1. Hydrology is a science which deals with the occurrence, distribution and movement of water on the earth, including that in the atmosphere and below the surface of the earth.
2. Water occurs in the form of vapour, on the surface as water, snow or ice and below the surface as ground water occupying all the voids within the geological stratum.
3. Except for the deep ground water, the total water supply of the earth is in constant circulation from earth to atmosphere, and back to the earth. The earth’s water circulatory system is known as the hydrological cycle.
4. Hydrological cycle is a process of transfer of moisture from the atmosphere to the earth in the form of precipitation, conveyance of the precipitated water by streams and river to ocean and lakes etc., and evaporation of water back to the atmosphere.

$$\text{Figure 15. Hydrological Cycle}$$ 5. The hydrological cycle consists of the following:

• Evaporation and Transpiration: The water from the ocean, lakes, rivers 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 the plants from their pores. Thus, the total evaporation (E), inclusive of the transpiration consists of:

  • Surface evaporation,
  • Water surface evaporation from river surface and from the oceans
  • Evaporation from plants and leaves
  • Atmospheric evaporation

• Precipitation: Precipitation may be defined as the fall of moisture from the atmosphere to the earth surface in any form.

• Runoff: 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 water surface, soil and vegetation and through transpiration by plants, and the remainder precipitation is available as runoff which ultimately runs to the ocean.