## How Much Water Is Available

1.2    Availability of water
Water is available in plenty on earth. More than three-fourth of the earth’s surface is covered with water in the form of seas, rivers and lakes. It is also found inside the earth’s crust Most of the water that we get from the wells comes from this source.
About the 71 % of the earth’s surface is covered with water.

1.3    Physical properties of water
Water possesses certain significant physical properties which are responsible for making it a vital liquid.
(i)    Nature :  It is a colourless, odourless and tasteless liquid.
(ii)    Freezing point :  Water freezes to ice at 0°C under normal atmospheric pressure that is 76 cm of Hg.
(iii)    Boiling point : Water gets converted to steam at 100°C under normal atmospheric pressure that is 76 cm of Hg. The boiling point of water is proportional to external pressure.
(iv)    Density : In general, solids possess greater densities than liquids. Therefore, for such liquids, the density decreases with an increase in temperature as the liquids expand on heating. However, in the case of water, it is different. Ice has lower density than water at the same temperature. As ice gets converted to water at 0°C its density increases and this trend continues till the temperature rises to 4°C. Beyond 4°C, water shows normal trend of decrease in density with increase in temperature. Therefore, density of water is maximum at 4°C (1 g /cc). From 4°C to 0°C, water undergoes expansion while all other liquids undergo contraction. Since this trend is opposite to the normal trend, this is called anomalous expansion of water.

(v)    Specific heat : The amount of heat energy required to raise the temperature of unit mass of a substance through 1°C is called the specific heat capacity of that substance. Specific heat capacity of pure water is 1 calorie / g / °C. Among all substances, water has the highest specific heat capacity.
(vi)    Latent heat : When a substance changes from one state to the other, certain amount of heat is required to overcome the intermolecular force of attraction. For instance, when ice of certain mass at 0°C is converted to water of same mass at 0°C, the heat supplied brings about conversion of state without change in temperature. This is stored as potential energy in the water molecules and is called latent heat of fusion and is equal to 80 cal/g.
Similarly, the conversion of 1 g of water to steam at 100°C requires 540 cal heat and thus called as latent heat of vapourization.
At any temperature between 0°C and 100°C, conversion of water to water vapour takes place rather slowly. This process is called evaporation. The rate of evaporation increases with increase in temperature.
(vii)    Conductivity :  Pure water is a poor conductor of electricity. However, the presence of dissolved salts renders normal water a good conductor.
(viii)    Solvent property : Water can dissolve most of the substances in it. Hence it is known as universal solvent.

1.4    Composition of water
Water is essential for life. Water is made up of hydrogen (H2) and oxygen (O2) elements.
The molecular formula for water is H2O.The ratio between hydrogen and oxygen in water is 1 : 8 by mass.

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## Forms of Water

1.6    Forms of water
Water can be found in all the three forms, i.e., solid, liquid and gas.
(i)    Solid : It occurs in the form of snow, ice crystal, ice caps at the poles of the earth, snow covered mountains and glaciers.
(ii)    Liquid : It occurs in the form of rain, oceans, lakes rivers and even ground water.
(iii)    Gas : The gaseous form is the water vapour i.e., present in the air around us.

1.7    Water cycle

The supply of water in nature does not run out. This is because water is continuously recycled in the water cycle.
(i)     Heat from the sun causes the water on the earth’s surface to evaporate. The  vapour rises, cools and condenses to form tiny water droplets. These droplets form clouds.
(ii)    The clouds get carried along by air currents. They cool and the droplets join to form larger drops.
These fall as rain.
(iii)     If the temperature in the region is very low, these water droplets fall as hail, sleet or snow.

(iv)     Some of the rain water flows along the ground as streams. Some soaks through the ground and then reappears as springs. Streams and springs join to form rivers. Rivers flow back into the sea. Thus, the water cycle is completed.

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## Groundwater as An Important Source of Water

(b)    Groundwater as an important source of water
If we dig a hole in the ground near a water body we may find that the soil is moist. The moisture in the soil indicates the presence of water underground. If we dig deeper and deeper, we would reach a level where all the space between particles of soil            and gaps between rocks are filled with water . The upper limit of this layer is called the water table. The water table varies from place to place, and it may even change at a given place. The water table may be at a depth of less than a metre or                          may   be several metres below the ground. The water found below the water table is called groundwater.

The rain water and water from other sources such as rivers and ponds seep through the soil and fills the empty spaces and cracks deep below the ground. The process of seeping of water into the ground is called infiltration. The groundwater thus gets            recharged by this process. At places the groundwater is stored between layers of hard rock below the water table. This is known as an aquifer.

Spring water : Springs are formed by percolation of rain water into soil. Springs supply water to wells and lakes.

Well Water : The rain water seeps through the soil and goes down and is stored over rocks or hard earth crust. On digging the well this underground water becomes available to us. This is known as well water. This water may not be pure and may                  contain  impurities such as suspended particles, bacteria and other microorganisms.

1.5    Sources of water
The natural sources of water are rain, springs, wells, rivers and seas.
(a)    Surface water : It is the water present on the surface of earth. The water present in oceans, seas, rivers, springs, etc. comes under surface water.
(i)     Rain water : Rain water is considered to be the purest form of natural water (distilled water) free from impurities.

When the water vapours go high up in the air they condense to form clouds. The water drops  come down as rain.

(ii)     River water : Rivers are formed by melting of snow on the mountain, and also sometimes from the rain water. River water is also not a pure source.
(iii)     Sea water : Sea water is the largest natural source of water. However, it is also the source of common salt and other important chemicals. It is the most impure form of water. All the impurities dissolved in river water are carried into the sea. Sea                       water cannot be used for drinking purpose because of high salinity and impurities.

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## Depletion of Water Table

1.9    Depletion of water table
Increase in population, industrial and agricultural activities are some common factors affecting water table. Scanty rainfall is another factor that may deplete the water table.
(a)     Increasing population :Increasing population creates demand for construction of houses, shops, offices, roads and pavements. This decreases the open areas like parks and playgrounds. This, in turn, decreases the seepage of rainwater into the                      ground. Moreover, a huge amount of water is required for the construction work. Often groundwater is used for this purpose.
So, on one hand we are consuming more groundwater, and on the other we are allowing lesser water to seep into the ground.

(b)   Increasing industries : Water is used by all the industries. Almost everything that we use needs water somewhere in its production process. The number of industries is increasing continuously. Water used by most of the industries is drawn from the                      ground.

(c)    Agricultural activities : A majority of farmers in India depend upon rains for irrigating their crops. Irrigation systems such as canals are there only in a few places. Even these systems may suffer from lack of water due to erratic rainfall. Therefore, farmers          have to use groundwater for irrigation. Population pressure on agriculture forces increasing use of groundwater day by day. This results in depletion of water table.

(d)    Scanty rainfall or draught conditions : During long periods of scanty rainfall or in the absence of rainfall, this balance is disturbed. Usage of underground water continues more or less at the same level while its replenishment becomes very less or                    almost nil. This results in the depletion of water table.

(e)    Deforestation : Deforestation results in the reduction of  forest area which allows the rain water flow rapidly into large water bodies. Seepage becomes less thereby leading to imbalance in the replenishment of underground water.

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## Distribution of Water

1.10    Distribution of water
India is a vast country and the rainfall is not the same everywhere. Some regions have excessive rain while some others have very little rainfall. Excessive rain cause floods, whereas the absence of rain results in drought. Therefore, some regions in                our country may have floods while others may suffer from drought at the same time.

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## Water Management

1.11    Water management
We have seen that most of the water that we get as rainfall just flows away. This is a waste of precious natural resource. The rainwater can be used to recharge the groundwater. This is called as water harvesting or rainwater harvesting.
Find out if the buildings in your neighbourhood have water harvesting systems installed.
We have at many places in India an age old practice of water storage and water recharge like the bawris. Bawri was the traditional way of collecting water. With time the bawris fell into disuse and garbage started piling in these reservoirs. However,                 because of the acute water shortage, people in these areas have had to rethink. The bawris are being revived. Today the situation is that inspite of scanty rains these places are managing their water needs well.
Water in the fields can use be used water economically. Maybe you have heard of drip irrigation. Drip irrigation is a technique of watering plants by making use of narrow tubings which deliver water directly at the base of the plant.

1.12    Uses of water
(i)    Essential constituent  for the reaction of photosynthesis which is the process supporting  life as well as making food for plants and animals.
(ii)    Medium for all biochemical processes taking place in plants and animals
(iii)    Medium for transport of nutrients and minerals to the living cells in living beings.
(iv)    Exchange of gases through blood by the process of respiration.
(v)    Germination of seeds.
(vi)    Growth of plants
(vii)    Regulation of body temperature.

Key words
1.    Aquifer : A water trap where groundwater is trapped between the layers of hard rock below water table.
2.    Depletion of water table : We are consuming more ground water for industrial, agricultural activities as human wastes and on the other hand, we are allowing lesser water to seep into the ground. Due to this reason, depletion of water table occurs.
3.    Drip irrigation : Drip irrigation is a technique of watering the plants by making use of narrow tubings which deliver water drop by drop directly at the base of the plant.
4.    Groundwater : The water that seeps into the  earth, gets collected between the layers of rocks and found below the water table, is called groundwater.
5.    Infiltration : The process of seeping of water into the ground is called infiltration. The groundwater gets recharged by this process.
6.    rainwater harvesting : The rainwater can be used to recharge the groundwater. This is referred as water harvesting or rainwater harvesting.
7.    Water recharged : The groundwater is recharged by the seepage of rainwater. The rainwater and water from other sources such as rivers and ponds seeps through soil and fills the empty spaces and cracks deep below the ground.
8.    Water table : The top most level of the groundwater, where all the space between soil particles is filled with water is called water table.

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## How Much Water Is Available

Chapter 16: Water: A Precious Resource

Water is a limited resource. It is needed for all living things and must be managed well to ensure we have enough for our needs and to protect our environment. Water is a precious resource. We can’t live without it.

How much water is available

There’s a whole lot of water on Earth! Something like 326,000,000,000,000,000,000 gallons (that’s 326 million trillion gallons) of the stuff (roughly 1,260,000,000,000,000,000,000 l) can be found on our planet. This water is in a constant cycle — it evaporates from the ocean, travels through the air, rains down on the land and then flows back to the ocean.

The oceans are huge. About 70 percent of the planet is covered in ocean, and the average depth of the ocean is about 12,100 feet (3,688 meters). Ninety-eight percent of the water on the planet is in the oceans, and therefore is unusable for drinking because of the salt. Less than 3 percent of the planet’s water is fresh, but about 1.6 percent of the planet’s water is locked up in the polar ice caps and glaciers. Another 0.36 percent is found underground in aquifers and wells. Only about 0.036 percent of the planet’s total water supply is found in lakes and rivers.

## Forms Of Water

Forms of water

Water exists in many forms, such as a liquid, a solid, as in snow and ice, underneath the land surface as groundwater, and in the atmosphere, as in clouds and invisible water vapour.

## Groundwater As An Important Source Of Water

Ground water as an important source of water

Ground water is an important source of water supply. 53% of the population of US receives its water supply from groundwater sources. Groundwater is also a major source of industrial and agricultural uses. We are withdrawing water from underground aquifers at a faster rate that it can be replenished.

## Depletion Of Water Table

Depletion of water table

Water Table depletion is caused when we pump water more rapidly than it can renew itself. With a booming population, the more frequently we pump water from the ground, the more difficult it is for the groundwater to recharge itself and provide us with the amount of water we need.

## Distribution Of Water

Distribution of water

Of all the water that exists on our planet, roughly 97% is saltwater and less than 3% is freshwater. Most of Earth’s freshwater is frozen in glaciers, ice caps, or is deep underground in aquifers.

## Water Management

Water management

Water management is the control and movement of water resources to minimize damage to life and property and to maximize efficient beneficial use. Good water management of dams and levees reduces the risk of harm due to flooding.

## What Role You Can Play

What roles you can play

Water harvesting.

Using water in adequate quantity.

Avoiding water pollution.

Using sprinkler system in plantation.

Reusing water, like giving water to plants which was used before for washing clothes.

## Effect Of Water Scarcity On Plants

Effects of water scarcity on plants

The effects of water scarcity are reduced photosynthesis, wilting of plants, reduced respiration, reduced transpiration, and alteration in their adaptation.