WATER RESOURCES The various sources of water can be divided into the following two categories:- (A) Surface sources such as (i) Ponds and lakes (ii) Streams and rivers (iii) Storage resources (dams) (B) Subsurface or underground sources such as (i) Springs (ii) Wells (open and tube-wells)

All of the above water resources are replenished by precipitation in various forms like rain, snow, hail, dew etc. Rainfall is the main source and forms the major constituent of the hydrological cycle. Water gets evaporated from the various exposed bodies of water depending upon the climatic factors like temperature, R.H. and the wind speed.

RH or the relative humidity is the ratio of the amount of water vapour present in the air to the maximum amount of vapour that air can hold at that given temperature. As the temperature goes up, the moisture retaining capacity increases and as the temperature goes down, it reduces.

That is the reason, why dew is formed at night when the temperature of the air falls down, and at that reduced temperature, the extra moisture (more than its capacity at that temperature comes out and is condensed in the form of dew). So if, for example, air can hold 3 gms of water vapour per Kg of air at 20°C and the moisture present in any sample of air (at this temperature) is 1.5 gms per Kg of air, then its RH will be- 50%. Lesser the RH of the air, higher the rate of evaporation as there is more deficits to be met with. Similarly, more is the difference of temperature (air and water) higher will be the rate of evaporation. Same is true with wind speed. Like this, depending upon the climatic factors, evaporation shall take place.

Rainfall The water vapours get collected in the atmosphere and behave like a gas and obey the various gas laws (Boyle‘s, Charle‘s etc.) under normal conditions. The concentration of water vapours keeps on increasing and when it goes beyond the maximum moisture retaining capacity of air at that temperature, it comes out in the form of precipitation. Generally, it happens when air is cooled (hence the water retaining capacity is reduced ).

There are three different methods by which the air mass gets lifted and gets cooled to cause precipitation, generally in the form of rain and sometimes as snow. 1.Cyclonic precipitation is by the lifting of an air mass due to pressure difference and consequent horizontal flow from the surrounding area. 2.Connective precipitation is due to the upward movement of the air that is warmer than its surroundings. 3.Orographic precipitation is the main cause of precipitation in India. Winds heavily laiden with moisture (monsoon winds) from the Bay of Bengal strike the southern slope of Himalayas, causing intense rains.

SURFACE SOURCES OF WATER SUPPLY These are the sources in which the water flows over the surface of the earth and is directly available as raw water like lakes, rivers or impounding reservoirs. Ponds and lakes are suitable for relatively small water supply schemes. Direct withdrawal from rivers may not be possible throughout the year, so it is best to construct a dam or barrage (any solid obstruction in the flow of river) and then use the water stored in the created reservoir. Generally a dam is constructed for generation of hydroelectricity, irrigation and a measure against floods and draughts. So it is a multipurpose project and water supply is a small part of the project. Actually, the total yearly demand of potable water (drinking water) is very less (say 1% of the water required for irrigation purpose). So the dams (reservoirs), canals and other systems mainly designed for irrigation are simultaneously used for water supply.

The ground water is deep at places of rainfall. Generally, the salts available in the soil gets dissolved in the ground water and they make it unfit for human consumption. Making water free of dissolved solids is very costly. So it is wise to construct some obstruction in uselessly flowing water to make it useful. Otherwise, the water will either ultimately flow into the seas and oceans, or will evaporate into the atmosphere. This is known as water management.

For making a permanent surface source for water supply a suitable type of dam is constructed which may be any one of the following: - (i)Earth Dam: An earth dam is made of earth (or soil). It resists the forces exerted upon it mainly due to shear strength of the soil. When the valley is wide and foundation is less strong, good soil is available; earth dam is a good option as it is more environments friendly. (ii) Gravity Dam: A gravity dam is made of stone and concrete, and resists the water pressure, uplift pressure, wind pressure etc. due to its weight. They are more suitable for narrow valleys with steep side slopes and strong foundation because of more weight. They are strong and costlier than earth dam. (iii) Rock fill Dam: A rock fill dam is like a combination of the two mentioned above. The inner segment consists of rock pieces, boulders etc. with an impervious outside membrane. They are less strong than the gravity dam and used for limited heights only, particularly when the rock pieces are abundantly available.

SUBSURFACE OR UNDERGROUND SOURCES The rainwater that gets infiltered and percolated inside the earth to form a uniform water surface is known as the ground water. This water is comparatively pure because of the natural filtration as it passes through the soil. Many impurities are retained by the soil through the actions like screening, sedimentation, adsorption etc. But it dissolves many soluble salts as it passes through the soil containing those salts. So the total dissolved solids of the underground water are much more than the surface water. The flow of rain water from surface of earth to the underground depends upon the porosity of the soil, the rate of water loss by evaporation, seepage to surface sources and withdrawal by us.

GROUND WATER YIELD The ground water is present in the water bearing stratum. Those soil formations through which it can be easily and economically extracted is known as aquifers. The top layer of ground water is known as water table. The ground water may come out by itself either through springs but generally extracted out through the wells. Larger diameter wells are known as open wells whereas the smaller diameter wells (30 cm or so) are known as tube- wells. Tube-wells are drilled by machines and fitted with blank or perforated pipes (strainers to draw water).

BASICS OF THE WATER SUPPLY SCHEMES Actually wherever a water supply scheme is framed (scheme means a system to draw water from suitable source, treat it and then supply it to the consumers) a comparison is made between all the possible solutions. Preference is given to the nearest possible source as the conveyance of water is very costly. The underground water is generally pure (from suspended impurities point of view because of natural filtration) but contains more dissolved salts. The lifting of water (pumping out from wells) also requires energy (electricity) whereas the filtration of surface water is a costly affair. So the environmental engineers in the public health engineering departments (water works) make schemes (plans) to supply potable (fit for drinking from all points of view, i.e. clarity, dissolved salts, and free from bacteria etc.) water to the consumers. The schemes are basically of two types, rural water supply schemes and urban water supply schemes depending upon the population of the habitation.

Characteristics of Water To ensure the quality of safe drinking water (potable water) the water is to be tested for its physical, chemical and bacteriological characteristics. Physical Characteristics: (i)Temperature: It can be measured by a thermometer. The temperature should be suitable to the human beings depending upon the climatic and weather conditions. An average temperature of 15°C is generally suitable. (ii) Turbidity: The muddy or cloudy appearance of clay or such other particles that presents hindrance to the path of light is known as turbidity. It may not be harmful but even then from aesthetical point of view it should not exceed the allowable value. The turbidity is measured by a turbidity rod or turbidity meter with physical observations and is expressed as the suspended matter in mg/l or ppm (part per million). The standard unit of turbidity is that which is produced by 1 mg of finely divided silica in one litre of distilled water. The turbidity in excess of 5 mg/l is detectable by the consumer and is hence objectionable.

(iii) Colour: The colour is imparted by dissolved organic matters from decaying vegetation or some inorganic materials such as coloured soils (red soil) etc. The algae or other aquatic plants may also impart colour. it is more objectionable from aesthetics point of view than the health. The standard unit of colour is that which is produced by one milligram of platinum cobalt dissolved in one liter of distilled water. Colour is measured in the labs by Nessler’s tubes by comparing the sample with the known colour intensities. (iv) Taste and odour: The dissolved inorganic salts or organic matter or the dissolved gases may impart taste and odour to the water. The water must not contain any undesirable or objectionable taste or odour. The extent of taste or odour is measured by a term called odour intensity which is related with threshold odour, which represents the dilution ratio at which the odour is hardly detectible. For domestic water supplies the water should be free from any taste and odour so the threshold number should be 1 and should not exceed 3. (v) Specific conductivity of water: The specific conductivity of water is determined by means of a portable dionic water tester and is expressed as micromhos per cm at 25°C. Mho is the unit of conductivity and is equal to 1 amper/ 1 volt). The specific conductivity is multiplied by a co-efficient (generally 0.65) so as to directly obtain the dissolved salt content in ppm.

( b) Chemical Characteristics (i)Total solids and suspended solids: The total amount of solids can be determined by evaporating a measured sample of water and weighing the dry residue left. The suspended solids can be determined by filtering the water sample and weighing the residue left on the filter paper. The difference between the total solids and the suspended solids will be the dissolved solids. (ii) pH of water: pH is the negative logarithm of hydrogen ion concentration present in water. The higher values of pH mean lower hydrogen ion concentrations and thus represent alkaline water and vice versa. (iii) Hardness of water: Hardness in water prevents the formation of sufficient foam when used with soap. It is caused by certain dissolved salts of calcium and magnesium which form scum with soap and reduce the formation of foam which helps in removing the dirt from clothes.

(iv) Chlorides: Chlorides are generally present in water in the form of sodium chloride and their concentration above 250 mg/l produces a salty taste in drinking water. (v) Nitrogen content: The nitrogen in water may occur in one or more forms of the following: (a) Free ammonia (b) Nitrites (c) Nitrates

vi) Metals and other chemical substances: Various metals and minerals may be present in water like iron, manganese, copper, lead, cadmium, arsenic, barium, selenium, etc. If the concentration of these metals and minerals exceeds the permissible limits they have certain harmful effects on the human health. (vii) Dissolved gases: Various gases like CO 2, O 2, N 2, H 2 S and CH 4 etc. may be present in dissolved form in water. H 2 S even in small concentration gives bad taste and odour. CO 2 indicates biological activity.