Evolutionary biology, one of the Biology Topics, investigates the mechanisms behind the origin and diversification of species.
What is groundwater and why is it so important?
A large number of people draw water from wells, tube-wells (bore-wells) and handpumps. So, wells, tube-wells and handpumps are the sources of water. An important question now arises : From where do these sources (like wells, tube-wells and handpumps) get water ? The answer is : They get it from water present below the surface of earth. This is called groundwater (though it is really underground water). Let us discuss this in detail.
If we dig a hole (or well) in the ground, we will find that after a certain depth, the soil which comes out of the hole is appreciably moist (or wet). This moisture in the dug up soil indicates the presence of water under the ground. If we dig deeper and deeper, we would reach a depth in the ground where all the spaces between the particles of soil, gaps between rocks and pores in permeable rocks are filled with water. This is the depth where the underground water starts. The upper level of water under the ground which occupies all the spaces in the soil and rocks, is called water table (see Figure). In most simple words, the upper level of groundwater below the surface of earth is called water table. Water table represents a depth in the ground below which soil and rocks are completely filled with water (or saturated with water). The
water table varies from place to place, and it may even change at a given place. For example, at some places the water table may be at a depth of less than a metre whereas at other places the water table may be several metres below the ground. How deep we have to dig into the ground to obtain groundwater depends on the water table at that place. If the water table at a place is high, we have to dig less into the ground to obtain groundwater. On the other hand, if the water table at a place is low, we will have to dig very deep into the ground to obtain groundwater.
The water table rises and falls depending upon the amount of rainwater which seeps into the ground, and on how much groundwater is drawn out (for irrigation and industry, etc.). If there is more rainfall and more rainwater seeps into ground, the water table rises. And if excessive amount of groundwater is withdrawn for irrigation and industries, then the water table falls. The water found below the water table is called groundwater. This water is held in soil and pores of permeable rocks under the ground. Thus, groundwater is not above the ground, it is deep under the ground. The groundwater is an important source of water for us. We will now discuss the source of groundwater.
The main source of groundwater is the rain. The rainwater and water from other sources such as rivers, lakes and ponds, seeps through the ground and fills the empty spaces between the soil and the rocks below the earth. The water keeps on seeping deeper and deeper into the ground through the soil and permeable rocks until it reaches an impermeable rock (hard rock). The impermeable rock does not allow water to pass through it. So, water starts collecting above the impermeable rock and fills the soil and permeable rocks completely with water up to the level known as water table. The underground layer of soil and permeable rocks in which water collects under the ground is called an aquifer (see Figure).
The aquifer is not a kind of underground pool of water. In aquifer, water is held between particles of soil, and in the cracks and pores of permeable rocks. The aquifer can be thought to be a giant underground sponge which holds groundwater in it. An aquifer is the water-bearing layer of the earth. The top of aquifer is referred to as water table. Aquifer stores and provides groundwater. Water from the aquifer can be pumped out with the help of tube-wells and handpumps, or drawn out through the wells. The rainwater which seeps into the ground ultimately reaches the aquifer and gets stored there.
We draw (or take out) groundwater for our use with the help of wells, tube wells and handpumps. This decreases the amount of groundwater. But when it rains, the rainwater seeps into the ground and the groundwater is recharged. If, however, we keep on drawing more and more groundwater for various purposes than can be recharged (or refilled) by rains, then the water table will go on falling. The water table will get deeper and deeper under the ground. If this happens, then many of the wells, tube wells and handpumps will dry up.
Before we go further, we should know the meanings of the terms : ‘depletion’ and ‘replenish’. ‘Depletion’ means ‘reduction’ or ‘lowering’ the amount of something. For example, ‘depletion of water table’ means ‘reduction in water table’ or ‘lowering of water table’. ‘Replenish’ means ‘to replace something which has been used’. In most simple terms, replenish means to fill up again.