The water cycle
Stores
97% of all earth's water is stored in oceans.
The remaining 3% is freshwater.
The majority of freshwater is stored in ice caps and glaciers (68.75 of all freshwater).
30.1% is stored as groundwater.
75% of freshwater on Earth is stored in a frozen state as part of the cryosphere.
Groundwater (aquifers) accounts for 0.7 of all water and 20% of all freshwater.
Just over 0.01% of all water on Earth is stored in lakes and rivers. The remaining water is stored in the atmosphere, in solid, fauna, flora and in humans.
Flows
Over 500,000km3 of water is in transfer between water stores due to a number of key processes.
Precipitation
Water and ice that falls from clouds.
Forms when vapour from the atmosphere cools to its dew point and condenses into tiny water droplets or ice particles to form clouds. Eventually these ice particles aggregate, reach a critical size and leave the cloud as precipitation.
Precipitation varies in character and this impacts the water cycle at the drainage basin scale.
Most rain on reaching the ground flows quickly into streams and rivers. But in high latitudes and mountainous catchments, precipitation often falls as snow and main remain on the ground for several months. Thus there may be a considerable time lag between snowfall and run-off.
Intensity is the amount of precipitation falling in a given time. High -intensity precipitation (e.g. 10-15 mm/hour) moves rapidly overland into streams and rivers.
Duration is the length of time that a precipitation event lasts. Prolonged events, linked to depressions and frontal systems, may deposit exceptional amounts of precipitation and cause river flooding.
Catchment hydrology
In some parts of the world (e.g. East Africa, Mediterranean) precipitation is concentrated in a rainy season. During this season river discharge is high and flooding is common. In the dry season rivers may cease to flow all together.
Condensation
Condensation is the phase change of vapour to liquid water. It occurs when air is cooled to its dew point where air becomes saturated with vapor resulting in condensation. Clouds form through condensation in the atmosphere.
3 clouds: cumuliform clouds, stratiform clouds and cirrus clouds.
Transpiration
The diffusion of water vapor to the atmosphere from the leaf pores (stomata) of plants.
It is responsible for around 10% of moisture in the atmosphere.
Like evaporation, transpiration is influenced by temperature and wind speed.
It is also influenced by water availability to plants. For example, deciduous trees shed their leaves in climates with either dry or cold seasons to reduce moisture loss through transpiration.
Evaporation
Is the phase change of liquid water to vapor and is the main pathway by which water enters the atmosphere.
Heat is needed to bring about evaporation and break the molecular bonds of water. But this energy input does not produce a rise of temperature in the water. Instead the energy is absorbed as latent heat and released later in condensation.
This process allows huge quantities of heat to be transferred around the planet: from oceans to continents: and from the tropics to the poles.
Infiltration, through flow, groundwater flow and run-off
Rain falling to the ground and not entering storage follows one of two flow paths to streams and rivers :
Infiltration by gravity into the soil and lateral movements or through flow to stream and river channels.
Overland flow across a ground surface either as a sheet or as trickles and rivulets to stream and river channels.
Two conflicting ideas explain the flow paths followed by rainwater. One relates to overland flow to the soil's infiltration capacity or the maximum rate it can absorb rain. Thus, it is argued that when rainfall intensity exceeds infiltration capacity overland flow occurs.
Cryispheric processes
Ablation is loss of ice from snow, ice sheets and glaciers due to a combination of melting, evaporation and sublimation.
Meltwater is an important component of river flow in high latitudes and mountain catchments in spring and summer.
Rapid thawing of snow in upland Britain in winter is a common cause of flooding in adjacent lowlands.