Roughly 70% of the Earth’s surface is covered with water. Most of this water is contained in our oceans. Some of it exists in our rivers and lakes. Much of it is frozen in the ice sheets which exist in the northernmost and southernmost parts of our planet. Some of it is found underground. Water is also found in our atmosphere as water vapour.
The Water Cycle
Through a variety of processes, water is constantly moving around the planet. Rivers flow into the oceans or lakes. Water evaporates from bodies of water (such as seas and lakes) or the land and enters the atmosphere. Some of the water in the atmosphere falls back to the Earth as rain, snow or even hail. Some water seeps into the ground to join the stores of groundwater. Some of the water in the soil is taken up by the roots of plants. Plants release water vapour into the atmosphere through their leaves.
This continuous movement of water from point to point on or below the earth’s surface and between the Earth’s surface and the atmosphere is known as the hydrologic cycle or the water cycle. The sun provides the energy which makes the water cycle work. The diagram below shows many of the processes involved in the water cycle.
It is important to bear in mind the meaning of the following terms as they relate to the water cycle:
- Evaporation is the process by which a liquid is converted into vapour. This occurs on the surface of a liquid.
- Transpiration is the process by which plants lose water through their leaves. Water vapour escapes through tiny openings called stomata in the leaves.
- Evapotranspiration is the loss of water from the land by evaporation and transpiration.
- Condensation is the process by which vapour is converted into a liquid. In the diagram it refers to water vapour being converted into water droplets in the atmosphere.
- Precipitation refers to any of the forms in which water falls from the atmosphere and reaches the ground including rain, snow or hail.
- Infiltration is the process by which water enters the soil.
- Throughflow: This is the movement of water diagonally downslope through the soil. Some of this water may eventually reach rivers.
- Groundwater flow: This is the slow movement of groundwater toward lower elevations. Some of this water may eventually reach rivers.
- Percolation is the process by which water moves downward through the soil.
Take a look at this informative video about the water cycle