It is estimated that the world contains about 1 400 million km3 of water. Of this water, 35 million km3 (2.5 percent) are freshwater.
The large amount of freshwater contained in ice caps, glaciers and deep in the ground is not accessible for use. Freshwater that can be used stems essentially from rainfall over land, generated through the hydrological cycle (see figure). Water is continuously recycled as a result of evaporation driven by solar energy. In this way, the hydrological cycle consumes more energy per day than that used by humankind over its entire history.
World water distribution | |||
Water volume (million km3) |
Percent of freshwater |
Percent of total water | |
Total water |
1 386 |
100.00 | |
Freshwater |
35 |
100.0 |
2.53 |
Glaciers and ice caps |
24.4 |
69.7 |
1.76 |
Groundwater |
10.5 |
30.0 |
0.76 |
Lakes, rivers, atmosphere |
0.1 |
0.3 |
0.01 |
Saline water |
1 351 |
97.47 | |
The average annual rainfall over land amounts to 119 000 km3, of which some 74 000 km3 evaporate back into the atmosphere. The remaining 45 000 km3 flow into lakes, reservoirs and streams or infiltrate into the ground to replenish the aquifers. This represents what is conventionally called "water resources". Not all of these 45 000 km3 are accessible for use because part of the water flows into remote rivers and during seasonal floods. An estimated 9 000 - 14 000 km3 are all that are economically available for human use, a teaspoon in a full bathtub compared to the total amount of water on earth.
Annual withdrawals of water for human use amount to about 3 600 km3. Part of the available surface water must be left to follow its natural course to ensure effluent dilution and safeguard conservation of the aquatic ecosystem. Exactly how much water needs to be left in a river will vary with the time of the year and many factors specific to each river basin. While a better understanding of the rivers' complex ecological services is still pending, instream flow needs are estimated at 2 350 km3 per year. Adding this amount to the amount withdrawn for human use results in 5 950 km3 of easily accessible freshwater resources that have been already subscribed. Taking into account demographic and water demand projections, the global water figures show a tightening water situation. Because both water and population are unevenly distributed, the situation is already critical in various countries and regions. Increasing areas of the world are suffering from freshwater shortages and competition among users is rising.
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The hydrological cycle with annual volumes of flow given in thousand km3  |