Water is all around us and is the molecule that makes life on Earth possible; we depend on water daily to hydrate us. The entirety of the planet's water is called the hydrosphere; amazingly, only a fraction of this is available for us to drink. This is because only 2.5% of the hydrosphere is freshwater, with the rest being saltwater in the oceans. Of this 2.5%, only a tiny fraction is available to humans, most stored in ice sheets, glaciers, or deep underground aquifers.
The definition of the hydrosphere
The hydrosphere encompasses all the water in the Earth's system; this includes water in the liquid, solid, and gas phases. Here are where you find water in each state:
Liquid: water found in oceans, lakes, rivers, and estuaries is in a liquid state. Groundwater in aquifers and soils is also in the liquid phase, and so is precipitation.
Solid: icebergs, ice sheets, glaciers, snow, and hail are all water in the solid phase, that being ice. The entirety of the planet's ice is called the cryosphere.
Gas: water in the gaseous phase refers to water vapour in the atmosphere. Water vapour can form mist, fog, and clouds; sometimes, it is invisible in the air.
All these different forms of water can be described as reservoirs of the hydrosphere, with the most abundant reservoirs being oceans and water vapour in the atmosphere.
Formation of the hydrosphere
Climate researchers have various theories about how the Earth gained water; most believe that asteroid impacts brought water to the Earth (these asteroids often contained vast amounts of ice that would have melted with increasing temperatures).
No water vapour was present when the Earth formed 4.6 billion years ago.
Other theories include water released from reactions between minerals in the Earth's crust and the consistent outgassing of this water into the atmosphere as water vapour (this would have taken much longer than asteroid impacts). Most scientists agree that a combination of these events caused the formation of the hydrosphere.
Outgassing is the release of a molecule in gaseous form that had previously been locked up. This could result from high temperatures, pressures, or a chemical reaction.
The characteristic of the hydrosphere
Here are some essential characteristics of the hydrosphere that you should know:
Solar energy from sunlight provides the power for water molecules to transition between different states.
The hydrosphere surrounds the Earth as water vapour.
The density of water changes with heat and salinity.
Freshwater from melting ice will reduce the density of salty waters.
Temperature decreases at higher latitudes as there are fewer particles at lower pressures (see hint).
The hydrosphere is an essential part of the Earth's system that sustains life.
Water is consistently cycling between the lithosphere, biosphere, and atmosphere.
Low pressures mean fewer particles in the same area. Therefore, fewer particles will collide, so they will have less kinetic energy and will be at a cooler temperature.
The water cycle
The water cycle is the circulation of water between the atmosphere, lithosphere, and biosphere. This circulation of the planet's water maintains the hydrosphere and makes water available to ecosystems and human populations. Here are the various stages of the water cycle.
Interaction between the hydrosphere and atmosphere
The first two stages of the water cycle, evaporation and condensation, involve interactions between the Earth's hydrosphere and atmosphere.
Evaporation
Infrared radiation (solar energy) from the sun warms water molecules and causes them to move around faster and gain more energy. Once they have sufficient energy, the intermolecular forces between them will break, and they will transition to the gaseous phase forming water vapour, which then rises into the atmosphere. Evapotranspiration concerns all the water vapour evaporated from soils and the stomata of plant leaves in transpiration.
Transpiration involves plants losing water molecules to the environment through their stomatal pores. Evaporation is the driving force behind this.
Sublimation is the direct evaporation of ice to water vapour molecules and occurs at low pressures.
Condensation
Water vapour molecules will rise to cooler regions of the atmosphere (they are less dense than air) and form clouds. These clouds will move around the atmosphere with winds and air currents. Once the water vapour molecules become cold enough, they will not have sufficient energy to remain as gaseous molecules. They will be forced to develop intermolecular bonds with the molecules around them and form water droplets. Once these droplets are heavy enough to overcome the cloud's updraft, they will transform into precipitation.
Acid rain is a natural and human-caused phenomenon that damages ecosystems, pollutes waterways, and erodes buildings.
Nitrous oxide and sulphur dioxide emissions can cause acid rain by reacting with water in clouds and forming nitric acid and sulphuric acid.
Acid rain has negative consequences for the hydrosphere: acid precipitation damages soils and aquatic ecosystems, reducing water circulation between the living and non-living components of the Earth.
Interactions between the hydrosphere and the biosphere
Precipitation, infiltration, and runoff involve interactions between the Earth's hydrosphere and biosphere.
Precipitation involves the atmosphere, hydrosphere, and biosphere!
Precipitation and infiltration
Condensed water droplets will fall as rain and seep into grounds and soils. This process is called infiltration and is much more efficient in porous materials like mud and soils. Water that runs far into the ground will be stored in aquifers that eventually rise to the surface to form springs.
Aquifers are networks of permeable rocks that can store and transport groundwater.
Runoff
Runoff is the natural process by which water travels downwards to sea level. Gravitational forces are the driving mechanisms behind runoff. Water transportation by runoff is essential in most biogeochemical cycles in transporting nutrients from the lithosphere to the hydrosphere.
The gradient of slopes, winds, storm frequency, and ground permeability affect the rate water runs off.
Figure 1: The Water Cycle, via Wikimedia Commons
Human impacts on the hydrosphere
The stability of the hydrosphere is pivotal in providing a consistent freshwater source for the human population. However, human activity is having a significant effect on the hydrosphere. Here's how:
Agriculture
Global agriculture is constantly expanding. With an ever-growing global population and increasing demands for food with higher consumption rates, reliable agricultural output is essential. To provide this, farmers will employ intensive methods that require massive amounts of water for heavy machinery and complex temperature regulation.
Irrigation systems that supply crops with water will suck water out of nearby rivers and lakes.
Land use and exploitation
Development in highly populated areas can devastate aquatic environments. Dams are built to block water flow and build infrastructure, while massive drainage systems dump masses of water and overflow alternative locations. Industrial development in coastal areas can reduce ground permeability and increase runoff rates, and deforestation can remove populations of producers that would contribute to water absorption from the soil.
Figure 2: Dams block water flow and disrupt aquatic ecosystems. via Wikimedia Commons
Pollution
Industrial and urban runoff is a massive threat to water bodies. The discharge will contain many poisonous chemicals.
Such as microplastics, hydrocarbons, and radioactive substances
These will kill off wildlife and reduce the circulation between the biosphere and the hydrosphere. The addition of these molecules may affect water densities and evaporation rates.
Influxes of nitrogen and sulphur will cause acid rain once evaporated, which can pollute waters and soils worldwide.
Climate change
Human-induced climate change is another way we are negatively impacting the hydrosphere. The release of carbon dioxide and other greenhouse gases from:
fossil fuel combustion,
agriculture,
deforestation,
and mass production.
This is adding to the greenhouse effect and warming the Earth's system.
Higher temperatures result in more liquid water evaporation and more water vapour released into the atmosphere.
Water vapour is a greenhouse gas, too, so it amplifies this effect and causes more global warming and evaporation in a positive feedback mechanism.
The Hydrosphere - Key takeaways
The hydrosphere encompasses the entirety of water molecules in the Earth's system. These can be solid (ice, hail, snow), liquid (ocean water), or gas (water vapour).
The water cycle circulates water between the different spheres and maintains water distribution around the hydrosphere. Critical processes in the water cycle are evaporation, condensation, precipitation, infiltration, and runoff.
Human impacts like intensive agriculture, land changes and pollution disturb water distribution between the spheres.
Climate change is impacting the hydrosphere too. Increasing temperatures are causing more water vapour to be added to the atmosphere, and as water vapour is a greenhouse gas, this effect is exacerbated.
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