Friday, November 13, 2009

Impact of global warming and climate change on groundwater.

Groundwater level of many areas of the world are going to affected.

Dr. Nitish Priyadarshi
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When astronauts first went into space, they were captivated by a vision of the earth that had never been seen before- an earth predominantly blue in colour. After photographs such as this were published, the earth became known as the ‘blue planet’. The earth looked blue because the oceans cover 71 percent of its surface.

Water is essential for life- all living organisms contain water. In fact most living organisms consist of over 60 percent water.

Through out his brief history on earth, man has depended on three basic natural resources for his survival: air, land and water. Of these three, none has been more abused and neglected by man than water. Yet without water, the land produces nothing; without water, life vanishes from the face of the earth. Civilizations arose and have prospered for millennia where water was abundant. The Nile valley is a prime example. When water disappeared, civilizations withered and died. Mesopotamia is a classic example.

The utter human dependence on water is exemplified by the fact that man can survive for five weeks without food, but he dies after only five days without water. If the water supply of a human being is cut off for just a few days, he will die of dehydration long before the water is completely evaporated from his body. Water vapor is lost with every breath, and we cannot avoid breathing.

In recent years due to increasing contamination of surface water bodies, dependency on groundwater for drinking and domestic purpose has increased many fold. But these water sources are now badly affected with global warming. Groundwater is depleting day by day due to effect of rising temperature and urbanization.

The excess soil moisture that saturates subsurface soil or rock and migrates downward under the influence of gravity. In the literal sense, all water below the ground surface is groundwater; in hydrogeologic terms, however, the top of this saturated zone is called the water table, and the water below the water table is called groundwater.

Groundwater is an important part of the water cycle. It comes from rain, snow, sleet and hail that soak into the ground. The water moves down into the ground because of gravity, passing between particles of soil, sand, gravel, or rock, until it reaches a depth where the ground is filled, or saturated, with water. The area that is filled with water is called the saturation zone and the top of this zone is called the water table. The water table may be very near the ground’s surface or it may be hundreds of meters below.

Although groundwater exists everywhere underground, some parts of the saturated zone contain more water than others. An aquifer is an underground formation of permeable rock or loose material which can produce useful quantities of water when tapped by a well. These aquifers may be small, only a few hectares in area, or very large, underlying thousands of square kilometers of the earth’s surface.

The effects of global warming are vast and cover every sphere of one’s life. Both the nature and the living beings are suffering from the effects of global warming. The water resources have been heavily affected by the global warming phenomenon.

In recent times, several studies around the globe show that climatic change is likely to impact significantly upon freshwater resources availability. In India, demand for water has already increased manifold over the years due to urbanization, agriculture expansion, increasing population, rapid industrialization and economic development. At present, changes in cropping pattern and land-use pattern, over-exploitation of water storage and changes in irrigation and drainage are modifying the hydrological cycle in many climate regions and river basins of India.

There have been observed changes in surface temperature, rainfall, evaporation and extreme events since the beginning of the 20th century. The atmospheric concentration of carbon dioxide has increased from about 280 parts per million by volume (ppmv) to about 369 ppmv and the global temperature of the earth has increased by about 0.6°C.

A warmer climate will accelerate the hydrologic cycle, altering rainfall, magnitude and timing of run-off. Warm air holds more moisture and increase evaporation of surface moisture.

If there is little or no moisture in the soil to evaporate, the incident solar radiation goes into raising the temperature, which could contribute to longer and more severe droughts. Therefore, change in climate will affect the soil moisture and groundwater recharge.

With the increase in temperature unconfined aquifers with water tables near ground surface frequently exhibit fluctuations. It can be ascribed to evaporation and/or transpiration. Both processes cause a discharge of groundwater into the atmosphere.

Groundwater supplies are recharged naturally by rain and snow melt. That means we are only able to abstract as much water as that being recharged, otherwise the groundwater supply will run into a “deficit”. It is therefore possible that we can run out of groundwater, at least until the supply has been recharged again. This recharge process can take months, years or even hundreds of years.

Due to warming and climate change rainfall trend has been badly affected worldwide. This change has adversely affected the groundwater recharge.

Water scarcity is expected to become an even more important problem than it is today. There are several reasons for this.

First, the distribution of precipitation in space and time is very uneven, leading to tremendous temporal variability in water resources worldwide. For example, the Atacama Desert in Chile receives imperceptible annual quantities of rainfall whereas Mawsynram, Assam, India receives over 450 inches annually. If all the freshwater on the planet were divided equally among the global population, there would be 5 000 to 6 000 m3 of water available for everyone, every year.

Second, the rate of evaporation varies a great deal, depending on temperature and relative humidity, which impact the amount of water available to replenish groundwater supplies.

The combination of shorter duration but more intense rainfall (meaning more runoff and less infiltration) combined with increased evapotranspiration (the sum of evaporation and plant transpiration from the earth's land surface to atmosphere.) and increased irrigation is expected to lead to groundwater depletion.

Third, as the temperature is rising demand for water is also rising especially in the countries like India, Africa, Bangladesh, South America etc. This demand has put extra pressure on the groundwater resulting depletion of the groundwater.

In a case study of Jharkhand state of India groundwater recharging is mainly dependent on rainfall. Though Jharkhand receives sufficient amount of rainfall (900 to 1400 mm/year) but from last several years the rainfall pattern is very erratic. From last two years Ranchi city the capital of Jharkhand state received sufficient rainfall but distribution of rainfall was not uniform. It rained heavily just for two to three days in the month of August and September which resulted in heavy runoff and less infiltration affecting groundwater level.

Projections of changes in total annual precipitation indicate that increases are likely in the tropics and at high latitudes, while decreases are likely in the sub-tropics, especially along its polar edge. Thus, latitudinal variation is likely to affect the distribution of water resources. In general, there has been a decrease in precipitation between 10°S and 30°N since the 1980s. With the population of low latitude regions increasing, water resources are likely to become more stressed in many regions, especially as global warming intensifies.

By 2100, water scarcity could impact between 1.1 and 3.2 billion people, says a leaked draft of an Intergovernmental Panel on Climate Change (IPCC) report due to be published in April 2007. The report focuses on the consequences of global warming and options for adapting to them. In February 2007 the panel released a report on the scientific basis of climate change.

The IPCC predicts critical water shortages in China and Australia, as well as parts of Europe and the United States. Africa and poor countries such as Bangladesh would be most affected because they were least able to cope with drought.

It is obvious that the projected climate change resulting in warming, sea level rise and melting of glaciers will adversely affect the water balance in different parts of world and quality of groundwater along the coastal plains. Climate change is likely to affect ground water due to changes in precipitation and evapo-transpiration.

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