Water occurs on the earth in all its three states - liquid, solid, and in gaseous form. Evaporation of water from water bodies, formation and movement of clouds, rain, streamflow and groundwater movement can be explained in terms of the hydrologic cycle. The hydrological cycle of the earth is the sum total of all processes in which water moves from the land and ocean surface to the atmosphere and back in form of precipitation. It is dependent on various factors and is equally affected by oceans and land surfaces.

Therefore, the clouds will precipitate onto land in the form of rain, hail, sleet, etc. A part of precipitation may evaporate back into the atmosphere, or may be intercepted by vegetation and other surface modifications. When the water reaches the ground, some portion of it will enter the earth's surface through infiltration which enhance the moisture content of the soil. Meanwhile, vegetation sends a portion of the water from the surface back to the atmosphere through the process of transpiration. When portion of the precipitation present above or below the surface of the earth is on variety of paths towards the stream channel, it is called runoff. Once it enters the stream channel, runoff will then become streamflow.

reflection:

• Hydrologic cycle is a multi-phase water cycle that involves the exchange of energy and phase change of water molecules on the earth's surface. It is important and necessary to study the hydrological processes due to its effects on climate and biogeochemical cycle. Evidently we can observe different processes under the said cycle. Due to large scale manipulation of water by humans, it is entirely critical for civil engineers to understand these hydrologic processes because of alterations in streamflow, global temperature change, changes in sea level and salinity of oceans, which may bring considerable harm to the entire ecosystem. With this in mind, we have to ensure that there should be available water to sustain the needs of all living organisms including plants, animals, humans, and the like.