Slide 4: The River Cuckmere (Sussex) enters the sea at Cuckmere Haven
The Water Cycle
Evaporation and transpiration:
Evaporation is when the sun heats up water in seas and oceans and turns it into water vapour or steam (water H2O turns into a gas). The water vapour or steam leaves the river, lake or ocean and goes into the air.
Transpiration is the process by which plants and trees lose water out of their leaves into the air.
Convection and condensation:
The water vapour is heated by the sun. In a process called convection it rises, because it is warmer than the air above it.
Water vapour in the air cools and changes back into liquid, forming clouds: this is called condensation.
Precipitation occurs when so much water has condensed that the air cannot hold it anymore. The clouds get heavy and water falls back to the earth in the form of rain, hail, sleet or snow. When the clouds meet cool air over land this acts as a trigger for the rain to fall.
Percolation and absorption: The rain falls on the sea, land and lakes.
On land the rain sinks into the soil and runs off hard rocks, and percolates (trickles or filters*) through the softer porous rocks like the chalk on the Downs. (* like a coffee percolator, water runs through the porous material but the coffee, or soil, doesn’t).
On land the water is used by the plants, crops and trees to grow. (Trees use lots of water.)
But the soil can only absorb so much water before it floods, forms bogs and marshes or is channelled into streams and rivers that flow out to sea.
Torrents, groundwater and aquifers: On hard rocks the water bounces straight off in torrents, which are fast streams that flow into rivers.
On chalk (like the downs) the water flows through the soft rock and is stored underground. Water stored underground is called groundwater and is stored in aquifers. The water is trapped in the soil and soft porous rocks, sometimes open to the air in ponds. Groundwater takes longer to flow out to sea by the way of rivers and streams.
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