The Hydrosphere

What is the hydrosphere?

The hydrosphere is one of the four major spheres of the Earth, encompassing all water on the planet– and there’s a lot! The surface of the Earth is approximately 75% covered by ocean. There are an estimated 1386 million cubic kilometers of water present on the planet, a number which includes both liquid and frozen water, and that which is present in lakes, rivers, and underground as groundwater. Water is also present in the atmosphere in vapor form, as clouds or fog. The hydrosphere is a unique feature of Earth (at least within our solar system), as although other planetary bodies such as Mars, Jupiter’s moon Europa, and Saturn’s moon Enceladus do have water present as frozen ice, liquid water is not present there. The term for the environment in which ice dominates most physical interactions is a ‘cryosphere’.

all water on the planet as a sphere
If all the water on Earth was gathered into three spheres. The largest is all water, the medium sphere is all fresh water, and the smallest sphere represents all surface water in lakes and rivers. Source: USGS.

One the main ways water differs from place to place is by salinity, or the amount of salt dissolved it contains. The oceans are, on average, 3.5% saline. Freshwater still has some amount of salt dissolved within, but is defines as water with a salt content of less than 0.05%. Between these two thresholds, water is called ‘brackish’, and is a mixture of saltwater and fresh water, often found in estuarine environments. Saltwater accounts for 97.5% of water, with freshwater making up to 2.5%. Of this 2.5%, 68.9% is locked up in ice and glaciers, 30.8% occurs as groundwater, and lakes and rivers make up just 0.3% of it.

The Water Cycle

The water cycle is a term used to describe how water moves through the environment and is transferred from one state to another. There are several different reservoirs in which water exists, and the water cycle depicts how water moves between these reservoirs. The easiest way to understand the water cycle is in terms of reservoirs and flows. Reservoirs are defined as places of storage, or where water remains for an extended period of time. Reservoirs include:

  • Streams and rivers
  • Oceans
  • Lakes
  • Groundwater and aquifers
  • Glaciers and polar ice caps
  • Rain, clouds, fog, snow

The majority of water on Earth spends most of its time in storage, and the majority of stored water is present in the oceans. Around 96.5% of water on Earth exists in the oceans. Throughout the history of Earth, the oceans have expanded and contracted as temperature and environmental changes occur. During the last ice age, up to 1/3 of the Earth was covered by glaciers and ice. This caused sea level to be around 120m lower than it is currently at the peak of the last ice age; about 20,000 years ago.

thunderstorm cloud from space
Clouds form when water vapor gas condenses in the atmosphere forming small liquid water droplets. Most of the moisture the condenses in the atmosphere will return to the ocean as rain. Source: NASA.

There are several different methods of transportation by which water may transfer from one reservoir to another. Evaporation is the process by which liquid water is converted to water vapor. Heat causes the bonds between water molecules to break. Evaporation will always occur in balance with condensation, but only when the rate of evaporation exceeds the rate of condensation will net evaporation will occur. Over the oceans, evaporation usually exceeds condensation and precipitation, which is why weather systems will often develop over the seas.

Condensation is essentially the reverse process to evaporation; as water molecules existing as water vapour cool, they will return to the form of liquid water. This is the process that produces clouds. When a water droplet forms, it is usually too small to fall as precipitation, but when a large enough accumulation occurs, the individual droplets come together as a raindrop and will overcome the updrafts keeping the cloud together. Precipitation of raindrops is more likely to occur if there is a central nucleus available for the water to condense around. Such nuclei include tiny dust or smoke particles. The amount of precipitation to fall in an area will depend on the specific geography of that region. For example, in northern India, the huge physical presence of the Himalayan mountain range causes northerly-migrating, equatorial, maritime air masses to be deflected towards the    north-west, an effect which produces seasonal monsoon rains in Bangladesh.

Interactions with Earth’s other systems

The hydrosphere has several important interactions with the neighboring spheres; the atmosphere, lithosphere, and biosphere. The atmospheric interaction has already been mentioned; evaporated water molecules exist as water vapor and clouds within the atmosphere, and the interaction between these two spheres is what produces weather systems on Earth.

The interaction between the hydrosphere and the lithosphere is also extensive. Water is an important component of weathering; the process by which solid rock is broken down into smaller pieces. Freeze-thaw weathering is an example of water-based weathering. Water infiltrates cracks in a rock as liquid water. When the temperature eventually drops, the water can freeze. When water freezes, it also expands. This expansion causes cracks to increase in size, and eventually leads to the mechanical failure of the rock.

Another interaction between the hydrosphere and the lithosphere is rock transport. Once rocks have been broken up into smaller pieces, they are often carried away by streams and rivers. Rivers will shape rocks into smaller and more rounded pebbles over the full course of the river, and the longer a river is, the smaller and more rounded the pebbles within it will become. The geography of a landscape can also be sculpted by its interaction with the hydrosphere. The seas erode the coasts through hydraulic action, creating caves, arches, and stacks.

There is also an interaction between the hydrosphere and the biosphere. There is a continuous transfer of energy, mass, and momentum between vegetated terrestrial ecosystems and the hydrosphere, and interactions occur on timescales ranging from hours and days to years and even decade-long cycles. For example, terrestrial plants will absorb liquid water through their roots. This water is used for the growth and reproduction of the plant. Water can also return to the atmosphere from plants through the process of transpiration, whereby water is essentially evaporated from plants via the underside of leaves.

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