Water

Availability of water on earth

We know that 71% of the earth is covered with water. Despite of the huge availability of water on the earth, the useful water is very less in amount. 97 percent of the water on the earth is salt water. However, only three percent is fresh water; slightly over two thirds of this is frozen in glaciers and polar ice caps. The remaining unfrozen freshwater is found mainly as groundwater, with only a small fraction present above ground as surface water or in the air. From the available fresh surface water 87% is found in lakes, 11% in swamps and 2% in rivers. You can see the distribution of fresh water in diagram shown below:

Do you have ever think that how much water we use in agricultural, industrial, household, recreational and environmental activities? Another interesting point of discussion is that the majority of human uses require fresh water and whether such a small amount of fresh water is sufficient to fulfill the needs of all people on the earth or not?

States of water

Water is known to exist in three different states; as a solid, liquid or gas. Liquid water is found in many places. You see liquid water coming out of the faucet, when it rains, and running in a river. Pure liquid water is free of salt, rocks, soil, and garbage. Ice, snow, and frost are examples of water in the solid state. Other examples of solid water are ice cubes, icicles, ice on a skating rink. Water in the liquid state may change to water in the gaseous state. Water evaporates to turn into a gas. Gases are colorless and odorless. You don't see anything because gas is invisible but sometimes you can hear it and smell it.

Water cycle

The water cycle is the journey of water from the land to the sky and back again on the land. The Sun's heat provides energy to evaporate water from the Earth's surface (oceans, lakes, etc.). Plants also lose water to the air by the process of transpiration. The water vapor eventually condenses, forming tiny droplets in clouds. Water vapor condenses into millions of tiny droplets that form clouds. Clouds lose their water as rain or snow, which is called precipitation. Precipitation is either absorbed into the ground or runs off into rivers. Some of the precipitation soaks into the ground. Some of the underground water is trapped between rock or clay layers; this is called groundwater. Water that was absorbed into the ground is taken up by plants. Plants lose water from their surfaces as vapor back into the atmosphere. Water that runs off into rivers flows into ponds, lakes, or oceans eventually returning to the seas as slightly salty water, where it evaporates back into the atmosphere. In this way, the cycle continues.

Sources of water

The main sources of water on the earth can be categorised as follows:
Rain, oceans, rivers, lakes, streams, ponds and springs are natural sources of water. Dams, wells, tube wells, hand-pumps, canals, etc, are man-made sources of water.
Rain water: Rain is the most important source of water on the earth. It replenishes the sources of surface water like rivers, ponds, oceans etc. It is the purest form of water but gets polluted as it comes down to the earth. Rain water collects on the earth in the form of surface water and underground water.
Surface water: The water present on the surface of the earth in the form of oceans, rivers, lakes, ponds and streams is called surface water. Surface water is naturally replenished by precipitation and naturally lost through discharge to the oceans, evaporation, evapotranspiration and sub-surface seepage. The water in rivers and lakes comes from rain and melting of snow on mountains. Rivers flow into the sea. The water in rivers, lake and streams contains dissolved salts and suspended impurities. Water from such sources can be consumed directly or after simple treatments. On the other hand, the water in seas and oceans is salty and contain lots of dissolved salts due to which it is unfit for the consumption as drinking water.
Underground Water: Some of the rainwater seeps through the soil on to the non-porous rocks below. This is underground water. The process of seeping of rainwater into the ground is called infiltration. Sometimes due to high pressure, this water sprouts out in the form of springs. It can be obtained by digging wells, sinking tube wells, etc. Typically, groundwater is thought of as liquid water flowing through shallow aquifers, but technically it can also include soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal water.
Frozen water: It is the water present on earth in the form of icebergs and glaciers. Several schemes have been proposed to make use of icebergs as a water source, however to date this has only been done for novelty purposes. Glacier runoff is considered to be surface water.