Category Hydrology

Is rain water hundred per cent pure? Can we use it as drinking water?

    All the natural water contains dissolved ions (positive valency anions and negative valency captions) within them.

            Rain water which is formed due to natural distillation process (i.e.) evaporation, condensation and precipitation. It is supposed to be in a very dilute solution with very less dissolved solids generally between 10-20mg/lit.

            All the dissolved ions present in them are in the range of 1-3 mg/lit. So it is considered to be pure and useful for drinking purposes. Moreover their PH is between 6.7 and 7.2.for consumption purposes. But the concentration varies from region to region.

            Near the coastal area due to sea spray the ions is higher, if more suspended particles in the air, they react with water molecules to give more dissolved ions.

            If the air is full of Carbon dioxide, Nitrogen dioxide and Sulphur dioxide, then they all dissolve in water to form an acid rain, which will cause more harm to vegetation an its PH is reduced.

            Pure rain water does not contain enough dissolved minerals and ions needed for the body. So after suitable modification and alteration the rain water can be used for consumption.

Is there a simple way to check the purity and genuineness of mineral water available in the market?

The Bureau of Indian Standards has prescribed the specifications for potable/drinking water.

            The mineral water is supposed to contain naturally occurring minerals within the permissible limits of potable water. Even though there are about 30 parameters which have to be checked, the most critical is the biological contamination of E. Coli bacteria. There are various methods for the identification of E. Coli, but it will take a minimum of 18 to 30 hours to know the result.

            Two ready kits are available wherein you have to just add 50 ml of water sample to a culture media (premixed chemicals). One such kit is developed by the Defence Food Research Laboratory (DFRL), Mysore, and the other by E. Merck, Germany. With these kits the results can be known in 18 hours.

            It is suggested that the Government should check all the parameters of mineral water available in the market on a random basis, as in the case of liquor, as testing at the consumer end is time consuming and costly.

            Dissolved salts in water affect its electrical conductivity and so a quick test to find the purity of mineral water would be to measure the conductivity.

            A number of conductivity meters are available in the market and the best one would be that with a conductivity range of 0-300 micro mhos per cm.

            The test can give quick results: if the water’s conductivity is around 200 micro mhos/cm, then the total dissolved solids (TDS) is about 100 ppm, if it is 250 micro mhos/cm then the TDS level is 125 ppm.

            These figures may be compared with the TDS values printed on the bottle’s label by the manufacturers.

            This will test for the amount of dissolved salts only and not for other contaminations such as by bacteria.

Why do water drops dance when poured on a hot plate?

    When a drop of water (at room temperature, 30 degrees centigrade) falls on a hot plate (at a temperature of say 150-200 degrees centigrade), the water begins to ‘dance’ around on the plate because of the formation of a layer of steam, at the interface of the drop and the late, which acts as a cushion. When the drop touches the plate, the bottom layer of water in the drop is suddenly heated to more than 100 degrees centigrade leading to the formation of steam at the interface.

            Steam not being a good conductor of heat, does not allow the plate’s heat to flow easily into the drop. So, as we move up from that interface, into the drop, the temperature falls down.

            The layer immediately above the steam will be of boiling water. In the outer top layer, called the free convection zone, water is heated slightly. At any point of time, the water tries to rest on the plate but the steam holds it up by acting like a cushion.

            As a part f the steam leaks out sideways, certain amount of water flows in to fill gap and gets vaporized. This process continues till the water drop is completely vaporized or the hot plate is cooled to below 100 degrees centigrade.

How dowels in the beach yield good drinking water?

    Beach sediments are generally enriches with sand and salt fractions which have a high porosity and permeability. When there is rainfall, fresh rain water percolates via this porous medium, saturate the sand medium and form fresh ground water table. The reasons for getting fresh water on the beach is that fresh water has a lower density (1000g/cm) than sea water and tends to float over the saline water (1.0255g/cm).

How is quantity of water in dams and reservoirs measured?

In a reservoir water is stored in an irregular shaped geological formation. The volume of such a formation is arrived with the help of contours and linear approximation formulae.

Contour is an imaginary line at the same elevation. Numbers of such contours are drawn before the formation of reservoir with the help of surveying techniques. The height difference between two such adjacent contours is called as contour interval. Area of two adjacent contours are arrived with linear approximation is or by an instrument called Plani Meter. By knowing the areas and their height difference (contour-interval) the volume is arrived at. The reservoir formation can be assumed as lot many of such segments with a contour on either side.

For example, if we consider the datum (bed formation) as +100m, and the reservoir level (say full reservoir level, FRL) is +110m and if the contour interval considered is 1m, we have 10 contour areas at +100m +101m, +102m… +110m. Volumetric measurement used to quantify the water storage in reservoir is TMC (thousand million cubic metres). These calculations are represented graphically as, water capacity versus reservoir elevation, commonly called as reservoir capacity curves. With such graphs, volume of water at any fractional elevation may also be obtained. Another point needing exposure here is with the increasing life of a reservoir. Flood brought silt may accumulate in the reservoir bed and reduces the reservoir capacity. This can be ascertained with the sounding techniques.








The quantities of water in dams or reservoirs are estimated by a survey method called contour survey, by the engineering surveyors. This ‘quantity survey’ is actually done prior to the reservoir construction. It can also be estimated while water is present.

In survey, the levels of the bed of reservoir are taken by leveling instruments and contours are drawn. This is called contour map. A contour is an imaginary line representing equal elevation points. The area enclosed by a contour is flat. This is simply true in the case of reservoir. Two successive contour lines at one foot interval provide the area of water at top and bottom for each segment or slice (or peel). The volume of each segment is the average area multiplied by the difference of contour height which is one foot. All volume of segments are added to get the total volume of water. This is how the volume is computed prior to reservoir construction or when dry.

            When water is present, the bed levels have to be taken under water. To get the bad level, equipment called Echo sounder is used. To know the point at which bed level is taken electronic equipment called total station is employed. Earlier a couple of Theodolites were used. A boat with Echo Sounder is navigated in water on predetermined parallel lines at specific intervals of 10 metre. Knowing the depth and location of each point of bed, levels are calculated and contour map is drawn. The volume is computed from contour map. Some surveyors calculate on simple cross sectional method also in which the water is divided into several sections like potato chips and all average sectional area is multiplied by distance between sections to provide the volume. There is no change in the method of data collection using boat.

Why does water appear white in a waterfall though it is colour less?

When all colours get reflected from the surface of an object, it appears white. In a waterfall, water drops can be thought of as suspended in air and they may form an inhomogeneous mixture of water and air. When light enters from a lighter medium (air) to a denser medium (water) some of it is reflected (by the surface) and the rest is refracted. In a waterfall, light suffer numerous reflections and refractions by the water drops because of the refractive index difference. As a result more light gets reflected. The light refracted by a layer on top would also contribute to reflection at the next layer of drops. Thus most of the light is reflected by a waterfall. This leads to whiteness. Mist, paper, water vapour, colloidal solution, clouds, talcum, snow and white paint (there are no white pigments in white paint) and sugar also appear white because of the same reason. White paint has transparent oxides of Zn, Pb, Ti suspended in transparent solution. To see waterfall white, light should not be directional (i.e.) it should be coming from all directions. If it is directional, one would see colours as in a rainbow.

How does water stored in a mud pot remain cool?

     Water stored in a mud pot remains cool because of a phenomenon called evaporative cooling.

            Mud pots by their very nature include a number of very minute pores through which water can slowly ooze out. In effect, these pores increase the surface area of water and consequently increase evaporation. For water to evaporate it requires some energy which it takes as heat from the water itself. This results in lowering the temperature of the water in the pot. Liquid water can turn into water vapour in two ways – boiling and evaporation. Boiling takes place only at a particular temperature called boiling point, but evaporation takes place at all temperatures. While foe boiling heat is supplied from outside, for evaporation heat is taken from the water itself.

   It is for the same reason, we feel cool when a wind blows and takes away the sweat from us.

          The molecules of water or for that matter any liquid are in constant motion but all of them do not move with the same speed (kinetic energy). Their energies vary over a small range and the temperature we measure represents only the average kinetic energy of all the molecules.

            Even when left undisturbed, the fast moving molecules escape from the surface and vanish into thin air. As a result, the mean kinetic energy or the temperature of the water is lowered. This process is facilitated by a large surface area as more number of molecules comes into contact with air. (That’s why coffee in a wide saucer cools faster than when kept in a narrow cup.) The pore in mud pots provide a large surface area for evaporation and thus bring about a significant cooling of water in the pot, especially when the outside temperature is hot as in summer.

            An interesting observation could be the fact that the water in the pot can never become ice. This is because of the fact that the pitcher is not a closed system as it can also take up heat from its surroundings. Thus an equilibrium temperature is reached when the process of heat loss and gain are balanced. Further conversion of water (even at zero degrees Celsius) into ice would require a further removal of heat (80 calories/gram) to bring about a phase change (from solid to heat). This will only be possible by using another liquid as a refrigerant.

          With the same principle Double layer earthen pots are also used for preserving food and vegetables as illustrated in the labelled diagram.