The Hydrosphere

The Hydrosphere - SQA Key definitions: 

• Aquifer - An underground layer of water-bearing material that can store significant quantities of groundwater.

• Condensation - The process of a vapour or gas turning into a liquid.

• Coriolis effect - The apparent deflection (to an observer) of a path due to the changing speed of rotation between the poles and the equator, caused by the Earth rotating independently of the atmosphere above.

• Evaporation - The process of turning from liquid into vapour or gas.

• Evapotranspiration - The process of water transfer from the Earth's land surface and vegetation back into the atmosphere.

• Global ocean conveyor belt - A constantly moving system of deep ocean circulation driven by thermohaline circulation and surface wind currents.

• Groundwater - Water that occupies pore spaces in soil and bedrock.

• Gulf Stream - A powerful, warm, and swift ocean current in the North Atlantic, flowing from the Caribbean towards Europe. 

• Infiltration - The physical movement of water through soil (relative to the soil’s porosity and permeability).

• Ocean circulation - A series of vast movements of water through the oceans of the Earth, driven by thermohaline effects and surface wind currents.

• Ocean gyre - A major circular surface ocean current, driven by surface winds and the Coriolis effect. 

• Percolation - The process of a fluid moving (usually downwards) through a porous material.

• Precipitation - Moisture that falls from the air to the ground (rain, snow, sleet, hail, drizzle, fog, mist).

• Runoff - The flow of water that occurs when rainwater, meltwater, or other sources flow over the Earth's surface, instead of percolating into the ground.

• Sublimation - The process of changing state from a Solid to a Gas, without first becoming a Liquid.

• Thermohaline circulation - The circulation of water in the ocean, caused by density changes linked to temperature and salinity. 

• Transpiration - The evaporation of water from plants’ leaves, stems or flowers. 

• Upwelling - The process of deep, cold, nutrient-rich water rising back up to the surface. 

The Hydrosphere

The hydrosphere consists of all of the water on, in or around the Earth. This includes water in the three states of matter; Solid (snow, ice, hail etc.), Liquid (rivers, lakes, oceans etc.) and Gas (water vapour in the atmosphere). 

There are several processes that form part of the hydrosphere, with the processes collectively making up the 'Hydrological Cycle'.  

The Hydrological Cycle

The 'Hydrological Cycle' describes how water changes state and moves through the environment. Water is not static but changes over time through a variety of processes. 

The diagram below shows an overview of the processes that make up the hydrological cycle :

A description of any cycle can start at any point, but in this case the starting point will be taken as the water present in an ocean. Sunlight falling on the ocean provides enough energy for some of the water to evaporate (to change state from a liquid to a gas). This  invisible 'water vapour' will cool as it gets higher, condensing to form small droplets of liquid water suspended in the air, forming clouds. The water within the clouds can then travel huge distances before falling as 'precipitation' (rain, hail, snow etc.) 

This precipitation can then move through the environment in a range of ways. Some will be stored for long periods of  geological time as snow or ice on mountains, within glaciers or within the polar ice caps. Other water will fall onto porous rock and move downwards into the ground (this is known as percolation), forming groundwater. In areas where the rock is very porous, large quantities of groundwater can be stored, forming an Aquifer. 

Alternatively, some water will fall on non-porous rock and flow along the surface (known as surface run off). When this surface runoff collects together whilst moving it forms streams and rivers, or if it becomes stationary it forms lakes. 

The water then can follow two paths; the surface runoff and groundwater can pass through rivers and lakes (or directly through the ground in a process known as 'throughflow') back to the ocean to begin the cycle again, or the water can be absorbed by vegetation (plants and trees). The water that is absorbed by vegetation then escapes from the leaves and stems of the plan (in a process known as 'transpiration'), again entering the air as water vapour.

In the hydrological cycle, there are many examples of water being stored for long periods of time. The diagram below shows how the Earth's water is distributed between the various 'water stores':-

As can be seen from this diagram, less than 1% of the water on Earth is easily accessible to living organisms, including Humans and, as such, liquid freshwater is a resource that must be managed very carefully and sustainably.

Ocean Currents : The Global Ocean Conveyor Belt

The water within Earth's oceans is not static; it circulates continuously through a series of vast movements, known as the 'Global Ocean Conveyor Belt'. The global ocean conveyor belt is driven by thermohaline circulation (caused by density changes linked to temperature and salinity) and surface wind currents. 

Link to live ocean currents

As ocean water in polar regions cools, it forms sea ice, drawing out the freshwater and causing the surrounding water to get saltier. This increases its density, and the cold water starts to sink. Surface water is pulled in to replace the sinking water, which then also becomes cold and salty enough to sink. This initiates the deep ocean currents driving the global ocean conveyor belt. 

In the North Atlantic, the 'Gulf Stream' (a powerful, warm, and swift ocean current in the North Atlantic, flowing from the Caribbean towards Europe) transports warm water away from the equator : 

As more warm water is transported north, the cooler water sinks and moves south of the equator down towards Antarctica. Eventually, the cold bottom waters return to the surface through mixing and upwelling (deep, cold, nutrient-rich water rising back up to the surface), continuing the ocean conveyor belt that encircles the globe. 

The water that returns through upwelling carries nutrients from the deep ocean to the surface waters, boosting the growth of primary producers, which supports the marine food web globally.

Ocean Currents : The Coriolis Effect

The ocean currents rotate in opposite directions in each hemisphere of the Earth. This is due to the 'Coriolis Effect' : 

The diagram above would lead to the conclusion that both ocean currents and storms would rotate clockwise in the Northern Hemisphere.

It is a bit more complicated than this, however :

Ocean currents do rotate clockwise in the Northern Hemisphere, but storms rotate anticlockwise, due to the following process : 

The outcome of the 'Coriolis effect' is that storms in the atmosphere will rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere : 

Ocean Currents : Gyres

An 'Ocean Gyre' is a major circular surface ocean current, driven by surface winds and the Coriolis effect. 

Global winds drag on the water’s surface, causing it to move and build up in the direction that the wind is blowing. The wind direction is influenced by the Coriolis effect, resulting in the deflection of major surface ocean currents to the right in a clockwise spiral in the northern hemisphere and to the left in an anti-clockwise spiral in the southern hemisphere. 

These major spirals of ocean-circling currents occur north and south of the equator, but not at the equator, as the Coriolis effect is absent there. 

The shape of a gyre is dependent on ocean currents, winds and continental location, as can be seen from the diagram below :

The circulating nature of ocean gyres traps marine debris and can distribute this over huge surface areas and throughout the top of the water column. This also leads to the accumulation (build-up) of marine debris in higher concentrations within the gyres than would have otherwise been seen. 

An example of this can be seen in the Pacific Ocean, in a region known as the 'Great Pacific Garbage Patch' :