THE WATER CYCLE AND WATER INSECURITY
Also known as the hydrological cycle
The Water/Hydrological Cycle: a closed system within which there are
inputs, outputs, stores and fows; is driven by solar energy and
gravitational potential energy
Effect of soar efe gy:a
- water is heated by the sun
-> it evaporates into the atmosphere to contribute to atmospheric water vapour
- also drives evapotranspiraton of water from the leaves of plants, which as a result draws
water up from the soil
Effect of g rvittriofra potefira efe gy:a
- when clouds become saturated with water vapour, water falls via gravity down to earth as
precipitaton
- water on the ground that is at higher elevatons ows towards land at lower elevatons
-> as it falls from high to low, grav. pot. energy is converted into kinetc, heat and sound energy
Water largely exists as water vapour in the atmosphere with
In the cryosphere water is the carrying capacity of the atmosphere directly linked to the
largely found in a solid state temperature – i.e the warmer it is, the more moisture the
[ice/snow] with some in liquid atmosphere can hold. However, water in the atmosphere also
form as meltwater and lakes exists in liquid and solid form (rain and snow/ice crystals)
On the land water is stored in rivers, streams, lakes and
In the oceans the majority of
groundwater in liquid form. It is also known as blue water –
water is stored in liquid form,
it is the visible part of the cycle. Water is also stored in
with a small fracton as icebergs
vegetaton and soil. It is known as green water – the invisible
part of the cycle.
, Some key: wo dsa
Stores – reservoirs where water is held, such as oceans, lakes, ice caps, etc
Fluxes – the rate of ow between the stores
Processes – the physical mechanisms that drive the uxes of water between the stores
Cryosphere – areas of the earth where water is frozen into snow or ice
Blue water – water is stored in rivers, lakes and groundwater in liquid form
Green water – water is stored in soil and vegetaton
Residence tie – the average tmes a water molecule will spend in a reservoir or store
Fossil water – ancient deep water from former pluvial [weter] periods
Evapotranspiraton – the evaporaton of water from vegetaton into the atmosphere
Groundwater fow – the slow transfer of percolated water underground through pervious or
porous rocks
Relative importance of stores:
The ocean is the largest store of the
Earth’s water – 97%
-> but this is not freshwater – so is
inaccessible for human use
Of freshwater, the largest store is
glaciers and ice caps
-> however, because so much of this is
at high alttudes and high lattudes, it is
largely inaccessible for human use.
The second largest is groundwater
-> some of which is stored as fossil
water and is very deep but accessible.
It is mostly used by countries who have
run out of groundwater to abstract. It is important to note that this store of water is non-
renewable. On the other hand, more surface groundwater is more accessible and is abstracted
for human use.
Of accessible su frce freshwater the largest store is ground ice
The second largest is lakes, and the third is soil ioisture
However, some accessible stores like lakes and soil moisture have the shortest estdefce ime
because water is so easily lost to the atmosphere by evaporaton, transpiraton and
groundwater ow
