Ground water and water table
To understand groundwater availability better, one must understand the various ways in which rainwater is distributed. A part of rainwater always...
To understand groundwater availability better, one must understand the various ways in which rainwater is distributed. A part of rainwater always evaporates and returns to the atmosphere as water vapor. Some of it is absorbed by the plants and trees and eventually return to atmosphere through the process of transpiration. A major part of the water joins streams, rivers and oceans. This process is called runoff.
A considerable portion of water received from rain or snow, however, percolates downwards into soil and rocks filling up joints and pores to form groundwater. Groundwater plays an important role in weathering and is also important as means of a natural water source. The amount of available ground water depends on the climate. In dry climate, the precipitation may quickly evaporate into the atmosphere and some moisture may percolate into the ground.
In moderately humid areas, water both runs off into streams and sinks into the ground. The proportion of rainfall absorbed as groundwater may depend on the season of the year. More important, however, is the nature of the rocks and how easily they absorb and retain water. Various rocks and soils differ greatly in their porosity and permeability; the amount of groundwater present and the depth in which it lies is governed by the following characteristics.
- Rocks have a lot of pores on their surfaces which absorb water.
- Permeable and pervious rocks are those that allow water to flow through them easily. Thus most of the rocks are also permeable.
- Some rocks are porous but are also impermeable unlike, for example clay is very porous but is impermeable.
- Granite is non porous but is permeable in nature. It is a good absorber of water.
The water table is the surface where the water pressure head is equal to the atmospheric pressure (where gauge pressure = 0). It may be conveniently visualised as the "surface" of the subsurface materials that are saturated with groundwater in the given vicinity. However, saturated conditions may extend above the water table as surface tension holds water in some pores below atmospheric pressure.
Individual points on the water table are typically measured as the elevation that the water rises to in a well screened in the shallow groundwater. The groundwater may result from infiltrating precipitation or groundwater flowing into the aquifer. In areas with sufficient precipitation, water infiltrates pore spaces in the soil, passing through the unsaturated zone. At greater depths, water fills most of the pore spaces in the soil, until the zone of saturation is reached.
In permeable or porous materials, such as sands and well fractured bedrock, the water table forms a relatively horizontal plane. Below the water table, in the phreatic zone, permeable units that yield groundwater are called aquifers. The ability of the aquifer to store groundwater is dependent on the primary and secondary porosity and permeability of the rock or soil. In less permeable soils, such as tight bedrock formations and historic lake-bed deposits, the water table may be more difficult to define.
The water table should not be confused with the water level in a deeper well. If a deeper aquifer has a lower permeable unit that confines the upward flow, then the water level in a well screened in this aquifer may rise to a level that is greater or less than the elevation of the actual water table. The elevation of the water in this deeper well is dependent upon the pressure in the deeper aquifer and is referred to as the potentio-metric surface, not the water table.
When plenty of water is available to augment the ground water supply the water table may rise, but in dry periods water table is lowered as ground water is lost through seepages or springs.
The groundwater stored in rocks is released onto the surface at points where the water table reaches the surface. A spring is simply an outlet for such water.
The water may gradually seep out of the rock or may gush out as a fountain. Springs are of several kinds due to the nature of rocks and the position of water table. The main types are described below.
- If the land is filled with permeable and impermeable rocks which alternate each other, water will end up at the impermeable layer.
- In well joint rocks, water may percolate into the inner layers until it finds an outlet from which it can move out.
- If the dyke or sill impermeable rock is intruded through permeable rocks, it causes the water table to reach the top and emerge as a spring.
- In limestone or chalk, permeable rocks alternate impermeable rocks where the water comes out through sip slope spring.
Unlike springs, wells are bored into the earth until the water is spotted. If the well is sunk only to the wet season depth of water table, water will be unobtainable when the water drops in dry season. When a well is bored, the water is raised through hand or motor.
It is a distinctive form of well where the rocks are folded into a basin like structure and the inner layer of the basin is either by clay or impermeable rocks. Due to gravity the water will travel till the end and is accumulated in the basin. The upper impermeable rocks will not allow escape of the water and inner impermeable layers store water in a place. Such rocks are called aquifers. After some time, the pressure decreases and water is pumped out.
The depth of artesian well differs from place to place. It can even supply water to a full village like the artesian wells of Great Plains of USA do or for sheep farming in Queensland and other parts of Australia. Groundwater might not be good for irrigation and human consumption due to the presence of high concentration of salts.