Water is the most abundant substance at the Earth's surface, with most (almost all) of it contained in the oceans, which cover nearly 71% of the surface area of the Earth. Oceans are by far the Earth's largest reservoir, but their water is salty (with salinity from 33 to 37 g kg~ ). If evenly distributed on the Earth-sized uniform sphere, water would form a layer of a depth of ^2.7 km.
The global water resources constitute approximately 1.385 billion km3 (Figure 1). This makes up 0.17% of Earth's volume. About 97.5% of global water resources are saline and only 2.5% are fresh. Saltwater stored in oceans is the prevailing portion (96.5%) of Earth's water resources (1.338 billion km ). The average ocean's depth is 3794 m and the mass of the oceans is approximately 1.35 x 101 t (about less than a quarter of a permille of the total Earth's mass). The second largest water store on Earth - glaciers and permanent snow cover - is very much smaller than the oceans, containing 24.4 million km3 of water (c. 1.72% of global water resources), that is over 50 times less than the ocean water. However, this solid water store (whose prevailing part is ice and permanent snow cover in the Antarctic, the Arctic, and mountainous regions) contains freshwater, making up most (about 69%) of the total freshwater resources. The third largest global water store is groundwater, containing 23.4 million km of water (1.7% of global water resources), but more than half of groundwater is not fresh. Fresh groundwater resources amount to approximately 10 530 000 km3 (0.76% of total global resources but 30.1% of total freshwater resources). Since the frozen hydrosphere (cryosphere), being the largest reservoir of freshwater, is not easy to reach by the humans, groundwater is the largest source of freshwater, which is readily available. All the lakes on Earth contain 176.4 thousand km3 of water (0.013% of total water) with freshwater constituting more than half of the total volume and 0.26% of total freshwater. Some 16.5 thousand km of water is stored in the soil (0.001 2% of total water, 0.05% of global freshwater), while, on average, all the rivers of the world carry in any time instant approximately 2120 km3 of water, that is only 0.006% of freshwater. The atmosphere itself is a large water store, with the total volume of stored water
Oceans 1.338 x 109
5 Ice snow 2.434 x 107
Lakes 1.76 x 105
- Groundwater 2.34 x 10 therein fresh 1.053 x 107
xmSafiiKmilwiii', therein fresh 9.1 x 10' Soil moisture 1.65 x 104 & Atmospheric water 1.29 x 104 Marshes 1.15 x 10'
Rivers 2.1 x 103 i Biological water 1.1 x 103
10000 100000 1000000 10000000 1E + 08 1E+09 1E + 10
Figure 1 Global water resources: distribution into water stores, in cubic kilometers. Gray bars represent saltwater, and stripped bars represent freshwater.
being about 13 000 km3 (0.001% of total water, 0.04% of freshwater). Wetlands contain approximately 11 500 km3 of water, that is, 0.000 8% of total water and 0.03% of global freshwater resources. Biological water has the global volume of 1120 km3 that is 0.0001% of total water and 0.003% of freshwater. Total freshwater resources are estimated to be in excess of 35 million km3.
Water is in a permanent motion converting from liquid to solid or gaseous phase, or back, with the principal processes being evaporation and precipitation, which are distributed very unevenly in space and time. Annual precipitation total largely depends on the latitude. Globally averaged latitudinal precipitation is highest near the equator and relatively high at the latitude around 60° (where upward lift of air masses is dominating). It is lower at the latitude around 30° and near the poles (where downward movement of air masses dominates). Also the altitude above the sea level is an important control of the amount of precipitation. Among further factors of importance are distance from source of water, exposition to prevailing wind, and large-scale landscape structure.
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