- The hydrological cycle of the earth is the total of all processes in which water moves from the land and ocean surface to the atmosphere and back in form of precipitation. The hydrological cycle is dependent on various factors and is equally affected by oceans and land surfaces.
- The hydrological cycle can be defined as the model of exchange of water over the surface of the earth from oceans via the atmosphere, land surface, and back to the oceans
It involves the following mechanism:
- Evaporation of water from oceanic water through insolation.
- Conversion of water into water vapour or humidity
- Horizontal transport of atmospheric moisture over the oceans and the continents by
- atmospheric circulation (advection)
- Release of atmospheric moisture in the form of precipitation
- Eventual transfer of water received at the earth’s surface to the oceans through
- different routes and processes such as surface runoff and rivers.
Mechanism of global hydrological cycle
- Heating of oceanic water by insolation, which transforms it into gaseous form-water vapour or moisture.
- It is then horizontally transported across the oceans and over the continents by atmospheric circulation
- The air is cooled because of its ascent and the moisture is released in the form of precipitation over the oceans and the continents.
Precipitation can be different ways:
- It can directly fall in the lakes, streams, other water bodies; this is called direct fall as is directly disposed off back into the oceans
- Some are intercepted by the vegetation, from this a portion is evaporated from the leaves and the remaining part is transported to the ground through the branches and stems of the plants as stem flow or aerial streams.
- A part of the rainfall reaches the ground directly as throughfall. Some portion of rainfall is lost to the atmosphere through evapotranspiration from the vegetation, while some are lost to the atmosphere through evaporation from the lakes, ponds and rivers.
While a good portion of rainfall reaching the ground surface becomes effective overland flow which reaches the streams as surface runoff. Some portion of rainwater received at the ground surface enters the soil through infiltration and thus forms soil moisture storage of which some portion is again lost to the atmosphere through evaporation and plant transpiration, some portion reappears as seepage and springs via through flow and interflow while some portion percolates further downward to form groundwater storage of which some portion reaches the channel through base flow, some portion moves upward as capillary rise to reach soil moisture storage and some portion is routed further down through deep transfer and enters the underlying bedrocks. The channel storage receives water from surface storage through surface runoff , from soil moisture storage through interflow and flow and from groundwater storage through base flow . Thus the input precipitation finds exit through two paths of output e.g.
(i) To the atmosphere through evaporation from rivers, lakes ponds, soil, evapotranspiration from vegetation and evaporation of falling rains
(ii) To the oceans through channel runoff or stream flow. This process is repeated every year to make the water or hydrological cycle at global scale effective.
It may be pointed out that though the different hydrological processes as elaborated above maintain the global hydrological cycle through the oceans, the atmosphere and the continents but out of the total moisture of the biosphere 95% is never available for the hydrological cycle because it is ( estimated quantity being 2,50,000 x 10(20) grams) locked in the rocks of the
The hydrological cycle is important to the transport and cycling of nutrients and energy. Though the interaction between vegetation and the hydrological cycle is very limited but the effect of hydrological cycle is enormous because the vegetation is an effective medium for the circulation of sediments and chemical elements through biogeochemical cycles in the biosphere and all these cycles become possible only through the movement of water.
Modification of hydrological cycle by man:
- Cloud seeding for induced precipitation, atmospheric pollution, modified atmospheric circulation, forest clearance.
- Additional input of water on ground surface is provided through irrigation of crops and effluent disposal from urban areas.
- Modification of interception storage by forest clearance and vegetation modification.
- Increase in surface runoff due to deforestation and cultivation and which is supplemented by additional input through channelled irrigation for cropland and effluent disposal from urban areas.
- Modification of infiltration through devegetation, urbanisation, afforestation and reforestation and irrigation.
- Soil moisture storage is positively affected by irrigation, planting of grasses and plants, and artificial recharge while it is negatively affected by land clearance through deforestation, burning of grasslands, urbanisation, etc.
- Extraction of groundwater for domestic use and irrigation purpose further modifies groundwater storage.
- What are the main steps in the hydrological cycle?
- The hydrological cycle consists of evaporation, condensation, precipitation, runoff, infiltration, groundwater flow, and transpiration.
- How does evaporation occur in the hydrological cycle?
- Evaporation is the process where water from the Earth’s surface (oceans, rivers, lakes, etc.) turns into water vapor due to heat from the sun.
- What is condensation in the hydrological cycle?
- Condensation is the transformation of water vapor into tiny water droplets or ice crystals, forming clouds in the atmosphere.
- What is precipitation in the hydrological cycle?
- Precipitation is the release of water from the atmosphere in the form of rain, snow, sleet, or hail.
- How does runoff work in the hydrological cycle?
- Runoff occurs when excess water on the Earth’s surface flows into rivers, lakes, and oceans, often leading to surface water drainage.
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