Hydrological Cycle

Hydrological Cycle Siva.D 2008EBT07 Bharathidasan university, Trichy – 620 024

The Hydrologic Cycle (Water Cycle)

 The path water takes as it circulates from

the land to the sky and back again. Water is recycled this way so we do not run out. So every time you get a drink of water remember you are drinking the same water the dinosaurs drank.

Parts of the hydrologic cycle  Evaporation  Condensation  Precipitation

Evaporation 

Evaporation is when the sun heats up water in rivers or lakes or the ocean and turns it into vapor or steam. The water vapor or steam leaves the river, lake or ocean and goes into the air.

Evapotranspiration 

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Evaporation often implicity includes transpiration from plants, though togather they are specifically refered to as “Evapotranspiration”. Here water given off through the pores of plants and animals joins the atmosphere as a vapor. Annual Evapotranspiration amounts approximately = 505,000 km³ of water From the ocean = 434,000 km³.

Condensation  Water vapor in

the air gets cold and changes back into liquid, forming clouds. This is called condensation.

Precipitation  Precipitation occurs when

so much water has accumulated that the air cannot hold it anymore. The clouds get heavy and water falls back to the earth in the form of rain, hail, sleet or snow.

Run-off 

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When the precipitation reaches the ground, several things can happen to it. First, it might be re-evaporated. For instance, we have all seen the mist rising off hot roads after a summer shower. If it isn’t re-evaporated, much of the water will become RUNOFF that goes into streams, lakes and rivers as it flows back to the ocean.

Infiltration 

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Some of the precipitation will be absorbed into the ground. This is called INFILTRATION. Once in the ground, the water can join the earth’s ground water supply. This is one of the world’s largest store house of water. The water could also be absorbed from the ground by the roots of plants.

Reservoirs  The largest reservoir is the collection of

oceans, accounting for 97% of the Earth’s water. The next largest quantity(2%) is stored in solid form in ice caps and glaciers.  The small amount accounts for approximately(75%) of all fresh water reserves on the planet. The water contained within all living organisms represents the smallest reservoir

Volume of water stored in the water cycle reservoirs Reservoir

Volume of water (106km³

Oceans Icecaps,glaciers Ground water Lakes Soil moisture Atmosphere Streams&rivers Biosphere

1370 29 9.5 0.125 0.625 0.013 0.0017 0.0006

Percent of total % 97.25% 2.05% 0.68% 0.01% 0.005% 0.001% 0.0001% 0.00004%

Residence times  The residence time of a reservoir within the

Hydrological cycle is the average time a water molecule will spend in that reservoir.  It is a measure of the average age of the

water in that reservoir, though some water will spend much less time than average, and some much more.

Average reservoir residence times Reservoir

Avg residence time

Oceans Glaciers Seasonal snow cover Soil moisture Ground water-shallow Ground water-deep Lakes Rivers Atmosphere

3,200 years 20–100 years 2-6 months 1-2 months 100-200 years 10,000 years 50-100 years 2-6 months 9 days

Effects on climate  The water cycle is powered from

solar

energy.  86% of the global evaporation occurs from the oceans, reducing their temperature by evaporative cooling.  Without the cooling effect of evaporation the green house effect would lead to a much higher surface temperature of 67°c, and a warmer planet

Effects on Biogeochemical cycling  While the water cycle itself a biogeochemical

cycle, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemical Runoff is responsible for almost all of the transport of eroded sediment and phosphorus from land to water bodies.  The salinity of oceans is derived from erosion and transport of dissolved salts from the land.

Cont…  Cultural Eutrophication of is primarily due to phosphorus, applied in excess in agricultural fields in fertilizers, and then transported over land and down rivers.  Both runoff and ground water flow play

significant roles in transporting nitrogen from the land to water bodies

 Runoff also play a part in the carbon cycle,

again through the transport of eroded rock and soil.

Don’t alter the water cycle  Agriculture  Industry waste  Alteration of the chemical composition of the

atmosphere  Construction of dams  Deforestation and Afforestation  Removal of ground water from wells

Cont…  Water abstraction from rivers  Urbanization  Contributing to climate change  Withdrawing large amount of fresh water  Clearing Vegetation and underground water  Polluting surface and underground water  Precipitation changes worldwide  Glaciers melt-loss of water source  Over population  Wild fires accelerate several cycles

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