What is the water flow

The global ecosystem

Seen from space, the earth is a blue planet - this is the color of the Waterwhich covers almost three quarters of its surface. Life is only possible when there is water. For this reason, space probes look for water on other planets - without water, no living beings will be found there. Water is abundant on earth, 1.4 billion cubic kilometers in total. However, the water is unevenly distributed over the earth and over the year, so that in many places there is either too much or too little water in many places.

water occurs on earth in liquid form, as ice or (here as a cloud) as a gas.
The photo shows an iceberg in Largo Argentino, from wikipedia, contribution “Water”,
accessed on June 17, 2006. Photo: Ilya Haykinson, License: Creative Commons
Attribution 2.0

The total amount of water on earth is estimated at 1.4 billion cubic kilometers (400; without the unknown amount that is enclosed in rocks deep inside the earth, more on this >> here). A good 97 percent of this is salt water; only 2.75 percent (38.5 million cubic kilometers) is fresh water. Although our very early ancestors, like all life on land, came from the oceans (see >> here) - the salty taste of our blood reminds us that this apple did not fall far from the tree - we cannot live on salt water today : we need fresh water. Most of this is in the Antarctic ice - a total of around three quarters of fresh water (29 million cubic kilometers) is bound in glaciers and ice sheets. And 98.5 percent of liquid fresh water (9.5 million cubic kilometers) is stored as groundwater in the cavities of the lithosphere - only a small part of it, and the small remainder of 144,000 cubic kilometers, 0.0001 percent of all water on earth, can be found easily accessible in rivers and lakes, in the ground, in living things and in the atmosphere. We owe this water to the earth's water cycle.

Only 0.0001 percent of the water on earth is there
easily accessible fresh water

The earth's water cycle

All water reservoirs are connected, they form the earth's water cycle. This creates fresh water from the huge salt water reserves of the oceans and thus ensures that the fresh water reserves are constantly renewed. The water cycle is driven by the sun: solar radiation allows water to evaporate from the oceans, lakes and rivers, the soil and living beings. This is how it gets into the >> atmosphere. The winds distribute the water vapor over the earth until it cools down somewhere, becomes liquid again and finally reaches the oceans, lakes, rivers and the ground as precipitation (rain, snow or hail), and from here into living beings . Part of the water seeps away and leads to the formation of new groundwater (see figure):

Scheme of the global water cycle. Own illustration, modified from Raven et al.,
Environment (1993), p. 82.

This water cycle moves enormous amounts of water: every year around 505,000 cubic kilometers of water evaporate, of which 434,000 over the oceans and 71,000 over the mainland. Of this water, 398,000 cubic kilometers of precipitation fall on the oceans and 107,000 on the mainland - in total, around 36,000 cubic kilometers of (fresh) water are transported from the oceans to the mainland every year. This water ultimately flows back into the sea via the rivers or as groundwater runoff. The constantly flowing water can remove hard rock, transport sand and rubble from the mountains to the plains and to the coast; in the event of storms or floods, it can also trigger natural disasters. In this way, water shapes the landscape of the earth. Living beings also play an important role in the water cycle, especially forests: Forests play the role of a sponge that holds back the water after rainfall and then gradually releases it again. Tree roots hold the soil in place, which stores water; Tree tops give mosses and other plants shade, which store water, and the trees evaporate water, which then falls again as precipitation - large forests generate part of their precipitation themselves and influence the water balance over a wide area.

The water cycle powered by the sun
relentlessly renews the fresh water supply

These above-mentioned figures for the water cycle also mean that the time that the water spends in the various reservoirs is very different: 434,000 cubic kilometers evaporate annually from the seas with 1.37 billion cubic kilometers, so the water is only completely replaced every 3,200 years . In contrast, there are only 13,000 cubic kilometers of water in the atmosphere - 505,000 cubic kilometers of precipitation mean that this water is completely renewed about every 9 days.

The origin of the water cycle

Liquid water only existed after the earth, which was hot in its early development, had cooled down so much that water became liquid and a first primordial ocean formed from the rainfall (see >> here). The water washed out minerals from the rocks, and together with the mineral salts from volcanic eruptions and dust from the deserts, these accumulated over time (mineral salts remain in the sea water during evaporation) - this is how the salt content gradually developed over the billions of years of the seas, which is between 3.3 and 3.7 percent today. (The result of this process also influences our climate: the salt content also determines the density of the seawater; and the density and the temperature are the driving forces for ocean currents such as the >> “global conveyor belt” - see >> Earth's climate.)

Life itself may have contributed to the fact that there is still water on earth today: The reaction of oxides from basalt rock with carbon dioxide from the air and water led to the formation of various carbonates, and hydrogen was released into the atmosphere. Hydrogen is so light, however, that the gravitational pull of the earth cannot hold it - it escapes into space. With the invention of photosynthesis, however, hydrogen was incorporated into the glucose molecule (for more on this see >> here); in addition, bacteria used hydrogen as an energy carrier. Without these “side effects” of life, there might be no more hydrogen on earth.

Water - a very special molecule

The Water molecule:
white = hydrogen,
red = oxygen.
The molecule has a negatively and a positively charged side.

Hydrogen bonds (dashed lines) allow the creation of a “network” that explains the special properties of the water.

The chemical formula of water - H2O - is probably the most famous in the world; and yet water still surprises chemists to this day. In many respects water is not “normal” - and that is a good thing (from the point of view of life). For a molecule of its size, water actually has a melting point that is much too high (0 ° C) and only evaporates at 100 ° C; that is the only reason why there are rivers and lakes on earth. “Normally” fabrics get denser the colder they get - water does not. Therefore, ice floats on the water (and living things under the ice cover can survive). The heat storage capacity of water is also much greater than a molecule of its size should - this is the only reason why ocean currents can make the climate in Europe so pleasant.

The reason for these "anomalies" lies in the structure of the water molecule. It has a negatively and a positively charged side. Water molecule: The molecule is built up like a V (see upper figure on the right): The oxygen atom sits on one side, the two hydrogen atoms on the other. These are positively charged, but the oxygen atom is negatively charged. And so the hydrogen atoms of one water molecule can attach to the oxygen atoms of another water molecule (via a so-called hydrogen bridge bond); and therefore they form associations that are larger and have different properties than the actual water molecule (lower figure). The researchers now know more than 70 "anomalies" of the water, and these occasionally give rise to esoteric speculations, which are mostly examples of "misguided extrapolations". But some are also the effects are also scientifically controversial - an overview of current issues is given on the (English) website Water Structure and Science.

Its properties also make water a central element of life: on the one hand, thanks to its polar structure, it can dissolve many substances and is therefore the universal solvent of life; on the other hand, thanks to its stable structure, it can also stabilize other substances, such as proteins or the genetic material DNA. How important water is can be illustrated by a number: The human body consists of around 60 percent water, the brain as much as 70 to 75 percent.

The hydrosphere in the concert of the spheres

Water and climate

In terms of energy, water is also a special molecule: a lot of energy is required to heat water. Therefore, water stores a lot of heat in lakes and oceans and has a balancing effect on the climate - near the sea or near large lakes, the climate is more balanced than far away from the water. And ocean currents, for example the >> global conveyor belt, transport huge amounts of heat from tropical regions to northern latitudes and make the >> climate more pleasant here. Evaporated water in the >> atmosphere also contributes significantly to the heat transport through the winds; During evaporation, even more heat energy is stored in the water vapor (the so-called “latent (as it cannot be felt) heat”); this is released when the water vapor becomes liquid again, i.e. when it rains. Furthermore, water vapor is a greenhouse gas; its share in the natural The greenhouse effect (more on this >> here) is around 60 percent. Clouds also influence the climate by reflecting sunlight on the one hand, but also holding back thermal radiation on the other. Whether clouds ultimately lead to warming or cooling depends on their altitude and on their level Shape down - deep, water-rich clouds cool, high ice clouds warm. The moisture transported in the clouds naturally also and above all determines the amount of precipitation, another important climate factor.

Water and rock

Water is one of the most important forces for erosion - precipitation removes mountains and makes entire islands disappear again; Tides and breaking waves shape the coastline. The water cycle thus shapes the face of the earth. It can even influence geological processes deep underground, as shown in the example of the Himalayas (more on this >> here).

Water and life

Water is the “matrix of life”: all physiological processes take place in an aqueous environment. Only when they come into contact with water do the proteins fold in the way necessary for their function; Biochemical reactions use water as a "lubricant" and for the transport of protons (positively charged hydrogen atoms). The DNA, the carrier of genetic information, only takes its famous double helix form when water is present. Water brings the nutrients from food into the cells and removes residues. The consequence: without water, most living things die much faster than without food (and the few exceptions fall into an inactive state); only the air is more important. Since the water in living beings is constantly being renewed, water - similar to the air, see >> here - connects all living beings with their environment and with each other.

In addition, water forms one for many living beings habitat: This applies first of all to the Oceanswhich occupy over seven tenths of the earth's surface (>> habitats of the ocean), and indirectly also for humans: the majority of the earth's population lives on the edge of the continents, no more than 100 km away from the oceans. This fact shows the importance of the oceans to humans (and also puts the oceans under stress from human activities). This applies to the various Freshwater habitatssuch as rivers and lakes, but also the water in the ground, which is indispensable for the living environment there. in the Soil water If nutrients are dissolved, it serves the plants to cover their water and nutrient requirements and supplies the microorganisms that can break down organic carbon as well as impurities. The water also makes them essential Wetlands which includes the lowlands of the great rivers, swamps, moors and wet forests. These areas are usually very productive and regulate the flow of water, thereby preventing or mitigating floods and promoting groundwater formation.

Through the precipitation, the water, in combination with the temperature, also has a significant influence on the vegetation on the mainland (>> Habitats - The mainland). The availability or the shortage of water have also increased over the course of the year evolution led to unique adaptations in living things that have learned to cope with drought, flooding and tides. In addition, the vegetation in turn influences evaporation; some ecosystems such as the tropical rainforests create a large part of their precipitation themselves.

The hydrosphere in us

The Human blood circulationn, the central distribution system for liquids and the substances dissolved in them. Figure changed from Wikipedia Commons (accessed July 21, 2007), license: Creative Commons attribuition 2.5

The interior of all living cells (previously called “protoplasm”) consists largely of water; around 60 percent of the water in a human body is found here. The other 40 percent circulate outside the cells, so around 90 - 95 percent of the liquid part of our blood consists of water. Overall, humans consist of around 60 percent water (babies 75 percent, old people 50 percent).

Water helps with the transport of nutrients in the blood, with the removal of unwanted substances (which are filtered out in the kidneys and excreted with the urine), has a role in numerous metabolic reactions and helps regulate our body temperature through sweating.

We take in water through drinking (a good half of the daily requirement) and with food (another good third), the rest is released during the metabolism.Too little water makes you feel thirsty; too much water causes the kidneys to secrete more water. In this way, the water balance remains in equilibrium as long as there is enough water available. Without water, however, humans can only survive for a few days ...

The water and humanity

The importance of water has led to the fact that water is described in many religions as the source of existence (the ritual bath of the Hindus in the Ganges, the ritual washing of the Muslims before entering a mosque and the Christian baptism remind of this); and that man has sought closeness to water throughout his history. The first human settlements were built where there was enough water (>> here), and many large cities were built on rivers or the sea. In the course of time the situation has often fundamentally changed: If Mesopotamia was still a green agricultural region 5,000 years ago (>> here), today the deserts of Iraq are located here; Tenochtitlàn, the capital of the Aztecs (>> here), was still on an island. Today Mexico City is a city with constant water problems. Water is scarce in many regions of the world - 40 percent of all households in sub-Saharan Africa are more than half an hour away from the nearest source.

Many of these problems were caused by humans. Deforestation destroys the storage capacity of ecosystems; Today, precipitation often no longer falls on the ground, in lakes or rivers, but on surfaces sealed by humans: on streets, parking lots or buildings. This precipitation no longer fills up the groundwater there, but is channeled into the rivers via the sewer system and reaches the sea with them. (In Germany, an estimated six percent of the land area is sealed, with around 120 hectares added every day - a significant proportion of the water is thereby withdrawn from the natural water cycle. More on the subject: >> here). The effect of soil sealing is reinforced by the fact that many rivers have been straightened and diked since the beginning of industrialization (>> more), which also accelerated runoff and reduced the formation of new groundwater. Only recently has the importance of natural water storage for ecosystems and the water supply of people been slowly recognized; in some regions of the world we are already beginning to restore wetlands and river systems to a natural state.

However, one can also try to remedy a lack of water by bringing water into the cities from regions that are far away and more abundant in water. Aqueducts have been part of human history for almost as long as the construction of cities (>> more); the construction of reservoirs as well, but reached new dimensions in the industrial age. Today, water shortages arise not only from the fact that there is not enough water, but also from the fact that the available water is polluted (>> more).

More about water:
The use of water by humans
A little history of water pollution
Ocean habitats

The global ecosystem - continue with:
The earth's climate - the hydrosphere (water)

To overview

© Jürgen Paeger 2006 - 2017

There is not only water on earthas the comets and the evidence of ice on the moon show in 2009. But in our solar system there is also liquid water, for example on the Jupiter moons Europa, Callisto and Ganymede and the Saturn moons Titan and Enceladus, and indeed many times the amount of water on earth. The water there is partially liquid because it is either prevented from freezing by violent tides (Europe, Ganymede), or its freezing point is lowered by dissolved substances (titanium).

Salt water can get through Desalination plants converted into fresh water. However, the current technologies cost a lot of energy - that makes desalination expensive - and the waste is brine, the disposal of which is a problem, especially at locations in Germany. In the future, the energy demand can be met in a more environmentally friendly way by solar systems.