Monday, July 26, 2010

Meteorological phenomena influences groundwater levels.

Rainfall is not an accurate indicator of groundwater recharge.
by
Dr. Nitish Priyadarshi
Water is essential to people and the largest available source of fresh water lies underground. Ground water is the part of precipitation that seeps down through the soil until it reaches rock material that is saturated with water. Water in the ground is stored in the spaces between rock particles. Increased demands for water have affected the level of under groundwater. The demand for water has increased over the years and this has led to water scarcity in many parts of the world. The situation is aggravated by the problem of water pollution or contamination. World is heading towards a freshwater crisis mainly due to improper management of water resources and environmental degradation, which has lead to a lack of access to safe water supply to millions of people. This freshwater crisis is already evident in many parts of world, varying in scale and intensity depending mainly on the time of the year.


Water-level changes can be divided into several categories. There are short-term changes that can only be seen when water-level measurements are made many times a day. There are long term changes that can only be seen after data are collected for many years. There are minor changes of only a few hundredths of a foot, and changes that are hundreds of feet.

Any phenomenon that produces a change in pressure on groundwater will cause the groundwater level to vary. Differences between supply and withdrawal of groundwater cause levels to fluctuate. Other diverse influences on groundwater levels include meteorological and tidal phenomena, urbanization, earthquakes, and external loads. And, finally, subsidence of the land surface can occur due to changes in underlying groundwater conditions.

Fluctuations due to meteorological phenomena:

Atmospheric Pressure:

Changes in atmospheric pressure can also cause groundwater levels to fluctuate. Atmospheric pressure is caused by the Earth’s gravitational attraction of air in the atmosphere. At sea level, the weight of the atmosphere exerts a pressure of about 14.7 pounds per square inch on the Earth’s surface.


Changes in atmospheric pressure produce sizable fluctuations in wells penetrating confined aquifers. The relationship is inverse; that is, increases in atmospheric produce decreases in water levels, and conversely.

For an unconfined aquifer, atmospheric pressure changes are transmitted directly to the water table, both in the aquifer and in a well; hence, no pressure difference occurs. Air entrapped in pores below the water table is affected by pressure changes, however, causing fluctuations similar to but smaller than that observed in confined aquifers. Temperature fluctuations in the capillary zone will also induce water table fluctuations where entrapped air is present.

Rainfall:

Rainfall is not an accurate indicator of groundwater recharge because of surface and subsurface losses as well as travel time for vertical percolation. The travel time may vary from a few minutes for shallow water tables in permeable formations to several months or years for deep water tables underlying sediments with low vertical permeabilities.

Precipitated water that reaches at the surface ground maybe partially discharge into streams as surface runoff or partially infiltrate into the ground. The latter further percolates into groundwater aquifers, eventually emerging in springs, seeping into streams to form surface runoff, or storing in subsurface. The soil stores infiltrated water to become soil moisture, and then it recharges to groundwater level if the soil is saturated. Nevertheless, it releases slowly as subsurface flow to enter the stream as baseflow during rainless period. This may also result from deeper percolation, evapotranspiration, or artificial discharge.

If no water supplies are continually provided from either rainfall or other sources of recharge, groundwater level would gradually decrease due to deeper percolation or evapotranspiration.

Furthermore, in arid and semiarid regions, recharge from rainfall may be essentially zero. Shallow water tables show definite response to rainfall where the unsaturated zone above a water table has a moisture content less than that of specific retention, the water table will not respond to recharge from rainfall until this deficiency has been satisfied.

Wind:
Minor fluctuations of water levels are caused by wind blowing over the tops of wells. The effect is identical to the action of a vacuum pump. As a gust of wind blows across the top of a casing, the air pressure within the well is suddenly lowered and, as a consequence, the water level quickly rises. After the gust passes the air pressure in the well rises and the water level falls.

Frost:

In the regions of heavy frost it has been observed that shallow water tables decline gradually during the winter and rise sharply in early spring before recharge from ground surface could occur. This fluctuation can be attributed to the presence of a frost layer above the water table. During winter water moves upward from the water table by capillary movement.
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