Inventory and Distribution of World’s Water Resources

In nature, water is distributed in different forms at different places from where it moves to other places, since water does not remain permanently anywhere. It evaporates and again reaches the earth in the form of rainfall. Water on earth remains stable for different durations. During the movement of water, its form keeps changing.

In oceans, water is found in the form of liquid. From solid (snow) and liquid (water), it evaporates and reaches atmosphere in the form of vapour. Heat of the sun plays a predominant role in changing the form of water depending upon climate and location. Heat of the sun is the maximum on the equator and minimum on the poles. Due to this reason, the snowline remains above 5,000 metre on the equator and near ground level on the poles.

Hydrosphere includes the total amount of water in all the three spheres lithosphere, atmosphere and hydrosphere. Normally it is presumed that only oceans are included in the hydrosphere, but along with oceanic water, surface water on the ground, underground water and water vapour in the atmosphere etc. also constitute hydrosphere.

Though vapour exists in the atmosphere on a small scale, it is very important because it forms the hydrological cycle of hydro­sphere, atmosphere and lithosphere. After evaporation from oceans, lakes, rivers, snow cover and transpiration from plants, it reaches the atmosphere from where it distributes in the form of rainfall to different water sources. During the last century, the movement of water has been affected due to unplanned disturbance with nature.

This has resulted in changes in nature and the time of distribution of rainfall on the surface of the earth. The latest examples are regular droughts in India and floods in Europe during the year 2002, which affected the area from Britain to Romania.

About 13, 84,120,000 cubic km water is found in different forms in the hydrosphere (Table 2.1). Its biggest part is found in oceans. While 77.23 per cent is in snow caps, icebergs and glaciers. Of the whole hydrosphere, 2.6 per cent (36,020,000 km’) is fresh water, 2.21 per cent is groundwater located up to a depth of 4 kms. The remaining water remains in soil, lakes, living organisms and atmosphere in small quantities.

Distribution of Water:

Water found in hydrosphere is distributed in different forms on earth. Its major portion (97.39) is distributed in saline form in seas, whereas fresh water is available in a small quantity. Water vapour is an important part of the atmosphere which plays an important role in the energy circle. Thus in normal water distribution, surface water, groundwater and oceanic water are included, whereas water vapour in the atmosphere completes many processes. Universal distribution of water remains in balance through the water cycle in which water vapour has its own geographical importance.

Distribution of water has not remained in equal proportion since ancient past. During the snow age, major portion of atmospheric moisture and oceanic water became frozen as ice which was converted into liquid form after climatic changes.

Thus, distributed in changed form, water resources are presently found in the following forms:

1. Oceans

2. Groundwater

3. Surface Water

1. Oceans:

Almost 70.87 part of the earth is watery out of which 97.39 per cent exists in the form of oceans, which contain 1,34,80,000,00 cubic km water. Thus, earth due to the presence of vast quantity of water, it is also called ‘Water Planet’. In the northern hemisphere, 40 per cent area is water and 60 per cent is ground. In the southern hemisphere, there is water on 81 per cent area and 19 per cent is ground. That is why the southern hemisphere is also called ‘Water Hemisphere’.

Pacific Ocean is the biggest among all the oceans, which contains 53.9 per cent of water of the world. Atlantic Ocean contains 24.9 per cent and Indian Ocean has 21.1 per cent share of world’s water (Table 2.2). Water of all the three oceans is saline. Except these oceans, major portion of water found in North Pole Ocean and South Pole Ocean exists in form of snow. Within the snow area, Arctic Ocean covers 3.97 per cent part of total water area.

Pacific Ocean is the most expanded water portion on earth which encircles almost one-third part of the earth. Its area is 16, 53, 84,000 sq km and average depth is 4,280 metres. The main coastal seas of this ocean are Bering, Okhotsk, Japan Sea, China Sea, Sulu Sea, Yellow Sea, Java Sea, Banda Sea, Celebes Sea, California and Aleutian sea.

The second largest part of the hydrosphere exists in the Atlantic Sea. It extends up to 8.2 crore sq. km. Its main coasts are Grand bank, George bank, St. Pierre bank, Seville island bank and Dagger bank. Its main bordering seas are Mediterranean Sea, Baltic Sea, Northern Sea, Black Sea, Aegean Sea, Adriatic Sea, Baffin Bay, Hudson Bay, Gulf of Mexico and Caribbean Sea.

The third great water area of the world is the Indian Ocean. This is bordered by Asian lands in the North, Africa in the west and Antarctica in the south. Pacific Ocean is in its east. Sixty per cent portion of Indian Ocean is between 4,000 to 6,000 metres deep. Its main bordering seas are Bay of Bengal, Gulf of Aden, Andaman Sea, Gulfs of Oman, Kutch and Cambay etc.

Large amount of water exists in the bordering seas of the oceans also. In the Pacific oceanic area, the main seas are Bering Sea, Okhotsk Sea, Japan Sea, China Sea, Zulu Sea, Yellow Sea, Java Sea, Banda Sea, Celebes Sea, and Gulf of California Aleutian. In the Atlantic Ocean, Baltic Sea, Northern Sea, Mediterranean Sea, Black Sea, Aegean Sea, Adriatic Sea, Baffen Bay, Hudson Bay, Gulf of Mexico and Caribbean Sea are found. Main bordering seas of Indian Ocean are Bay of Bengal, Mozambique Channel, Arabian Sea, Red Sea, Persian Gulf, Gulf of Adan, and Andaman Sea etc.

2. Groundwater:

Water existing in voids and fissures of rocks is called groundwater. It is regulated by quantity and speed of rainfall, timings of rain, quantity of evaporation, temperature, slope of land, dryness of air, voids and non-permeability of rocks, vegetation cover, and water absorbing capacity of the soil. Surface flow goes underground through percolation.

Groundwater is 0.58 per cent of total water resources and out of total fresh water (2.6%) it is 22.1 per cent, which is located up to 4 kilometers (in-depth). Since it is found below the ground, it is also called sub-surface water.

Storage of Groundwater:

Water received from different sources on surface of the earth goes underground after entering through voids, cavities, fissures and bedding planes and takes the form of groundwater. After percolation downwards when water collects in rock voids, it is called saturation of rocks. When rock is fully saturated, it is called saturated zone. The part of saturated rocks below the surface is called saturated zone and its upper part is called groundwater or water table.

According to location, groundwater below the surface is found in three zones, which are as follows:

Non-Saturated Zone:

The upper most part of groundwater is called non-saturated zone where air is filled in voids of rocks. Hence, it is also called ‘Zone of Aeration’. In this zone, the water exists only during rainy period. In the remaining periods, rocks cannot be saturated with water.

Saturated Zone:

After percolation from the non-saturated zone, water reaches the middle portion and fills voids of rocks completely, resulting in rise of the water level. This zone gets maximum water during rainy period, whereas in the dry season the water table again goes down. Thus, this zone having higher water table during rainy season and lower water table during dry season, is also called ‘Intermittent Zone of Saturation. The zone below it remains permanently saturated, which is called ‘Zone of Permanent Saturation’. The saturation zone is also called ‘Preatic Zone’.

Rock Flowage Zone:

Below the surface of earth, there is a definite area where voids of rocks are closed due to the weight of rocks. Because of this the water cannot infiltrate further. Because the water cannot percolate further, it collects there. This collected water is known as ‘Aquifer’. Such water is assumed to be at the depth of 16 kilometers from the ground. The water existing at this great depth can flow, and this zone is called ‘Flow Area’.

Sources of Groundwater Availability:

Groundwater exists below the ground in different zones due to layers of non-permeable rocks. The upper part of groundwater is known as surface water level from where water enters through voids. The portion where water flows is known as aquifer.

Here, sources of groundwater implies such sources from which groundwater is received on the ground, whereas in Chapter 4, sources of ground­water are those source points like meteoric water, connate water, and magma water sources, from which groundwater is stored. Here, water cannot move so fast that it becomes sufficient for springs or other forms.

It is clear that due to different nature of internal structure ground level, groundwater exists at different places in different quantities, which comes to the ground from the various sources. There are two broad ways by which groundwater may be discharged. One is by artificial methods and the other is by natural methods.

First, the natural methods, these include:

1. Evapo-transpiration, particularly in the low lying areas where the water table is close to the ground surface.

2. Effluent seepage into surface water bodies, especially where the aquifer flows into a water body such as a lake or a river.

3. Leakage through aquacultures into the adjacent aquifers, and

4. Spring flow

Second is the artificial abstraction.

As for artificial abstraction, it is a story all of you must be familiar with. It has adversely affected storage in many areas. Ground­water when pumped from wells and boreholes causes a decline in the height of the water surface in the wells and results in the formation of a cone of depression around it. Successive abstraction from a large number of wells over a long period of time results in the gradual lowering of the water table or the piezometric surface.

Springs:

That part of underground water which comes out on the ground from some natural void, is called a spring. Springs move according to the structure of layers of rocks. Rain water infiltrates through permeable rocks under the ground but when non-permeable rocks come in the way, that water cannot move and it starts flowing upwards by the side of non-permeable rocks. This water appears on the surface of the earth in the form of springs. In hilly or sloppy areas, this collected water sprouts out from raised permeable rocks in the form of a water source on the surface of the earth.

On the basis of permanency of water, these springs are divided into two categories:

1. Permanent Springs:

When non-permeable rocks below the surface of the earth are in inverted position and the water table is high, groundwater flows out permanently due to availability of slope.

2. Temporary Springs:

These springs exist due to unequal water level below the surface of the earth and they depend on rainfall.

Based on nature of water, springs are of following types:

1. Common Springs:

These water sources are of normal depth which give fresh, cold and potable drinking water.

2. Thermal Springs:

They are deeper in size which emit out hot water. Such sources are found in California Park.

3. Warm Springs:

Temperature of these springs is even higher than boiling point because they are very deep. About 4,000 such springs are located at Kulu and Manikarn in Himachal Pradesh (India) as well as Yellowstone National Park of United States of America.

4. Mineral Springs:

These sources are mainly found in volcanic areas from where water mixed with minerals like sulphur, salt, etc. comes out, is of medicinal value. Sahstradhara near Dehradun in Uttaranchal, Atari in Orissa and Chhindwara in Madhya Pradesh are such sources of water.

Springs are an interesting feature. The groundwater discharge through springs occurs in areas where the upper surface of the zone of saturation intersects the ground surface. Springs are different from seepage. Springs appear in the form of concentrated discharge of groundwater.

A seepage area on the other hand, is the slower movement of groundwater of the surface. K. Bryan has classified springs into several categories. First is the ‘Depression Spring’ in which the groundwater flows to the surface from a permeable aquifer. Then there is the ‘Contact Spring’ that forms where the contact plane between a permeable and impermeable rock intersects the ground surface in such a way that the groundwater is deflected to the surface.

The Contact Spring is found at the foot of limestone or chalk escarpment, foot of a scree slope and also along a fault line. ‘Fracture Springs’ occur when a system of interconnected minor faults lead the groundwater to the surface. Another type is the ‘Tubular Spring’ in which groundwater moves through lava tubes or interstices in solution enlarged in limestone. And, last but not the least, is the spring which arises from hydrostatic pressure.

Well:

To get water from the collected water area located below the surface of the earth, wells are dug in permeable rocks below which, due to non-permeable rocks it is full of water. Wells based on rains are less deep and they become dry in summer. Deeper wells are helpful for water supply for a longer period.

Artesian Well:

Artesian wells are natural water sources from which water automati­cally appears on the surface of the earth. For their construction, two types of favourable structures are required. First, a synclinal or dome shaped inverse structure, and second, a Monocline structure. The region from where the water of artesian wells moves upwards, is called aquifer.

There should be non-permeable rocks, below as well as above aquifer, whose nature is similar to an aquiclude. Its lower level prevents seepage of water, whereas the upper level prevents water from evaporation. The part of the aquifer opening on the surface is called drainage area. From where the water entering through the permeable layer, reaches the central portion.

By, and by as soon as the permeable layer is full, the ‘Hydrostatic Pressure Head’ is formed. Now, if a well is dug from the surface to the depth of the aquifer, then water of the aquifer would appear automatically on the surface as water source due to hydrostatic pressure. This forms an artesian well.

Geysers:

In the language of Iceland, ‘Gosir’ means ‘Sponter’. According to Holmes, “Geysers are sources of hot water from which after some intervals hot water and vapour come out with speed.” Temperature of such water remains up to 75°C to 90°C. There are 100 geysers in the world including ‘Old Faithful’ in the world famous Yellowstone National Park situated in Wyoming the state of United States of America. Many geysers are found in Iceland and New Zealand.

3. Surface Water:

Water on the surface of the earth is found both in fixed as well as moving forms. Water is found in different shapes on the basis of form. On high mountainous areas it is found in the form of snow caps and icebergs, and in lower parts it is found in liquid form. Surface water is distributed on different continental parts.

More than two-third part of fresh water is found in the form of snow Antarctica, and that is why the geographical position of the continent is different among the seven continents of the world. On the other hand, there is no permanent snow area in Australia.

Except Antarctica and Greenland, snow is found in other places which are above the snow line. The height of the snowline is different at different places. It is 5,000 to 6,000 metres at the equator, 4,300 to 5,300 metres at the Himalayas and 666 metres at South Chile and South Greenland.

Thus, water on earth is distributed in different forms, the description of which is as follows:

Snow Fields:

Above the snowline that part is called snow area where snow remains permanently frozen. Such parts are called ‘Permanently Snow Fields or Permanent Frost’. Except the continent of Australia, permanent snow fields are found on all the remaining continents of the earth.

At present, 24 per cent area of the whole earth is permanently snow covered area which extends to about 15 million sq kms. Ninety-six per cent of glaciated ice cover is found in thick masses in Antarctica and Greenland. All these areas are spread over the Arctic Ocean and the Antarctic continent, where polar type climate (E type climatic group presented by Koppen) is found.

Summer temperature here remains below 10°C, which is the hottest month here. The coldest and hardest snow covers are in parts beyond the Tundra, which are called permanent snow caps. Being located in Polar parts, they are also called ‘Polar Ice Caps.’ The Arctic portion is called Internal of Greenland. Arctic and Antarctic regions are included in it. Regional snow cover here is visible in the form of mountains which are called ‘Nunatak’.

The Arctic:

This region includes Arctic Ocean and the nearby areas in which the extended northern part of Canada, Russia, Greenland, Scandinavia, Iceland and Alaska are the main areas. Permanent snow cover or pack ice is found here. Icebergs break from glaciers and the reach the sea, which float at 100 metres above sea level. In April 1912, The Titanic ship of United States of America had collided with such a pack ice and drowned.

Antarctica:

This is the fifth great continent of the world which is situated in the midst of the Antarctic circle. It is the coldest continent of the world. The lowest temperature was 88.3°C as recorded at Vostok station on 24th August 1960. About 95 per cent of its part is snow covered which is about a 2,000 metre thick layer.

Maximum thickness of snow is 4270 metres. Ninety per cent of the total fresh water resources of the world are situated in the Antarctica continent in the form of snow. Due to global warming, this snow is slowly melting, which will bring changes in the sea level also. Antarctica plays an important role in regulating world climate and oceanic ecology system where first forest life habitation is found on the earth.

Glaciers:

Above the snow line, there is located a vast mass of ice which is called glacier, which moves due to gravitational force. The movement remains in the snow area on high latitude and high mountainous parts. Glaciers are formed due to accumulation and compaction of ice above snow line. Glaciers of all types on the whole earth have been estimated to be between 70,000 to 2, 00,000.

Types of Glacier:

Glaciers are of different types according to process of formation, size, form, situation and extension.

As per size and location glaciers are of bur types:

1. Ice Cap Glaciers

2. Alpine or Valley Glaciers

3. Piedmont Glaciers

4. Continental Glaciers

1. Ice Cap Glaciers:

These are combinations of Ice Cap Glaciers, Alpine Glaciers and Continental Glaciers, which are encircled by mountains and high plateaus. Alpine glaciers flow on steep slopes. They are also called mini continental glaciers. Snow lumps extending for less than 50,000 sq. km are called snowcaps. ‘Svalbard (Spits bergan) Island’ of Norway is such a type of glacier.

2. Alpine or Valley Glaciers:

In mountainous regions, when Tee Cap Glaciers’ move to lower slopes because of gravitational force, they are called ‘Valley Glaciers’. Because its study was done first on Alps Mountain, it is also called ‘Alpine Glacier’. Such glaciers are found in abundance in the Himalayas (Asia) and the Rocky Mountains (North America).

These glaciers become narrower as and when they flow downwards. Valley Glaciers are found above the snow line. Valley Glaciers start with accumu­lation of snow at the peak of the valley in Cirque that coalesce into glaciers by and by.

3. Piedmont Glaciers:

They are extended Glacier sheets, which are formed at the foot of mountains by merging and coalescence of Alpine Glaciers. Such types of Glaciers are found specially in Alaska. Malaspina is the main Piedmont Glacier of Alaska which is spread over in 3,900 sq. km.

4. Continental Glaciers:

They are vast covers of dense snow, which encircle the entire ground surface of their area. They develop as an extended sheet due to continuous accumulation of snow in a vast area. They are also called ‘Ice Sheet’. In Antarctica, there is an extension of 13, 000, 00 sq. km of Conti­nental Glaciers. An area of 18, 00, 00 sq. km in Greenland is also covered by Ice Sheet. Near Gulf of James, the thickness of ice is 3,000 metres.

The above four types of glaciers that have been described above are normally classified. The remaining minor glaciers are described below:

1. Ice Sheet:

Extension of Ice Sheet is in more than 50,000 sq km area. It is scattered in a leveled dome shape and formed by accumulation of ice. At present, there are two wide Ice Sheets, namely Greenland Ice Sheet and Antarctica Ice Sheet, where 11 and 85 per cent ice is situated respectively. Apart from these many minor ice sheets exist in Arctic Canada, Iceland and Norway.

2. Ice Cap:

It is a small ice sheet which extends to less than 50,000 sq. km area. It is a permanent ice mass that is found in high mountains and high altitude. Burnace Ice Cap and Baffin Ice Cap are the main Ice Caps.

3. Ice Dome:

This is the central portion of ice sheet and ice cap.

4. Outlet Glacier:

Outlet Glacier is a stream which flows from ice cap and ice sheet. Subsequently, it merges with a Valley Glacier.

5. Ice Shelf:

This is a floating ice sheet adjacent to coastal parts. It flows independently without any friction with base level.

6. Ice Field:

This is a mass of pack ice or a wide area of sea ice which is specially found in the polar areas.

7. Cirque Glacier:

Cirque Glacier is a part of minor ice.

Surface Water Bodies as Snow Fields:

Except liquid water found in rivers and lakes on the ground, a large water deposit are also found on the ground in the form of snow. They are found in the Arctic area of the North Pole, and Greenland and Antarctica in the South Pole. Apart from them ice caps on high mountains and glaciers are also important water deposits. Ground areas of Canada, Russia, Greenland, Scandinavia, Iceland and Alaska in the Arctic water region are included in it.

Among them, Greenland is the biggest snow-covered area. Antarctica continent in southern hemisphere is the fifth biggest continent where 2,000 metre thick snow-cover exists in 95 per cent of its region on an average. About 90 per cent fresh water of the earth is found here but due to increase in world temperature, this snow is melting.

Apart from Arctic and Antarctic, some quantity of water is also found in the form of ice caps and glaciers on Himalayas, Alps, Rocky, Andes, Kilimanjaro and a few other mountains. These ice deposits become source place of many rivers and help in supply of water.

It is clear from the geographical description of the interrela­tionship between hydrosphere and the process of hydrological cycle that out of the water found in nature only a limited part becomes available for use in the biosphere because its greater part is saline and 80 per cent of total water is frozen as snow. In present times, when the population is growing fast, there is a increasing demand for water on one side while on the other side, there is deterioration in the quantity and quality of water.

Glaciers that contain a large part of the fresh water deposit of the world are melting due to rise in temperature of the world and these glaciers are directly or indirectly connected with seas. The snow of Antarctica is melting. At a conference held in March 2002, scientists of London reported that Larsen-B glacier linked with Antarctica since past, which was 1,500 years old and vast snow deposit spread over in 1,250 sq miles area had broken, in addition people have made rivers, lakes and oceans as places for disposal of waste.

Continuously increasing agricultural area and consequent excessive exploitation of groundwater for irrigation have resulted in acute water crisis. It is clear that looking at the disturbing water balance and deterioration in quality of water, a strategy for its management has to be worked out so that water is able to maintain its balance in nature.

Rivers:

Flowing water deposit in any definite way on the surface of the earth is called a river. Rain water flows on the earth in different forms which is called run-off. When this same water starts flowing from a higher plane to a lower plane in a definite sequence due to gravita­tional force, it takes the shape of a river. Its development takes place by and by. First of all, the rain water flows in the form of sheet flow.

While flowing in the form of sheet flow, it follows the slope very soon and flows in the form of a rill. When rills combine and flow together, they become a gully. When gully becomes deeper, it becomes a rivulet and subsequently with combination of many rivulets, a river is formed.

The source of rivers are spring, snout of a glacier, water collected from rain on the earth, lakes etc. Slope of the ground, rock structure and climate decide the nature and form of rivers. Rivers occupy an important place in the ground level water sources.

They originate from high mountain areas, snow covered peaks, lakes etc. and flow in speed over ground level. Except independent flow, they also receive interflow at many places like water received from percolation, groundwater springs, natural soil lines and infiltration.

Thus, many streams combine to take the shape of a river and merge into a sea at the end. Many rivers also fall into lakes, for example, Volga and Ural rivers fall into the Caspian Sea, whereas some rivers are also inland. Ghaghar River emanating from the Himalayas vanishes in the Thar Desert. Area of drainage basin, shape of drainage basin, total rainfall, and intensity of rainfall and surface of drainage basin control the flow of rivers.

Lakes:

Lakes are important water deposits in which 0.017 per cent part of water on earth is stored. Lakes are of different kinds according to nature and quality of water. Among them non-saline and fresh water lakes, saline or saltish lakes and snow lakes are important.

According to location, there are mountain lakes, plateau lakes and lakes of plain area. Lakes like Ladoga, Onega, Aral Sea and Baikal etc. are of plain area in Western Russia, whereas Kokonor (Ching Hai) of Central Asia, Tania of Africa, Teetikaka of South America, and Mansarovar Lake of Tibet are mountain lakes.

Seventy-five per cent water out of the fresh water of the lakes of the world is found in great lakes of North America and Baikal Lake. Caspian Sea is also a closed lake where water is said to have been stationary for the last 200 years. Its main reason is the fast rate of evaporation.

Lakes are formerly glaciers, action of volcanoes and destruction etc. Rudolf Lake in the Rift valley of Africa, Albert, Edward and Tanganyika are the longest lakes, and Baikal is the deepest fresh water lake. Caspian Sea is the longest lake of saline water. Sambhar and Chilka are vast saline lakes of India. Lunar lake of Maharashtra is a crater lake.

Great Lakes:

The series of five lakes of North America is known as ‘Great Lakes’ they are located in the United States of America and Canada. These lakes are Superior, Michigan, Huron, Erie and Ontario. All the five lakes are spread over as inter-connected. Superior is the greatest and Ontario is the smallest of these lakes.

Great lakes are spread over an area of 2, 44,650 sq. km. Fresh water is found in these lakes. Lake Superior is the biggest fresh water lake of the world whose area is 82,350 sq. km. It is spread in Minnesota, Wisconsin and Michigan States of the United States of America over 53,000 sq. km. The remaining area of this lake is in Canada.

Geographers opine that the formation of these Great Lakes about 15,000 years old was due to melting of glaciers. The size of these lakes has changed due to many sudden activities on the earth. Presently, these lakes are used mostly for internal water transport. So canal links Lake Superior and Huron, whereas Wallend canal is in the middle of Ontario Lake. Many ports and cities have developed along the banks of these lakes. Due to availability of water and transport, steel manufacturing centers have grown around them.

Buffalo, Cleveland and Detroit on the banks of Erie Lake, Chicago, Gary etc. on the bank of Michigan Lake and Milwaukee port on Ontario have developed as important cities. Toronto ports etc are on the bank of Lake Superior. Duluth and Minnesota are main steel manufacturing centers.

Atmosphere:

About 0.001 per cent water out of the total water found on the earth and 0.04 per cent of the total available fresh water (2.26% of the total water) exists in the atmosphere. It has been estimated that if total water in the atmosphere condenses and falls on earth in the form of rain, a layer of 2 mm will collect on earth.

This water is equal to the water available in the rivers of the world. It has become clear from different estimates that duration of water in the atmosphere is about 10 days. On this basis, atmospheric humidity exchanges it with ground level about 40 times.

Due to this deficiency, atmosphere remains in mobile position, due to which it remains as an extended linkage with other hydrological systems. In the process of hydro- logical cycle of the world, atmosphere functions as an important link.

Sub-Surface Water:

Minerals and rock pieces scattered over the upper part of the ground surface of the earth in an unorganized group is called soil. Soil includes fine clay, salt granules and sand particles. Many times pebbles and boulders are also mixed in it. Soil scientists have considered a few feet thick surface of the ground as soil from the point of view of agriculture.

The United States Agriculture Department has decided the maximum depth of soil as 1.5 metres (5 feet) in the soil map. In this soil, both the forms of water, namely, sub-surface water and ground­water are found. From the geological point of view, based on availability of water, both these forms of water are different because both are found in different zones.

Groundwater zone is also called saturated zone because groundwater fills all the voids of rocks as well as soil. Upper portion of saturated zone is called water table. It is clear that soil water is found in un-saturated zone which is situated above the water table. It is also called Vadoz zone. Here water is filled in some pores of the soil, whereas air is filled in other pores.

Soil water is found in unsaturated zone. This unsaturated zone performs two types of functions. Firstly, it serves as a storage reservoir for soil water and secondly, it moves water downwards for groundwater and evaporation from surface to atmosphere.

It is thus clear that the geological structure of unsaturated zone affects the quantity of groundwater as well as soil water. But, from here, both upward as well as downward movement affects the quantity of soil water. Movement of the water on surface as well as in the saturated zone happens on account of gravitational force. From the surface, water moves downwards through the process of infiltration. Infil­tration is the flow of water from surface to the soil below.

Process of Infiltration:

Below the surface of the soil, permeability of water through rock granules or pores is called infiltration. Many a times, it is presumed to be a synonym for seepage. Seepage is slow flow of water on the surface of soil through some other pore. Infiltration can be slow as well as fast.

When rain first falls on dry soil, water enters the soil by molecular forces. First, the water makes soil granules wet along-with mineral surface through hydrogen bound. Subsequently, it reaches the minor pores of the soil.

Rate of infiltration depends on viscosity of water and perme­ability of soil. As viscosity increases (with decrease in temperature), water starts flowing in slow speed. It is clear that with increase in temperature, rate of infiltration is higher and it is less with decrease in temperature. Permeability is that quality of porous materials, which regulates flow of liquid in any material. Water can flow with speed in sand as compared to clay because sand is more permeable than clay.

Effect of Rainfall Intensity on Infiltration:

Dry soil immediately soaks drops of water as soon as it rains. During normal rain, rate of infiltration is high and sufficient rain water enters the soil. But in case of intense rainfall, effect of drops of water specially on uncovered soil, is not good.

When intense rain drops fall on naked soil, soil granules rearrange themselves and infiltration of water in soil does not taken place. Consequently, water collects on the surface and starts flowing on the surface. Flow of water also expedites soil erosion. Apart from intensity of rain, quality of water also affects soil. After weathering action on the surface, many minerals mix in the soil and along with water they prevent the affect of the downward flow.

Soil Water:

Soil water is that part of sub-surface water which is found in the unsat­urated zone between ground level and water table. It enters the soil through the process of infiltration. After entering soil this water either is stored there or comes upwards through evaporation or transpiration. Apart from evaporation or transpiration, it can also flow towards the saturated zone. During intense rain or storm, sub-surface water flows through inter-flow also.

Horizontal flow of water immediately below soil surface is called inter-flow. But inter-flow is not really soil water. Soil water is an important constituent of hydrological cycle which is helpful for vegetation on ground level and increases the quantity of groundwater below ground level. Unsaturated zone is found in the form of storage for soil water.

The water existing in the pores or voids or open spaces is called soil water. The greater portion of soil water enters by infiltration of rain water. Use of such water is made by plants through root osmosis. From the view point of water-holding capacity, two aspects of soil are important.

One aspect is porosity and other aspect is permeability. Porous and permeable soil immediately absorbs rain water and becomes dry early whereas, compact soil has less porosity because its soil granules are very near to each other. Water is a universal solvent which, after dissolving necessary nutritious substances, supplies them to different parts of the plants according to requirement.

Famous soil scientist Briggs has classified soil water in three categories:

1. Gravitational Water

2. Hygroscopic Water

3. Capillary Water

1. Gravitational Water:

Water is found in the soil in different forms. When percolation of water takes place into the soil due to gravitational force, it is called Gravitational Water or Independent Water. Water is filled to a certain level, after which its percolation is not possible.

This level is called Zone of Saturation. Quantity of gravitational water in the soil is related to many situations of which quantum of rainfall and place of storage of water are important. Gravitational water works inside the soil in different forms. Firstly, it affects the soil colour, structure and composition.

As soon as gravita­tional water enters and moves into the soil, it takes the fine granules of soil together with it. This it is called ‘eluviation’ of particles of upper soil. After the water collects clay granules of sub-soil along with it while vibrating. This process of soil is called ‘illuviation’. Thus gravitational water is also an agent for transfer and mixing of soil granules from one layer to another layer.

Its effect is that the upper soil gets filled with bold particles because the finer granules are trans­ferred to the lower soil after dissolving with water. Due to this, the upper soil becomes more compact and of higher density than the lower soil.

Gravitational water, through affecting chemical composition of the soil, also affects composition, colour and structure of the soil. Thus gravitational water while moving downwards mixes the nutrients of upper soil, in the lower soil at the depth after dissolving. Taking nutrients of upper soil towards the lower soil is called ‘leaching’. Thus, problem of water logging is also created by over saturation of the soil with water.

2. Hygroscopic Water:

This water is received by dry soil through absorption of atmospheric water vapour. If dry soil is heated at 105°C for about 8-12 hours, some part of water would go out from the soil. This minimum quantity of water is called Hygroscopic Water.

This water is in the form of a thin film which remains around soil granules or living organisms by adhesion force. This water destroys evaporation and there is no movement in it. It is also called waste water because living organisms of soil cannot utilize it. The structure of soil, quantity of clay material, humidity and temperature determine its quantity. Clay contains more hygroscopic water as compared to sand. It increases with increase in humidity and reduces with increase in temperature.

3. Capillary Water:

Capillary water is attached to soil granules in the form of a thin film on all sides. Such water forms a ring around soil granules.

The size of this ring is affected by humidity because when the quantity of humidity increases, its size also goes on increasing. When this layer of water amalgamates with the nearby soil granules, most of the voids are filled with water, whereas in the remaining voids, air exists. Capillary water is retained by surface tension, and like hygro­scopic water, it does not stick to soil granules but it maintains stiffness to the extent that gravitational force of earth cannot separate it.

Atoms of capillary water are independent, mobile and in liquid form. Due to this reason, it evaporates easily. Capillary water is also known as ‘easy water’, because in spite of being stuck to soil granules, trees and plants can utilize it.

Its quantity is decided by the following factors:

1. Structure of the Soil:

Structure of the soil affects capillary water’s retaining capacity because if it is sandy soil, voids and pores are not monocular in it and capillary water is found in little quantity, whereas in clay due to monocular pores, water retaining capacity is greater.

2. Organic Matter:

Any soil having more organic matter will have more water retaining capacity, because humus itself has more capillary water retaining capacity.

3. Surface Tension:

Capillary water increases with increase in surface tension and reduces with increase in temperature.

4. Size of Soil Particles:

If soil granules are more fine, clay will have more capillary water retaining capacity, whereas bold soil granules have less capillary water retaining capacity.

Thus, capillary water is affected by the above factors and maximum quantity of capillary water retaining capacity develops. This can be called maximum capillary water retaining capacity.

Factors Affecting Soil Water:

1. Soil Texture:

Flow of water is affected by size of soil granules. Water can infiltrate easily in sandy soil whereas, it cannot do so in clay soil. Clay soil absorbs more hygroscopic water. If soil particles are finer, capillary water would move faster. Gravita­tional water can flow in soil having bold particles.

2. Humidity:

With increase of humidity in atmosphere, absorption of hygroscopic water capacity increases. Its quantity is more in monsoon season and less in dry days.

3. Rainfall and Temperature:

Intensity of rainfall also affects soils. If rainfall is in slow speed, capillary water can be used by plants but with more intensity of rain, water moves very fast to the lower layers of soil by gravitational force and cannot be used by plants.

4. Soil Structure:

Soil structure plays an important role in vibration of water in the soil. If soil is more compact, infiltration of water would be slower, whereas in loose soil, water would enter with faster speed. Thus, there is lesser infiltration in soil with fine granules.

Thus, besides the above factors, ice, dew and irrigation also affect soil water. Nature of soil water is changeable. Whenever it is used by plants for transpiration, its place is occupied by air. Thus, soil water and air both are interchangeable.