systems approach
In order to understand the operation of the hydrological cycle (also known as the natural water cycle) a systems approach is useful.
Systems approaches study hydrological phenomena by looking at the balance of inputs and outputs, and how water is moved between stores by flows.
Three concepts are key to understanding how water cycling operates:
1 Stores (stocks), which are reservoirs where water is held, such as the oceans.
2 Fluxes, which measure the rate of flow between the stores.
3 Processes, which are the physical mechanisms which drive the fluxes of water between the stores.
The global hydrological cycle
- a closed system
- driven by solar energy and gravitational potential energy.
- In a closed system there is a fixed amount of water in the Earth-atmosphere system (estimated at 1385 million km3).
- A closed system does not have any external inputs or outputs, so this total volume of water is constant and finite.
- However, the water can exist in different states within the closed system (liquid, vapour and solid) and the proportions held in each state can vary for both physical and human reasons.
- For example, in the last Ice Age more water was held within the cryosphere in a solid form as snow and ice; as less was held in the oceans, sea levels dropped considerably - over 140 m lower than they are today.
- Recent climate warming is beginning to reverse this with major losses of ice in Greenland and, more recently, Antarctica, and significant rises in sea level
- At a small scale, humans have built numerous water storage reservoirs to complement natural lakes in order to increase the security of their water supplies.
- there are four major stores of water, of which the oceans are by far the largest: they contain an estimated 96.5 to 97 per cent of the world’s total water.
The next largest stores occur in the cryosphere (1.9 per cent), and then terrestrial surface groundwater.
The atmosphere is by far the smallest of the significant stores.
The global water cycle
Details of the main global water stores; note that numbers are rounded so the totals may not
add up to 100
Blue water:
Water is stored in rivers, streams, lakes and groundwater in liquid form (the visible part of the hydrological cycle).
Green water:
Water stored in the soil and vegetation (the invisible part of the hydrological cycle).
Water cycle
- the major fluxes are driven by key processes such as precipitation, evaporation, cryospheric exchange, and a run-off generation (both surface and groundwater).
- These fluxes have been quantified, with the most important being evaporation from the oceans and precipitation on to land and the oceans.
- residence times are the average times (it is an estimate, hence you will find considerable variation depending on source used) a water molecule will spend in that reservoir or store.
residence times for different stores
- Residence times impact on turnover within the water cycle system.
- Groundwater, if it is deep seated, can spend over 10,000 years beneath the Earth’s surface.
- Some ancient groundwater, such as that found deep below the Sahara Desert - the result of former pluvial (wetter) periods - is termed fossil water and is not renewable or reachable for human use.
- Major ice sheets too (such as Antarctica and Greenland) store water as ice for very long periods, so the figures in the table represent an average.
- Ice core dating has suggested that the residence time of some water in Antarctic ice is over 800,000 years.
- Conversely, some very accessible stores, such as soil moisture, and small lakes and rivers, have much shorter residence times.
- Water stored in the soil, for example, remains there very briefly as it is spread very thinly across the Earth.
- Because of its accessibility it is easily lost to other stores by evaporation, transpiration, groundwater flow or recharge.
- Atmospheric water has the shortest residence time of all, about ten days, as it soon evaporates, condenses and falls to the Earth as precipitation.
- There is a strong link between residence times and levels of water pollution: stores with a slower turnover tend to be more easily polluted as the water is in situ for a longer length of time.
Precipitation:
The movement of water in any form from the atmosphere to the ground.
Evaporation:
The change in state of water from a liquid to a gas.
Residence time:
The average times a water molecule will spend in a reservoir or store.
Fossil water:
Ancient, deep groundwater from former pluvial (wetter) periods.
Transpiration:
The diffusion of water from vegetation into the atmosphere, involving a change from a gas to a liquid.
Groundwater flow:
The slow transfer of percolated water underground through pervious or porous rocks.
Accessible water for human life support
- 96 to 97 per cent of global water is stored in the oceans - only around 2.5 per cent occurs as fresh water..
- Around 69 per cent is locked up in snowflakes, ice sheets, ice caps and glaciers found in high latitudes and high-altitude locations.
- This water supply is largely inaccessible for human use, although some streams in mountain areas are ‘fed’ from ice and snow as melt water.
- Another 30 per cent occurs as groundwater, some of which is very deep seated as fossil water and, therefore, also inaccessible.
- This leaves only around one per cent of fresh water which is easily accessible for human use.
- Figure 1.2c includes all sources of surface water, including ground ice and permafrost, which are very difficult to access.
- Figure 1.2d shows only fresh water that is accessible to humans with current levels of technology - note the importance of lakes and soil moisture.
- Rivers, which are currently the main source of surface water for humans, constitute only 0.007 per cent of total water.
- It is not surprising that there are so many concerns and disputes about the usage of this tiny, precious fraction.
- As with any global overview, the differences between places are masked and, in terms of availability of water, it is a very unequal world.
- It is also notable that technology is being used widely to extend the availability of fresh water supplies, for example by desalination of ocean water.
