Hydrology

Hydrology is the science that deals with the occurrence, circulation, and distribution of water of the earth and its atmosphere.

HYDROLOGICAL CYCLE

• The hydrological cycle is a global sun-driven process whereby water is transported from the oceans to the atmosphere, from the atmosphere to the land, and then back to the sea. 
• The processes constituting this cycle extend from an average depth of about 1 km in the lithosphere (the crust of the earth) to a height of about 15 km in the atmosphere. The hydrological cycle has no beginning or end. 
• A convenient starting point to describe the cycle is in the Oceans.
• Water in the oceans evaporates due to the heat energy provided by solar radiation. The water vapor moves upwards and forms clouds. While much of the clouds condense and fall back to the oceans as rain, part of the clouds is driven to the land areas by winds. There they condense and precipitate onto the landmass as rain, snow, hail, sleet, etc.
• A part of the precipitation may evaporate back to the atmosphere even while falling. Another part may be intercepted by vegetation, structures, and other such surface modifications from which it may be either evaporated back to the atmosphere or moved down to the ground surface. 
• A portion of the water that reaches the ground, enters the earth's surface through infiltration, enhances the moisture content of the soil, and reaches the groundwater body. 
• Vegetation sends a portion of the water from under the ground surface back to the atmosphere through the process of transpiration. Groundwater may ultimately be discharged into streams or may emerge as springs. The hydrological cycle is usually described in terms of six major components: Precipitation (P), Infiltration (I), Evaporation (E), Transpiration (T), Surface Runoff (R), and Groundwater flow (G).
• Some infiltrated water may merge to surface-water bodies as interflow, while other portions may become groundwater flow.
• After an initial filling of depression storages and interception, overland flow (surface runoff) begins provided that the rate of precipitation exceeds that of infiltration.
• For computational purposes, evaporation and transpiration are sometimes lumped together as evapotranspiration (ET).
• The magnitude and duration of a precipitation event determine the relative importance of each component of the hydrological cycle during that event. 
• During storm events, evaporation and transpiration may be of minor considerations, but during rain-free periods, Evapotranspiration becomes a dominant feature of the hydrological cycle.

Note :

Evaporation is the transfer of water from a liquid state to a gaseous state, i.e., it is the conversion of liquid to the vapour phase. Precipitation is the deposition of water on the earth's surface in the form of rain, snow, hail, frost, and Interception is the short-term retention of rainfall by the foliage of vegetation. 

Infiltration is the movement of water into the soil of the earth's surface. 

Percolation is the movement of water from one soil zone to a lower soil zone.

Runoff is the volume of water drained by a river at the outlet of a catchment and so on.

Transpiration is the soil moisture taken up through the roots of a plant and discharged into the atmosphere through the foliage by evaporation. Transpiration occurs during photosynthesis Storage is the volume of water that gets stored in natural depressions of a basin