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GLOBAL WATER BALANCE,

ICE SHEETS

ENVIRONMENTAL SCIENCE:

CONCEPT & APPROACHES

SUBMITTED BY MINIMOL TOMY II MSC ZOOLOGY ROLL NO: 213

PRESENTATION

GLOBAL WATER BALANCE

DEFENITION:

 Water balance. The cyclical movement of water between the atmosphere and the ground surface, considering precipitation, evaporation, and runoff (Whittow, 1984 ).  Water balance is the most important integral physiographic characteristic of any territory—it determines its specific climate features, typical landscapes and opportunities for human land use. Assessment of mean long-term water balances of large regions at a sufficient accuracy depends on reliable estimation of the major water balance components— precipitation, evaporation and runoff (surface and subsurface).

 (^) Water balance is the ratio between water inflow and outflow estimated for different space and time scales, i.e. for the Earth as a whole, for oceans, continents, countries, natural-economic regions, and river basins, for a long-term period or for particular years and seasons. Water balance is the most important integral physiographic characteristic of any territory, determining its specific climate features, typical landscapes, possible water management and land use.  (^) The water balance of each continent (except Antarctica) is given separately for the areas of external runoff and internal runoff (endorheic areas) where precipitation is completely lost to evaporation. All balance components are estimated by independent methods which provide a computation of a balance discrepancy and thus assessment of reliability of the obtained results.

Water balance equation for any land area and any

time interval (without taking account of the above

minor components) is as follows:

P + R’s + R’un = E + Rs + Run ± ΔS

where: P is precipitation; E is evapotranspiration;

Rs and Run, R’s and R’un indicate surface and

subsurface runoff from some land area and

surface and subsurface water inflow to the land

area, respectively; ΔS is water storage change in

the area.

 The hydrological cycle

The hydrological cycle describes the movement

of water within a ecosystem, for example a

watershed (see figure).

 If the subsoil layer is saturated by water or if pipes exists, the infiltrated water runs as interflow downward inside the soil; some amount of water percolates deeper into the bedrock and enters the groundwater reservoir. The top of this zone is termed the water table. Water which is stored as groundwater in pores and fissures runs with long delay as base flow to the receiving water; the precipitation that reaches the soil surface and was not absorbed by the soil may be stored temporary in puddles on the surface if the upper soil is impermeable or saturated with water. It becomes unchannelled overland flow which runs directly from the slope to the Gina river (receiving water body) if rainfall continues

ICE SHEETS DEFENITION In glaciology, an ice sheet , also known as a continental glacier , [1] is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km 2 (19,000 sq mi). [2] The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide Ice Sheet covered much of North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern South America.  Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km 2 are termed an ice cap. An ice cap will typically feed a series of glaciers around its periphery.

1. Antarctic ice sheet  (^) The Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km 2 (14 Mm 2 ) and contains 30 million km 3 of ice. Around 90% of the Earth's ice mass is in Antarctica, [3] which, if melted, would cause sea levels to rise by 58 meters. [4] The continent- wide average surface temperature trend of Antarctica is positive and significant at >0.05 °C/decade since 1957.  (^) The Antarctic ice sheet is divided by the Transantarctic Mountains into two unequal sections called the East Antarctic ice sheet (EAIS) and the smaller West Antarctic Ice Sheet (WAIS). The EAIS rests on a major land mass but the bed of the WAIS is, in places, more than 2,500 metres below sea level. It would be seabed if the ice sheet were not there. The WAIS is classified as a marine-based ice sheet,

meaning that its bed lies below sea level and its edges

flow into floating ice shelves. The WAIS is bounded by

the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers

that drain into the Amundsen Sea.

2. Greenland ice sheet

The Greenland ice sheet occupies about 82% of the

surface of Greenland, and if melted would cause sea

levels to rise by 7.2 metres.

[4]

Estimated changes in the

mass of Greenland's ice sheet suggest it is melting at a

rate of about 239 cubic kilometres (57 cubic miles) per

year.

[7]

These measurements came from NASA's

Gravity Recovery and Climate Experiment (GRACE)

satellite, launched in 2002, as reported by BBC News in

August 2006.

How Ice Sheets Form

Ice sheets formed like other glaciers.

Snow accumulates year after year, then melts.

The slightly melted snow gets harder

and compresses. It slowly changes texture from

fluffy powder to a block of hard, round ice

pellets. New snow falls and buries the grainy

snow. The hard snow underneath gets

even denser. It is known as firn.

As years go by, layers of firn build on top of each

other. When the ice grows thick enough—about

50 meters (165 feet)—the firn grains fuse into a

huge mass of solid ice. At this point, the glacier

begins to move under its own weight.

Ice sheets tend to be slightly dome-shaped and spread

out from their center. They behave plastically, or like a

liquid. An ice sheet flows, oozes, and slides over

uneven surfaces until it covers everything in its path,

including entire valleys, mountains, and plains.

Compression and geothermal energy sometimes cause

the bottom of an ice sheet to be slightly warmer than

the ice above it. The bottom of the ice sheet melts,

causing the ice above it to move at a faster rate than

the rest of the ice sheet. These fast-moving glaciers are

called ice streams

Ice streams can move as quickly as 1,000 meters (.

mile) every year. The slightly warmer, softer ice of the

ice stream is where most of the ice sheet's crevasses

are located.

 (^) The Greenland ice sheet is much smaller than the Antarctic Ice sheet, only about 1.7 million square kilometers (656,000 square miles). It is still the second- largest body of ice on the planet.  (^) The Greenland ice sheet interacts much more dynamically with the ocean than the Antarctic ice sheet. The annual snow accumulation rate is more than double that of Antarctica. Glacial melt happens across about half of the Greenland ice sheet, whereas it is much more isolated on the far western part of Antarctica. Greenland's ice shelves break up much faster than those surrounding Antarctica.  (^) Both the Antarctic and Greenland ice sheets have caused the land under them to sink. Eastern Antarctica is about 2.5 kilometers (1.6 miles) below sea level because of the colossal weight of the ice sheet above it.



If the continental glaciers were to suddenly disappear,

the landscape of Antarctica and Greenland would

change drastically. Antarctica would shrink by more

than a quarter as the western part shrunk into the sea.

Greenland, the largest island in the world, would

become an archipelago, a chain of small islands only

connected by waterways.



Melting Ice Sheets



Scientists from around the world are concerned that

the Earth’s temperature is increasing very rapidly,

causing glaciers, ice caps, and ice sheets to melt.

Glaciers melt when ice melts more quickly than firn

can accumulate. Because they are so large, melting ice

sheets can affect the climate of ecosystems in the entire

world.