Hydrology is the science which deals with the occurrence, distribution and movement of water on earth, including that in the atmosphere and below the surface of earth.

The earth’s water circulatory system is known as the hydrologic cycle. Hydrologic cycle is the process of transfer of moisture from the atmosphere to the earth in the form of precipitation, conveyance of the precipitated water by streams and rivers to ocean and lakes etc., and evaporation of water back to the atmosphere.

The hydrologic cycle consist of following processes

1. Evaporation and Transpiration:-

   The water from the surface of ocean, river, and lakes and also from the moist soil evaporates. The vapours are carried over the land by air in the form of clouds. Transpiration is the process of water being lost from the leaves of plants from their pores.

2. Precipitation:-

   Precipitation may be defined as the fall of moisture from the atmosphere to the earth surface in any form. Precipitation may be in two forms a) Liquid precipitation b) Frozen precipitation.

3. Run Off:-

   Runoff is that portion of precipitation that is not evaporated. When moisture falls to the earth’s surface as precipitation, a part of it is evaporated from the water surface, soil and vegetation and through transpiration by plants, and the remainder precipitation is available as run off which is ultimately runs to the oceans through surface or subsurface streams.