1.3 The hydrological cycle or water cycle

The hydrological cycle, which is also called the water cycle, is the process where water is recycled again and again between its stores. It starts with precipitation, which is the delivery of water from the atmosphere to the Earth's surface. Precipitation happens as a result of condensation, where water molecules in the clouds condense into larger and larger water droplets. When they have enough mass, they start to fall out of the sky as rain, sleet, hail or snow arriving on the Earth’s surface.

Of this water, some will infiltrate into soils where it can be taken up by plant roots. Plants send this water up through their stems, and deliver it from their leaves back into the atmosphere. This process is called transpiration.

Some water will evaporate back into the atmosphere because the water is heated up so the water molecules change from liquid to gas. This process is called evaporation and happens on land, rivers, lakes and oceans. Together these processes of transpiration and evaporation are called evapotranspiration. However, some water that falls on land percolates through the soil, and collects in aquifers as groundwater.

Other water will actually flow over the land surface into rivers, streams, lakes and oceans.

Described image
(Source: Open University (2019) ‘Water and Human Health’ based on Houghton, J. (2004) Global Warming (3rd edn) p. 155, Cambridge University Press)
Figure 4 The cyclical process of precipitation, evaporation and transpiration.

Question 6

Using the following two lists, match each numbered item with the correct letter.

  1. Transpiration

  2. Evaporation

  3. Evapotranspiration

  • a.What is the process called where water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants?

  • b.What is the process called where precipitation returns back to the atmosphere because the water sits on soils and is heated up so that the water molecules change from liquid to gas?

  • c.What is the process called by which plants take up water that infiltrates soils and return it to the atmosphere?

The correct answers are:
  • 1 = c
  • 2 = b
  • 3 = a

Internal renewable water resources comprise the average annual flow of rivers and groundwater generated from precipitation that falls within a country. Surface water and groundwater can be thought of as having inputs, outputs and storage. The natural input for surface water is precipitation. The outputs are the release of the water back to the ocean, either through river discharge, or back to the atmosphere through evaporation.

Surface water is stored by being intercepted by plants, and accumulating in lakes and rivers, as well as through infiltration and percolation to be stored as groundwater. The natural input to groundwater is this seepage from surface water. The natural outputs from groundwater are springs and seepage to the oceans.

Groundwater storage is generally much larger (in volume) than surface water because of its slow rate of turnover. This difference makes it easy for humans to use groundwater unsustainably for a long time without severe consequences. Moreover, in coastal areas, human use of groundwater may cause the direction of seepage to the ocean to reverse, which can cause soil salinisation.

Humans can also cause groundwater to be "lost" (i.e. become unusable) through pollution. Humans can increase the input to a groundwater source by building reservoirs or detention ponds.

Question 7

The water table in a particular region is always at the same level

a. 

True


b. 

False


The correct answer is b.

Feedback

The water table within any region fluctuates up and down, depending on the season and depending on what sort of pressures that water table is under.

1.2 What are water resources?

1.4 Water Resources in Myanmar