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Home › Grade7 › Grade 7 CBSE › Science › Water-A Precious Resource › Water-A Precious Resource Notes › Sources of water

Sources of water

SOURCES OF WATER

  • Precipitation in the form of rain or snow provides fresh water to our planet Earth.
  • Most of the fresh water returns to the oceans through rivers flowing across the globe.
  • A small portion of it is absorbed by the soil and is stored underground.
  • A still smaller portion is stored in natural (lakes and ponds) and man-made (tanks and reservoirs) water bodies.
  • Various sources of water can be divided into two main categories:
  1. Surface water
  2. Underground water or subsoil water

 

Surface Water:

  • Water present on the surface of the Earth is called surface water.
  • It can further be classified into three categories depending upon the 'purity' of water.

   Rain water

   River and Lake water

   Sea and Ocean water  

 

Rain water:

  • Rainwater is the purest form of water.
  • As water from the seas and rivers evaporate to form water vapour due to the heat of the sun, it leaves behind all the impurities.
  • When precipitation occurs, the first showers dissolve certain gases present in air and also bring suspended impurities along with it. Subsequent showers, however, contain pure water.

River and Lake Water:

  • The water in these water bodies comes either from rainfall or melting of snow (glaciers) on the mountains.

Sea and Ocean water:

  • Oceans are a huge store of water.
  • But the water is salty and is not fit for domestic or agricultural use.

 

Underground Water or Subsoil Water:

  • Groundwater is the water under the ground where the soil is completely filled or saturated with water.
  • Rainwater seeps through topsoil and layers of rocks like limestone, sand and gravel, and gets collected on top of non-porous layers.

Aquifer

  • The top level of this underground water is called the water table.
  • Underground water is also called an aquifer.
  • The place where the water table meets the land surface, water may come out of the surface in the form of natural spring and flow into a lake, stream or an ocean.
  • Groundwater that meets the land surface also keeps rivers, streams, and lakes filled with water. Wells can also be drilled to take out the underground water.