Where does water come from? The hydrological cycle
On Earth, water is distributed in the seas, rivers or lakes in a liquid state; in the glaciers near the poles and mountains in a solid state; and in the clouds, in a gaseous state. Basically, the water cycle consists of the movement of water from one place to another, changing its physical state: from liquid to gaseous or solid state, or from gaseous to liquid state, depending on environmental conditions.
Content
The hydrological cycle: how the water moves in Earth
The water cycle describes the presence and movement of water in and on the Earth. The Earth's water is always in motion and constantly changing state, from liquid, to vapor, to ice, and back again. The water cycle has been going on for billions of years, and life on Earth depends on it. In fact, it can be argued that the Earth would be an inhospitable place if the water cycle did not take place.
Figure 1 shows schematically the water cycle. This figure will be very useful to understand the explanation that will be presented later, so do not lose sight of it.
Figure 1 The hydrological cycle (Source: https://www.usgs.gov/special-topic/water-science-school/science/).
The water cycle does not start at a specific location, but for this explanation it will be assumed that it starts in the oceans. The sun, which drives the water cycle, heats the water in the oceans, which evaporates into the air as water vapor. Rising air currents carry the vapor to the upper layers of the atmosphere, where the lower temperature causes the water vapor to condense and form clouds. The air currents move the clouds over the globe, the cloud particles collide, grow and fall as precipitation.
Some of this precipitation falls as snow, and accumulates in ice sheets and glaciers, which can store frozen water for millions of years. In warmer climates, the accumulated snow melts when spring arrives. The melted snow runs over the land surface as meltwater and sometimes causes flooding. Most precipitation falls in the oceans or on land, where, due to gravity, it runs over the surface as surface runoff.
Some of this runoff reaches the rivers in depressions in the land; in the flow of the rivers the water is transported back to the oceans. Runoff and groundwater that gushes to the surface accumulates and is stored in freshwater lakes.
Not all rainwater flows into rivers, as much of it is absorbed by the soil as infiltration. Some of this water remains in the upper soil layers, and returns to water bodies and the oceans as groundwater discharge. Another part of the groundwater finds openings in the earth's surface and emerges as freshwater springs.
Groundwater found at shallow depths is taken up by plant roots and transpired through the leaf surface, returning to the atmosphere. Other infiltrated water reaches deeper soil layers and recharges aquifers (saturated subsurface rock), which store large quantities of fresh water for long periods of time. Over time, this water continues to move, and some of it will return to the oceans, where the water cycle closes... and begins again.
Distribution of water in the planet
How is water distributed on our planet? According to Gleick (1996), of the total water estimated to exist on Earth, 1,386 million cubic kilometers, about 96% of the total, is salt water. Of the total freshwater, 68% is confined in glaciers and snow. Thirty percent of freshwater is in the ground. Surface sources of fresh water, such as lakes and rivers, account for only about 93,100 cubic kilometers, which represents 0.007% of the total water. Table 1 shows an estimate of the distribution of water on the planet, highlighting the contribution of the different sources.
Table 1 Estimation of the distribution of water on the planet (Source: adapted from Gleick, 1996).
Source of water | Volume of water, in cubic kilometers | Percentage of fresh water | Total percentage of water |
Oceans, Seas and Bays | 1,338,000,000 | — | 96.5 |
Ice caps, Glaciers and Perpetual Snows | 24,064,000 | 68.7 | 1.74 |
Groundwater | 23,400,000 | — | 1.7 |
Fresh | 10,530,000 | 30.1 | 0.76 |
Salted | 12,870,000 | — | 0.94 |
Soil moisture | 16,500 | 0.05 | 0.001 |
Ground ice and permafrost | 300,000 | 0.86 | 0.022 |
Lake | 176,400 | — | 0.013 |
Fresh | 91,000 | 0.26 | 0.007 |
Salted | 85,400 | — | 0.006 |
Atmosphere | 12,900 | 0.04 | 0.001 |
Marsh water | 11,470 | 0.03 | 0.0008 |
Rivers | 2,120 | 0.006 | 0.0002 |
Biological water | 1,120 | 0.003 | 0.0001 |
Total | 1,386,000,000 | – | 100 |
Despite the proportion shown in the table above, rivers and lakes are the main source of water used by the population on a daily basis.
Importance of hidrological cycle
The water cycle is fundamental for the maintenance of life on Earth, and for the sustenance of all terrestrial ecosystems. It also determines climatic variation and interferes with the level of rivers, lakes, seas and oceans.
As human beings, we have an enormous responsibility to preserve the proper functioning of the water cycle, for ourselves and for future generations.
References
- “The Water Cycle” (2021) Water Science School. USGS, USA. Disponible en https://www.usgs.gov/special-topic/water-science-school.
- Gleick, P. (1996) “Water Resources”. In Encyclopedia of Climate and Weather, ed. By S.H. Schneider, Oxford University Press, New York, Vol. 2, pp. 817-823.
