Availability of water
Water is the most widely occurring substance on this planet. Globally distributed by the hydrological cycle, driven by the energy cycle, the circulation of water powers most of the other natural cycles and conditions the weather and climate. Water has shaped the earth’s evolution and continues to fashion its programmes. This is the water precipitated from the atmosphere on to land, where it may be stored in liquid or solid form, and can move laterally and vertically and between one phase and another phase by evaporation , condensation , freezing and thawing on the land surface. This water can travel at widely differing velocities usually by predictable pathway’s which can slowly change with time. We are aware that about 71% of  the earth’s surface is covered with water. Almost all the water on the earth is contained in the seas and oceans, rivers , lakes, ice caps, as groundwater and in the atmosphere. However, most of this water is not fit for human consumption.