The Hydrosphere

The Hydrosphere: Definition and Distribution

The hydrosphere is defined as the water that covers the Earth's surface. Water in the Earth can be found in three different states: liquid, solid (called ice) and gas (called water vapour).

70% of total Earth's surface is covered by water. Most of this water is salt water, located in the seas and oceans. In fact, 97% of total water is salt water. The other 3% is fresh water that can be found in different locations and states. 

68.8% of total fresh water is solid water: ice and snow. This kind of water can be found, above all, in the poles, but also in high mountains as snow covering the ground or glaciers (large and erosive rivers made of solid water).

30.1% of fresh water is groundwater, water located in pores of deep rocks, underground currents, aquifers, etc.  

0.45% of total fresh water can be found in the atmosphere, as water vapour or tiny drops of water in the clouds. Other 0.45% can be found in living beings: water is the main component of all the known living things in this planet.

Finally only 0.3% of total fresh water is surface fresh water. Only this percentage is really available for regular human consumption. 

87% of surface water is located in lakes and other similar wetlands. 11% is located in swamps. Only 2% can be found in rivers and other surface currents, such as torrents or streams. 

Water: Properties

Water is a substance with some relevant characteristics and properties that make it essential to understand its role in the planet and living things.

Universal solvent

Water is called universal solvent, because it can dissolve many different substances. It can dissolve nearly any kind of salt and many organic substances. It can also support many chemical reactions, or in other words, many chemical reactions can occur in water.

This is one of the reason why water is the most abundant component of living things. All the chemical reactions essential for in living beings take place in water. Water, besides, dissolves many organic or inorganic substances that make up the living beings.

Specific heat capacity

Water has a high specific heat capacity. This means that water preserves heat, so it warms and cools quite slowly.

This characteristic is related to how seas and oceans prevents from abrupt climatic changes of the atmosphere.

High surface tension

Surface tension is the force of the water surface that makes it extend on a surface as much  as possible. Water has a relevant surface tension. As a result, it has a great force of adhesion and also a great cohesion.

This is related to how water flows, its viscosity and its erosive capacity.

Sea Water

Characteristics

97% of total water on Earth is salt water located in seas and oceans. The main characteristic of this water are:

• Components and salinity.
• Temperature.

Components and salinity

Apart from water molecules (H₂O), sea water has other components. The most relevant ones are salts and electrolytes, that are related to the salinity. But sea water has other components, such as gases and organic molecules.

Sea water has different dissolved gases. Oxygen (O₂) is one of the most abundant dissolved gases and it is essential for living beings that need this molecule for respiration. Nitrogen and carbon dioxide are quite abundant too.

The organic molecules dissolved in sea water come from the vital activity of living beings or from several human activities (in this case, we talk about contaminants).

Salinity is defined as the concentration of salts in the water. Salts are inorganic substances. The most abundant one is, by far, sodium chloride (NaCl). Sea water has higher concentration of salts than surface water or groundwater. Due to this, sea water is also called salt water.

The concentration of salts, above all of sodium chloride, is one of the most important characteristics of sea water. Different oceans have different salinity. We can say, in general, that the colder the sea is, the lower amount of salts it has. Due to this, The Artic Sea is characterised by its low salinity. The Dead Sea is  the sea with highest salinity.

The average salinity of sea water is around 35 grams of salt per litre of water. Although the essential components of salt are, as we have just studied, sodium and chlorine, there are other components, such as magnesium, calcium, sulphur or potassium.

Temperature

The temperature of sea water depends on the latitude (distance from the equator) and the depth.

As far as latitude is concerned, sea water temperature decreases with latitude. In other words, it is hotter near equator and it is colder in the poles. This fact is related to some water movements called sea currents.

As far as depth is concerned, the water temperature decreases with depth. At 1000 meters deep, the temperature is lower to -2°C. In fact, it is still liquid because pressure and salinity prevents it from freezing.

Sea water movements

The most relevant sea movements are:

• Waves.
• Ocean Currents.
• Tides.

Waves

Waves are surface movements of water in seas or oceans. They mainly result from wind wind flowing over the seas. Waves can move from thousands kilometres before reaching the coast. 

Waves can have different sizes depending on the strength and direction of the wind. Wave size also depends on the coast features.

Waves are relevant erosive factors. They are related to the formation of cliffs and beaches and how they evolve or change.

Waves are, besides, very important because they oxygenate the water.

Ocean Currents

Ocean currents are continuous directed movements of seawater. They are large masses of water that move like rivers through the oceans.

These currents are mainly produced by the winds and differences in the temperature and salinity of the water. These differences cause variation in the density of the water and this variable density is the main reason for its movement.

The Coriolis effect is another relevant factor. This effect comes from the centrifugal forces caused by the Earth's rotation. The centrifugal forces affect, above all, the fluids, such as atmospheric gases and seawater. It affects the direction and trajectory of the current.

There are also vertical currents. These are vertical movements of seawater. There are two different currents, the surface current that involves the first four hundred meters, and the deep current that involves water deeper than four hundred meters. These two layers of seawater are nearly independent and water does not tend to move from one layer to other. In fact, some water properties, such as salinity and temperature, change abruptly causing a sort of boundary between these two regions. 

Tides

Tides are periodical rises and falls of seas levels. There are one or two high and low tides per day that are caused by gravitational forces exerted by the Moon and the Sun. The Moon's gravitational effect is much stronger than the Sun's one.

The amplitude of the tides depend on two factors: the relative position of the Moon and the Sun and the characteristics of the shore.

Tides are higher when the Moon and the Sun are aligned with the Earth, and are lower when  they are not in a line.

The structure of the shore is also a relevant factor. When the shore directly connects to an large ocean, tides tend to be higher. Tides in the Cantabrian Sea, for instance, are higher than tides in the Mediterranean Sea because the Cantabrian Sea is directly connected to the Atlantic Ocean, that has much more water mass than the Mediterranean Sea and as a result, the effect of the Moon and the Sun is also bigger.

Fresh Water

It is called continental water, and its main characteristic is the low concentration of dissolved salts. There are several types of fresh water:

• Solid water (ice).
• Groundwater.
• Lakes.
• Wetlands.
• Streams, torrents and rivers.

Solid Water

It is the most abundant type of fresh water. It can be found in glaciers, caps, ice sheets or icebergs.

Solid water is exclusive to cold places. The atmospheric temperature depends on the altitude and the latitude and the coldest locations are at high altitude or high latitude. Due to this, ice and snow can merely be found in covering high mountains or near the poles. In fact, mountains above four thousand meters and polar regions have permanent snow or ice. 

Glaciers are characteristic structures made of a permanent mass of ice in continuous movement. The ice in the glaciers moves, flows like the water in a river, but very slowly. Although it is not a fast movement, it has an extreme erosive power. They characteristically wear deep and wide U shaped valleys.

Groundwater

Groundwater is defined as the water that can be found beneath the Earth's surface. It is stored in soil pores, fractures of rocks or in underground rivers or lakes.

Groundwater can sometimes form underground caves and galleries.

When it emerges to the surface, it can form springs, seeps, wells or wetlands. It usually feeds rivers, streams and lakes.

The line that separates the lower parts of the soil filled with water to the upper parts of the soil with no water in the pores is called phreatic level. 

Lakes

Lakes are large masses of water surrounded by land. They can be fed by rivers, glaciers or groundwaters. Rivers and groundwaters can also drain lakes.

They are always located in basins or depressions and they can have variable size and depth, from few square metres to many square kilometres. The largest lake of the world is the Caspian Sea, that is 371,000 Km2. 

Wetlands

Wetlands are areas permanently or seasonally saturated with water. There are many different types of wetlands:

• Swamps: forested wetlands.
• Marshes: wetlands covered by herbaceous plants.
• Bogs: wetlands rich in discomposed plants and other decaying materials.
• Fens: wetlands dominated by grass and sedges.

Rivers and Torrents

Natural water courses of freshwater. Rivers are permanent water courses. Torrents and streams are intermittent or discontinuous water courses with variable courses.

They are responsible for surface runoff, they transport water from the land to the sea. They are also a relevant erosive factor, not as strong as glaciers but much more frequents. They wear V shaped valleys in he upper course and U shaped valleys in the lower course.  

Water Cycle

The water cycle includes all the process that promotes changes in water and its transformations from one type of water to another.

• Evaporation: the Sun warms water, transforming it into water vapour. This process occurs, above all, in seawater, because it is extremely abundant and covers 75% of Earth's surface. But it also takes place in freshwater.
• Evapotranspiration: the water from living beings also evaporates. It is a low amount of water.
• Condensation: the atmospheric water vapour is condensed, transforming the vapour into tiny drops of water that made the clouds.
• Transport: the wind moves the clouds. This is specially relevant in the sea, so clouds formed above the sea due to the evaporation and condensation can move from the sea to the land.
• Precipitation: when the tiny drops of water of the clouds cool and condense, forming bigger drops that fall. This process can occur in many different places, but is very usual in higher zones of the atmosphere and when the clouds shock with the mountains. The water falls forming rain, snow or hail.
• Surface Runoff: this is the movement of the surface water from the land to the sea. Surface runoff forms rivers, streams and torrents.
• Infiltration: the infiltration is the movement of water from the Earth's surface to deeper parts of the ground, forming groundwater.
• Deep Runoff: this is the movement of groundwater to the sea. Due to this, groundwater returns from the land to the oceans.