycle: evaporation from the oceans and land, condensation into clouds, precipitation, and return by runoff and infiltration. The average time that a water molecule stays in a store varies enormously: days in the atmosphere, weeks to months in rivers, years to centuries in lakes and groundwater, and thousands of years in ice sheets and deep groundwater.

What is surface water?

Surface water is found above the ground in rivers, streams, lakes, ponds, reservoirs and wetlands. It is replenished quickly by rainfall and snowmelt, and it is the most visible and most used source for cities, industry and irrigation. The stores in lakes are much larger than in rivers: of the fresh surface water, lakes hold much more than rivers, which contain only about 0.5 percent of the surface fresh water at any time, though they carry a large flow. Surface water is easily polluted and varies with the seasons and droughts. Glaciers and snowpacks in mountains act as natural reservoirs, which release meltwater in summer to rivers such as the Indus, Ganges and Rhine, so their decline is a concern.

What is groundwater?

Groundwater is the water that fills the pores and cracks in soil and rock below the surface. Rain soaks in (infiltration) and moves down (percolation) until it reaches the water table, the top of the saturated zone. A rock layer that stores and transmits water well, such as sandstone, gravel or limestone, is an aquifer; a layer that blocks it, such as clay, is an aquiclude. Water is taken out through wells and boreholes or comes out naturally at springs. Unconfined aquifers are recharged by local rainfall, while confined aquifers lie between impermeable layers and the water is under pressure, so it may rise in an artesian well. Groundwater is cleaner than surface water, and it is the main source of drinking water for about half of the world’s population, but it is renewed very slowly. Over-pumping lowers the water table and may cause land subsidence, salt water intrusion near the coast, and dry wells. Some deep aquifers contain “fossil water” that was stored thousands of years ago and is effectively not renewable, such as the Nubian Sandstone Aquifer in North Africa.

Fresh water stores compared

Worked example: how much water is accessible?

1. Fresh water: 2.5 percent of 1,386 million km³ is 0.025 × 1,386 = 34.65 million km³, or about 35 million km³.
2. Ice: 68.7 percent of 35 million km³ is 0.687 × 35 = 24.0 million km³.
3. Groundwater: 30.1 percent is 0.301 × 35 = 10.5 million km³ (some of it salty or too deep to be used).
4. Surface and other fresh water: 1.2 percent is 0.012 × 35 = 0.42 million km³.
5. As a share of all water: 0.42 ÷ 1,386 = 0.0003, or 0.03 percent. If all the water on Earth were 1,000 liters, the easily accessible surface fresh water would be only about 0.3 liters, a small cupful.
6. Per capita comparison: a family uses 400 liters of water per day; per year this is 400 × 365 = 146,000 liters = 146 m³. A lake of 1 km³ = 1,000,000,000 m³ would supply 1,000,000,000 ÷ 146 = about 6.8 million such families for a year, if no water were lost to evaporation and no refilling was required.

How did we learn about the water cycle?

Ancient thinkers such as Aristotle suggested that rivers are fed by water from underground caves, and the idea that rain and snow supply rivers was developed by Pierre Perrault and Edme Mariotte in France in the 17th century, who measured rain and river flow in the Seine basin. In the 19th century, Henry Darcy’s experiments (1856) established the law that describes the flow of groundwater through sand. Hydrology developed as a science in the 20th century, and satellites such as GRACE (launched 2002) now measure changes in groundwater and ice by sensing small changes in gravity.

Which mistakes are common?

Mistake 1: Thinking that fresh water is plentiful because Earth is covered by water

Most water is salty, and most fresh water is frozen or deep underground.

Mistake 2: Picturing groundwater as underground rivers and lakes

It is mostly water held in the pores of rocks and sediment. Caves with flowing water are the exception.

Mistake 3: Assuming groundwater is always renewable

Recharge can take centuries or longer, and fossil aquifers are being mined faster than they refill.

Mistake 4: Confusing the volume in a store with the flow

Rivers hold only a small volume at any time, but they carry large flows because the water is renewed quickly.