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.

Ranchi city of India is facing acute water crisis.

Water has decreased in Dams, Ponds and Rivers.

by

Dr. Nitish Priyadarshi

Surface Water has reduced in Kanke Dam

Flow of water in Kanke Dam has also reduced

All the small rivulets flowing through Ranchi city has dried up.

No water in rivulet

Water reduced in one of the important ponds of Ranchi city.

No water in local ponds

Inhabitants of Ranchi city of Jharkhand state of India are facing acute water crisis. Most of the dug wells and deep wells and the corporation taps of this populated area have run dry forcing people to consume polluted surface water. Extensive deforestation, urbanization and industrilisation has led to uneven spread of rainfall, on which the water supply from the dams to the city area is depended. Even the ground water table has been affected due to uneven rainfall. From last few years rainfall due to western disturbances during winter season has shown decline trend. This rainfall earlier used to recharge groundwater which helped to maintain water table in peak summer season.

This summer season most of the ponds and small rivulets flowing through the city has dried up due to unbearable hot temperature. Earlier Ranchi was known for its cool climate even in the summer season. Temperature rarely went up to 41 degree centigrade. But now it is very common. It is a fact that before 1990s whenever temperature rose to 35 degree centigrade during day time, Ranchi plateau used to get evening rainfall which helped to recharge the ground water table as well as surface water table. But now this phenomenon has stopped. Now continuous dry season last for more than six months. From last few years the temperature is showing increasing trend.

The main reasons for this water scarcity are as follows:

Dams: There are three major dams (Kanke, Rukka and Hatia) in Ranchi city which is now filled with sediments affecting water table. Due to these sedimentations the storage capacity of all the three dams have decreased many fold. From the year of their constructions silts have never been removed which has not only depleted the water table but also made dame water contaminated.

Topography: General elevation of Ranchi city is 600 m. above the mean sea level and has a flat to gently undulating topography with occasional ridges. The Ranchi plateau gradually slopes down towards south east into the hilly and undulating region of Singhbhum. Due to this uneven topography the rain water are lost through surface runoff resulting in less water percolation below the surface. The thin soil layer of Ranchi plateau which is becoming more thin due to weathering is gradually loosing its water retaining capacity.

Geology and Urbanisation: The process of urbanization and industrilisation from last 20 years has caused changes in the water table as a result of decreased recharge and increased withdrawal. Many of the small ponds which were main source of water in the surrounding areas are now filled for different construction purpose affecting the water table. Lots of DEEP- BORING in the Ranchi city has also forced the water table to move down as well as Ranchi plateau consists of metamorphic rocks which are relatively impermeable and hence serve as poor aquifers. They bear groundwater only in their weathered top portion which rarely exceeds 10 meters.
Rainfall factor: Though Ranchi receives sufficient amount of rainfall (1000mm to 1200 mm every year) but it is not an accurate indicator of groundwater level changes. Recharge is the governing factor (assuming annual withdrawals are constant); it depends on rainfall intensity and distribution and amount of surface runoff. As Ranchi is the plateau area waste of rainwater in the form of surface runoff varies from 35% to 40%. So recharging is not so good.

Many houses have been built over the recharge area which are the major source of water to unconfined aquifer below, because an unconfined aquifer is one in which a water table varies, depending on areas of recharge and discharge and pumping from the wells.
Among the natural drinking water sources are ponds and lakes that too has their life span, is being threatened by early Eutrophication due to sedimentation, which hampers bottom discharge processes. Pollution growth of Algal bloom, weeds in the remaining ponds of Ranchi has made water unfit for domestic use imparting fowl smell.

The state of Jharkhand, although claims to be a store house of minerals in India, is not so rich in water resources. Most of the areas are occupied by the hard rock, which is in general protracted drought prone areas.

Hard rock which occupies major portion of the Jharkhand plateaus are devoid of primary porosity and occurrence and movement of groundwater is controlled by the joints, fractures and fissures present in them.

The need of the day is to conserve every drop of water and recharge the depleted aquifers. Mere slogan shouting will not carry us far. The time is now ripe for action with the involvement of the people who are beneficiaries.

Half- hearted attempts are being made at rainwater harvesting in Ranchi city and with much publicity given in the media. The scheme can benefit thousands of acres of arid lands, if small ponds are excavated at vantage points or small check dams are build in at particular intervals in the different rivulets to conserve soil moisture.

Remedies:

1. Identification of the recharge area or catchment areas in and around Ranchi city to ensure no further construction on those lands.
2. Construction of artificial water reservoirs in suitable area in and around Ranchi to collect the rain waters.
3. Cleaning of ponds, lakes, rivulets and dams periodically to rejuvenate their capacity of storage of water.
4. Providing proper consultancy to determine suitable areas for deep boring and hand pumps.
5. To ensure regular safe drinking water supply from the Municipal to stop reckless deep boring, because in absence of water supply people are forced go for alternative source of drinking water.
6. Water conservation measures from domestic level.
7. Metering and pricing of water.
8. Region wise, in-depth study of the water balance.
9. Further planning of plantation and aforestation to ensure regularity of water cycle.
10. Fostering an awareness of water as a scare resource and its conservation as an important principle-through NGOs.