(i) Hydrology: It may be defined as the science which deals with the depletion and replenishment of water resources. Hydrology deals with the occurrence and distribution of water over and under earth’s surface. It is helpful for managing the water resources.
(ii) Surface runoff: It is that portion of precipitation (rain fall) which actually flow towards stream, lake, river or ocean. Run-off can be possible only when the rate of precipitation is more than rate of absorption of water in soil and also evaporation losses. Run-off = Total precipitation – total evaporation of water The collection of water in lake, river and ocean depends upon the surface run-off.
(iii) Evaporation: It is the transfer of water from liquid state to vapor state. It includes all the rainfall that is returned to atmosphere from the land and water surfaces.
(iv) Precipitation: Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. It is mainly of two types:
a) Liquid precipitation (Rainfall)
b) Solid precipitation (Snow, Hail)