The Hydrological Cycle | Engineering Hydrology - Civil Engineering (CE) PDF Download

Hydrological Cycle

(also known as the water cycle) is the journey water takes as it circulates from the land to the sky and back again.

The sun’s heat provides energy to evaporate water from the earth’s surface (oceans, lakes, etc.). Plants also lose water to the air – this is called transpiration. The water vapour eventually condenses, forming tiny droplets in clouds.

• When clouds meet cool air over land, it causes precipitation (rain, sleet, or snow) to fall, returning water to the land or sea.
• Some of this precipitation seeps into the ground.
• Underground, water gets trapped between rock or clay layers, known as groundwater.
• Most water flows downhill as runoff, either above ground or underground, and eventually makes its way back to the seas as slightly salty water.

This Information page provides an understanding of the hydrological cycle.  It describes the principal stages of the cycle, with a brief description of each stage.  A diagram gives a clear visual explanation.  The links between the hydrological cycle and the duties of a water utility to supply clean water and dispose of dirty water are also explained.

What is the Hydrological Cycle?

• The total amount of water on Earth and in its atmosphere remains constant, but the water is always moving.
• Oceans, rivers, clouds, and rain are always changing and moving. They all contain water.
• Rain and flowing rivers constantly move water in an endless cycle.
• This continuous movement and preservation of Earth's water, as it travels from land to sky and back, is known as the hydrological cycle or water cycle.

How does the Hydrological Cycle work?

The stages of the cycle are:

• Evaporation
• Transport
• Condensation
• Precipitation
• Groundwater
• Run-off

Evaporation: Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas. The sun’s heat provides energy to evaporate water from the earth’s surface. Land, lakes, rivers and oceans send up a steady stream of water vapour and plants also lose water to the air (transpiration).
Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.

Transport:  The movement of water through the atmosphere, specifically from over the oceans to over land, is called transport. Some of the earth’s moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets.

• Clouds move around using different forces like the jet stream, breezes, or other ways.
• A typical cloud that's 1 km thick has only a small amount of water for rain, but the air holds much more moisture.
• Most of the water moves as water vapour, which is a common gas in the air. Satellites can see this vapour even if we can't.

Condensation: The transported water vapour eventually condenses, forming tiny droplets in clouds.

Precipitation:  The primary mechanism for transporting water from the atmosphere to the surface of the earth is precipitation.
When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is triggered and water returns to the land (or sea). A proportion of atmospheric precipitation evaporates.

Groundwater: Some of the precipitation soaks into the ground and this is the main source of the formation of the waters found on land – rivers, lakes, groundwater and glaciers.

• Some of the underground water gets trapped between layers of rock or clay, known as groundwater.
• Water that seeps into the ground moves downwards until it reaches impermeable rock, then flows horizontally.
• The areas where water moves horizontally are called aquifers.
• Groundwater rises back to the surface through these aquifers, eventually draining into lakes, rivers, and oceans.
• In certain cases, groundwater can flow upwards in artesian wells.
• The movement of groundwater is much slower compared to surface run-off.

Run-off

• Most of the water that comes back to land flows downhill as run-off.
• Some of it goes into the ground and fills groundwater while the rest, as river flow, goes back to the oceans where it disappears in the air.
• When the amount of groundwater goes up or down, the water table goes up or down too.
• If the ground below is completely soaked, flooding happens because all extra rainwater stays on top.
• Different surfaces can hold different amounts of water and soak up water at different speeds.
• When a surface becomes less able to soak up water, more water stays on top, which can lead to more flooding.
• Flooding is quite common in winter and early spring because frozen ground doesn't soak up water, turning most rain and meltwater into run-off.