Unit - 5

Ground water Hydrology

Q1) Explain Occurrence & distribution of Ground water?

A1)

• "Ground water Hydrology" is a science which deals with the occurrence, distribution and movement of water which has infiltrated and passed down into the earth. It occurs, beneath the water-table in the soil, when it gets saturated.
• Ground water is a part of the hydrological cycle i.e., in a form of spring when it came up, it gives rise either to a stream lake or a dug well and the cycle restarts like evaporation → precipitation →→surface flow → infiltration and so on.
• Ground water is an important source of irrigation in India and also it makes supply to industries and domestic water supply through the municipalities.
• The groundwater originates from the surface water (which depends upon the precipitation) when the surface water goes down into the surface it is called "Ground water Recharge".
• Depending upon the sources of recharge; we can classify then as the Natural recharge i.e., through precipitation and surface flow.
• The stored water on the surface i.e., ponds, lakes etc. and Artificial Recharge i.e. Through the excessive irrigation, seepages through canals, leakages from the reservoirs or tank, or the water which has been applied on the ground surface for augment of the ground water storage.
• When the ground water comes back to the surface it is called as Discharge of water. This happens either naturally i.e., when the ground water comes near the surface, it joins the surface flow in the form of a spring.
• It also happens artificially i.e., when we lift the water from the tube well or a dug well for irrigation or for domestic purpose, it is an artificial discharge of the ground water. Some of the ground water gets lost through the transpiration frame the plants in the form of vapour.

Q2) What is Specific yield of aquifer?

A2)

• We must note that higher porosity does not mean high yields of water. The quantity of water which can be obtained from a geological formation is expressed in terms of specific yields.
• Specific yield can be defined as, "Specific yield (Sy) of a soil or rock is a ratio of volume of water (after saturation) can be drained by gravity to its own volume". The specific yields also is expressed in percentage.

If Sy =Specific yields.

Then, Sy = × 100

Where in,

Sy = Specific yields,

= Volume of water drained

V = total volume of the soil (or Rock)

Q3) What is Movement of Ground water?

A3)

• The groundwater flows underground till it reaches the discharge zone i.e., the area where the water is above the land surface e.g., the springs are the visible discharge zones.
• The non-visible movements of ground water are, through the seepage into the wetlands or when it contributes to the stream flow.
• The velocity of the ground water: It moves very slowly from the Recharged Areas to the Discharged areas. The velocity or the movement of the ground water is measured in feed per day.
• In case of large opening i.e., fractured basalt or coarse gravel. The movement of ground water is same as the movement of surface water.
• Transmissivity can be defined as "It is the rate at which the groundwater flows, horizontally through an aquifer".

Q4) What is Darcy’s law?

A4)

• The velocity of flow of water in an aquifer is very low and so Darcy's law can be applied in such case.
• The law states "The rate of flow, per unit area of an aquifer, is proportional to the gradient of potential flow in the direction of flow".
• It can be expressed by the following equation.

V = K×i=- K

Where is,

V = velocity of flow (in m/s)

K = coefficient of permeability

i = Hydraulic gradient =dh/dt

• The negative sign in the equation indicates that the flow is in the falling head.
• The coefficient of permeability (K) is also called as hydraulic conductivity and has the dimension as those of velocity of flow which depends upon the fluid property and as the property of aquifer soil sample K is expressed as below.

K =

Where in,

K = Hydraulic conductivity

c = A constant factor

d = Average pore size of the material;

= Density of material

g= Gravitational acceleration

μ = Coefficient of the dynamic velocity of water the velocity of flow through the soil sample 'V' is called as Darcy's velocity, discharge velocity or apartment velocity.

Q5) What is Permeability?

A5)

• The term permeability can be defined as "It is a measure of ability of rock material to transmit water.
• It is proportional to the degree of interconnection of voids between particles in the rock, larger open spaces in the rocks favour a higher degree of interconnected of spaces"
• This is a natural filtration process of ground water so the more permeable rock yields a good quality of ground water.

Q6) What is Safe Yield of Basin?

A6)

• We must note that higher porosity does not mean high yields of water. The quantity of water which can be obtained from a geological formation is expressed in terms of specific yields.
• Specific yield can be defined as, "Specific yield (Sy) of a soil or rock is a ratio of volume of water (after saturation) can be drained by gravity to its own volume". The specific yields also is expressed in percentage.

If Sy =Specific yields.

Then, Sy = × 100

Where in,

Sy = Specific yields,

= Volume of water drained

V = total volume of the soil (or Rock)

Q7) Explain Steady State Flow to Wells in Unconfined Aquifer?

A7)

• The Fig. a well of radius r. It has completely penetrated the unconfined aquifer. Let us assume H as the thickness of this aquifer (measured from the impermeable strata upto the initial level of the water table)
• When the well is getting pumped the constant rate for a long period of time, and the water level has been stabilized. To develop the drawdown curve.
• At this stage let h be the depth of the water in the well (measured above the impermeable strata).
• Let R be the radius of influence (which is measured from the centre of the wall to a point where the drawdown is inappreciable).
• Consider the origin at a point O at the centre of the well at its bottom; and let the co-ordinates of any point P, on the drawdown curve be (x, y) and if a vertical cylindrical surface passing through the point P and surrounding the well, located at its centre is considered then as the area of portion of the cylindrical surface which is lying within the aquifer, below the point P is equal to (2 x b).

Fig.: A well penetrated an unconfined aquifer

• If (dy / dx) is the hydraulic gradient at P. Then from the law the rate of flow of water (the discharge) through the cylindrical surface is equal to [k (dy/dx) 2 xy). The rate of flow of water is equal to the well discharge and so,

Or Q

• By integrating both the side of this Equation (i), between the limits, at x = r, y = h at the well and at x = R, y = H at the extremity of the area of influence, we get

=

Or

Q8) Explain Steady State Flow to well in Confined Aquifer?

A8)

• In the Fig., it has been shown that a well of radius r, is fully penetrating the confined aquifer. Let 'B' is the thickness of this aquifer, measured between the top and bottom impervious strata and H is the height of the initial piezometric surface, measured above the impervious strata at the bottom.
• When this well is pumped with a constant rate of Q for a long period of time so that the water level in the well has been stabilized to develop the drawdown curve (as shown in the Fig.).
• At this stage let 'h' be the depth of water in the well, measured above the impermeable strata at the bottom. Let R be the radius of the influence.
• Let (x, y) be the coordinates of any point P on the drawdown curve, with respect to origin O at the centre of the wall at its bottom.
• It 2 vertical cylindrical surfaces passing through point P and surrounding the well, located at its centre, is considered, then the area of portion of the cylindrical surface which is lying within the aquifer, is equal to (2π x b).

Fig.: A well penetrated a confined aquifer

• If (dy/dx) is the hydraulic gradient at P then the cylindrical surface, is equal to [k (dy / dx) 2 x b] which is equal to the well discharge

Q = ... (II)

• Integrating both the sides of the Equation (2) i.e., Q= , between the limits, at X = r, y= h at the well and x = R, y = H at the extremity of the area of influence, we get,

Or

Or

• This Equation is known as Equilibrium Equation or Thiem Equation. If s is drawdown at the well then, as s= (H-h) this Equation can be expressed as,

Q9) What is Specific Capacity of wells?

A9)

• It is defined as the discharge per unit drawdown in a pumping well. So, the specific capacity can be calculated by dividing the discharge by drawdown in a pumping well. So, if s the drawdown and Q is the well discharge, then

Specific Capacity = Q/s   … (1)

From the Equation,

S= BQ+CQn

So, the Specific Capacity,

   …. (2)

Or   ….(a)

• This Equation i.e. (a) shows that the specific capacity of a well is independent of the well discharge, in fact it gives only the theoretical specific capacity because in reality, the well loss is not equal to zero (except at very) low pumping rates, when this loss may be negligible
• The specific capacity of a well is the measure of its productivity and so larger the specific capacity, better the utility of the well.
• As indicated by the Equation (a) for a given discharge a well may be assumed to have a constant specific capacity but in reality, it is not so, as, this capacity varies with the time.
• The reduction in the specific capacity of a well may be due to an increase in the well loss, which is an effect of clogging or deterioration of the well screen.
• In case of an unconfined aquifer due to lowering of the ground water level, the transmissibility gets reduced which may reduce the specific capacity of the well.

Q10) What is well efficiency?

A10)

• It is defined as, "a ratio of the actual capacity (Q/s) of a well, measured in the field and the theoretical specific capacity (Q/BQ)." It is always expressed in percentage, is shown below.

×100

Or

Introducing Equation (S= BQ+CQn) in above Equation,

• The Equation shows that in case of well, if the well loss is more the well is less efficient.

Q11) What is Tube well?

A11)

• It is a well of a long pipe, sunk into the ground, and which intercepts one or more, water-bearing strata. The diameter of the tube well is much smaller than the open wells (it ranges between 80 mm and 600 mm). On the basis of the depth, the tube wells can be further classified into two as.

1. A shallow tube well
2. A deep tube well

• The shallow tube wells, generally have the maximum depth of 30 meters and have the maximum yields (discharges) of about 20 m³/hrs.
• The deep tube well, may go as deep as about 600 metres and have the yields more than 800m3/hrs.
• Different methods, like boring, driving and melting are used for the construction of shallow tube wells. While for the deep tube wells the drilling methods are used (This drilling can be carried out by cable tool method, or Hydraulic rotary method or Reverse rotary methods.)
• The tube wells can be classified as:

1. Strainer type tube wells
2. Cavity type tube wells
3. Slotted type tube wells

Q12) What is strainer type of tube well?

A12)

• It is the most commonly used and widely accepted type of tube well.
• In this type, the pipe which used to be introduced into the ground, is an assembly of strainer pipes or strainers and the ordinary pipes (which are known as Blind pipes) are alternately placed.
• They are arranged in such a manner that the strainer pipes rest against the water bearing strata i.e., aquifer while the blind pipes rest against the impervious strata i.e., aquicludes.
• The strainer pipes consist of a fine wire. Mesh (or screen), which is wrapped around a slotted pipe (perforated) with a small annular space between two.
• The total area of the opening of these slotted pipes is kept equal to the opening of the opening of the wire mesh, so, no change. In the velocity of flow occurs between these two.
• The annular space between the pipe and wire mesh is necessary because otherwise, the wires of the mesh may cover a large portion of the area of openings of the pipe
• The water enters through the pipes, through the wire mesh and so, the sand and other particles, larger than the size of the mesh are prevented from entering in the well pipe. This allows larger velocities of flow.
• The size of the opening of the wire mesh is generally kept equal to Do to Do of the surrounding soils.
• Some strainer pipes have only slotted pipes with the wire mesh wrapped around them. In such cases, the slots are of small size (so. They can prevent the entry of sand particles into the well pipe).
• Following are the basic requirements of strainer pipes;

1. It must resist to corrosion and deterioration.
2. It must have enough structural strength to prevent collapse.
3. It must have the stability to prevent excessive movement of sand into the well.

• This type of well is not suitable for a very fine sand strata, as, if the sand is to be prevented in entering the pipe, the size of the mesh will have to be made very small but this will make the strainer to get chocked. If the size of the mesh is Increased the well will have the discharge of sandy water.
• This type of tube well draws water either from an unconfined aquifer of unlimited extent or from one or more confined aquifers, which are lying one over another one (Refer Fig.).
• At the bottom, the well is plugged by concrete.
• In case of Abyssinian tube well, the diameter of the well pipe is kept equal to 38 mm i.e., 1.5 inches and the strainer is provided only for a length of about 1.2 to 1.5m. i.e., 4 to 5 feet.

Fig.: Strainer type tube well

Q13) What is Open well?

A13)

• These are the wells which have a large size (diameter) but have low yields (discharges) and by the name it is clear that they are dug wells and have less depth.
• The diameter of the dug well, vary between 1 and 10 metres. In most of the cases the discharge i.e., fields of these wells is about 20m³/hr or less.
• The depth of the open well, may vary between 2 and 20 meters.
• The wells of these open wells are built of bricks or stone masonry or by precast concrete rings. The thickness of these walls varies between 0.5 and 0.75 metres It depends upon the depth of the well.
• On the basis of the depths of these wells, they can further be divided into two as,

1. A shallow open well
2. A deep open well

Fig.: Shallow and Deep open well

Shallow Open Wells:

• These wells are confined to the top water bearing strata and they draw their supplies from the surrounding (Refer Fig.).

Deep Open Wells:

• These wells are confined to the impervious strata and they draw their supplies from the previous formation which lies below the impervious strata, through the bore holes, made in the impervious strata.
• This impervious layer has clay, cemented sand, kankar and other hard material (It is called as Moda layer). But this term of moda layer is not used to the layer of hard material, which lies above the water table.
• The main function of this moda layer is to give structural support to the open well, which is resting on its surface. Because the pervious formation below this moda layer, contains a large quantity of water, the discharge i.e., the yields of the deep open wells are always greater than that of the shallow open well.
• We must note down that the terms shallow well and Deep wells are purely technical i.e., sometimes the shallow open wells have more depth than the deep open wells. So, these names are the indication of the source of supply of water and not the actual depth of the well.

Q14) What is Water Logging & Drainage?

A14)

• In case of agriculture land when due to over watering, the normal circulation of air within the soil pores is totally cut off, it is called as "water logged condition".
• This affects the fertility of soil productivity of the land and it leads to reduction in the yields per ha and total production of the crop is also reduced.
• The depth, at which the water table make the water logging conditions, depends upon, the height of the capillary fringe and the type of crop.
• The capillary fringe means the height to which the water will rise above the normal water table due to the capillary action. (It is more in fine textured soil and is less in the coarser soils). Generally, the height of the capillary fringes varies between 0.9 and 1.5m.
• The problem of water logging of the agricultural fields is very acute in Northern plains of India mainly in Uttar Pradesh and Bihar.
• This is mainly, due to fertile plain areas of Ganga basin and a huge, good network of surface canal irrigation. Recently, the reports of water logging have been received from the Chambal project in Rajasthan and Madhya Pradesh.
• As per the national reports, the total land getting affected by water logging in various state of India, is about 3 million hectres out of which 1 million and is shared by are single state i.e., Punjab, the state that leads the green revolution.
• So, it is high time to note down the short- and long-term effects of water logging in the command area and steps must be taken to control this huge loss of the natural resources like soil and also of surface water.

Q15) What are the causes of water logging?

A15)

Following are the main causes of water logging. Feast of men is man-made only).

1. Seepage from the nearly channel.
2. Over irrigation of the fields.
3. Inadequate surface drainage system.
4. Obstruction of man-made constructions in the natural, drainage i.e., by construction of a darn over a river, creates artificial reservoirs. Which create the problem of back water spread to create water logging.
5. Obstruction of sub-soil drainage. i.e., when the construction of cause ways, bridges is carried, which need very deep foundations. They create problem to sub-soil drainage.
6. Nature of soil: In some soil the water holding capacity is high e.g., clayey soils. They create the problem of water logging.

Q16) What are the effects of water logging?

A16)

• It converts the good fertile land into infertile land.
• It reduced the production capacity of the land through the reduction in the yields of the crops. And the total production from the Seld moons of a small field near Patiala in Punjab within a period of about 10 years, to Vicks or cu gri down from 746 kp (fin wheat from 1344 kg/ha to 896 kg/ha.
• Following are the important factors responsible for the reduction in the yields of crops:

1. Absence of aeration of soils in root zone of a plant

• The plants get the necessary nutrients from the soils, through the process of nitrification of sorts: which is carried by some microorganisms which break up the compounds into simple forms which the plants can absorb.
• For their survival, these microbes need oxygen (they can act in the anaerobic conditions) when land get water logged the oxygen is lost from the soil, the microbes cannot function under these conditions the proportion of nutrients required by the plants, is not generates So the yields are reduced.

2. Difficulties in the process of cultivation

• When the ground level reaches the surface of the agricultural field no process of farming is possible.

3. Growth of water

• As the water is easily available, the growth of water-weeds becomes very fast.

4. Increase in the level of salts

• When the water table come near the surface, all the dissolved salts, which were taken down by the process of percolation are brought up, by the capillary action.
• It is called as salivation of soils which converts the good fertile soils into bad, non-fertile soils in Bihar and Uttar Pradesh, due to over irrigation, all the good, fertile soils also the canals have become saline, and useless for any cultivation.

Q17) What are the Preventive & Curative measures of water logging?

A17)

• Following steps, if taken in time, the water logging can be prevented,

1. If the channels are properly lined, in the bed and sides the seepage can be controlled to control further effect of water logging.
2. By keeping the full supply level of the channel, blow top level the channel, over flowing of the canal water can be controlled.
3. By providing intercepting drains along the channels, the extra amount of water can be handled of properly.
4. By making the people aware about the proper use of water, over watering which causes water logging can be prevented. This can be done by making the canal water costly or by changing the cropping pattern.
5. By providing a proper drainage system (by developing drains), the water logging conditions can be avoided.
6. By providing a system to pump out the water in the field, the water logging can be controlled.
7. By keeping the field open for one season (without cropping it) to allow the water to get evaporated to reduce the percentage of excess water it is called as Efflorescence.

Q18) What are the types of drains?

A18)

• The drains can be classified on the basis of their location as,

1. Surface Drains
2. Sub-surface Drains

Surface Drains:

They can be subdivided into two as. Shallow Surface Drains and Deep Surface Drains

1.Shallow Surface Drains

• They are used to take away the extra water. Provided through imigation and it also helps to have a quick dispose off the storm water.
• By this method, the rate of percolation is reduced. They are not useful to relieve the land which is already water logged.

Fig.: The Shallow Surface drainage

2. Deep open surface drains

• They used to take out the extra water from the sub-soils and so they can be used as a method of prevention of water-logging.
• They are used for the reclamation of the wet lands.

Fig.: The deep surface drainage

Subsurface Drains:

• These drains are useful for both i.e., for prevention water logging and for relieving the land, under water-logging conditions.
• These are tile drains, of porous earthen ware. They are laid, below the ground level, putting each other with open joints and are covered by earth. So, they do not make use of the fertile land or they do not disturb the farming activities.
• Generally, they are spaced with a distance of 15 to 45 m. The usual diameter of the tile drain is 100 mm. These drains are located about 0.3 m lower than the desired water table. They are laid, with a gradient, steeper than 1 in 500.
• The closed drains are designed to carry the seepage water and their capacity is determined by the general rate of infiltration (we already have discussed the methods used to calculate the rate of infiltration, in chapter I of this book).
• Spacing of closed drains: As already seen, the usual spacing of these drains is between 15 and 45m, the closer spacing is being used for the soils have low permeability and for the soils which are very highly permeable, they are spaced away from each other’s, keeping the range between 15 ta and 45 m.

Q19) What is Alkaline & Saline lands?

A19)

• Before we use any method to reclaim the land, we must consider the following conditions:

1. We must test the soil to know where the soil is Saline or Alkaline or it is both Saline and Alkaline.
2. Through the testing of the soil samples. We must know the percentage of salinity or Alkalinity.
3. The quality and the quantity of water available to be used for leaching operation.
4. General topography of the area.
5. General soil characteristics of the area.

Method of Reclamation of Salins and Alkaline Soils:

• The well-proven method to reclaim the alto ad Alkaline soil is the lenching method. It necessary to have an efficient surface and sub surface drainage system to bring down the ground water table. It should go well below the hat zone of the crop.
• This is done by stopping surface runoff, le control the infiltration. Once the ground water able is pushed below the root zone.
• The land to be reclaimed is flooded with abundant water (the depth of this supplied water should be about 150 to 250 mm above the surface). This flooded water washes down the salts. This process is called as leaching method.
• This extra water, dissolves the alkaline salts and is either percolates to meet the ground water table or it may be drained out by the surface drains.
• This process is repeated for a number of times with some interval to allow the land to dry up due to dryness of the surface soil, cracks are formed which allow the next flow of water to be percolated easily which carries the alkaline salts.
• These soils which are reclaimed should be used to cultivate the crops like rice, which is resistant once the salinity is brought under control other crops can easily be practiced.
• For reclamation of Alkaline soils is a long process. It needs to replace the excess quantity of exchangeable sodium elements corresponding Calcium (Ca), Calcium or magnesium chloride (CaCl, or MgCl₂) (Na) by Calcium Sulphate (CaSO,) which popularly called as Gypsum, are used for this process.
• When Gypsum is added into the soil having sodium carbonate it reacts chemically to form Na,80, This can easily be washed out, by using the flooding method to either leach out or to drain out the excess alkaline salts. Like Gypsum pyrites also is used to reclaim the soil.

Q20) Explain Warabandi?

A20)

• It is followed by Punjab, Haryana, Uttar Pradesh, Rajasthan, Bihar states, in North India. Most of the irrigation schedules are Run-off River Schedules:

1. In this method, the irrigation water is allocated among the farmers in proportion to their land holdings i.e., the size of the fields. This imposes water scarcity condition.
2. The utilization of irrigation water is decided by the farmers themselves.
3. In this method, the water is carried from the source by the main canal. Which feeds two or more branches and the supply is operated by rotation.
4. Depending upon the available water, supply may be full or may not be full.
5. The branch canal supply water to a large number of distributaries, which must run at full supply level, by rotation.
6. The distributaries supply water to water courses called as Cut through ungated fixed change outlets. They are called as adjustable proportional modules (A.P.M.)
7. By using the Time Roster, the water allocated to the farmers.
8. Each unit of cultivable common area is allocated a certain rate of flow. It is called as Water allocation the value of water allowance at the water course head is generally 2.5 to 3 cusecs per 100 of C.C.A.

Q21) Explain Rotational Water Supply System?

A21)

• It is a rigid method, followed in India with constant frequency either with constant amount or with varied amount.
• The water in this method is supplied at a fixed time or at the prearranged schedules. The rotation system is used between,

      A: Two water users.

      B: Two or more groups of water users.

      C: The outlets.

      D: The distributaries and minors.

      E: Definite section of the main canal.

Advantages of Rotation Method:

1. It is better and beneficial in the command area.
2. If all the off takes are running at the same time with no clubbing of off takes, it has smaller c/s of main canal.
3. It is better for irrigation staff for the supervision and for organizing larger areas.
4. It can provide the required periodical maintenance.

Disadvantages of Rotation Method:

1. If by any reason, the rotation (of water supply) is delayed it causes water stress; which finally reduces the crop yields.
2. This does not provide any control time for the individual irrigation.
3. The farmer has to adjust to the rotation schedule (because the supply of irrigation water is controlled by authorities).

• The rotation method is known by different names in different area of our country.

      A: Shejpali or Rigid Shejpali or Rotational Water Supply (R.W.S.) in the state of Maharashtra.

      B: Warabandi in North India.

      C: Varavaram in Tamil Nadu

Q22) Explain Participatory Irrigation Management?

A22)

• With all the efforts taken by the various state governments constructing small, medium and major dams to provide controlled water supply to the large number of farmers to increase the food production, has not yield the expected results.
• So, for getting better results, it is necessary to develop Participatory Irrigation Management (PIM) unless the farmers participate in this programme. The expected results cannot be achieved.

Need of Farmers' Participation:

• It would reduce the operational cost and also as they are the actual user, they can handle the irrigation task more efficiently than any other outside agency.
• As they are owners of water, the unauthorized use of water, miss use of water, wastage of water etc. would get reduced. This would help to increase the efficiency of the irrigation system.

Objectives of Farmers Participation in Irrigation Management:

1. To entrust the Irrigation Water Management to the actual users.
2. To allow the farmers to take the decision about crop to be cultivated and water to be supplied to the crop.
3. To achieve the equity in water distribution as the result of community management.
4. To make the farmers aware about the need make use of irrigation water economically.
5. To create good relation among the farmers and also between the farmers and government officials.

Q23) Explain Co-operative water distribution system?

A23)

• On the basis of this, in Maharashtra state the farmer's Co-operative Irrigation schemes have been developed. Due to the typical topography and non-perennial rivers.
• The water is supplied through the dams, by the canals (mains) and the lift irrigation schemes are developed to carry the irrigation water through the pipe lines. It is then distributed among the members of the co-operative society.
• The water bill, Electricity bill, the maintenance cost, piping etc. is equally shared by these members. This has helped the farmers, previously having only rainfed crops.
• Now due to such co-operatives they have started cultivating cash crops and the industrial crops like cotton, sugarcane, oil seeds etc.
• For the last few years these irrigation co-operatives are in danger.
• This mainly because of the followings:

1. Non payment by the members.
2. Shortage of water in the reservoir.
3. Less or no-maintenance of the supply system.
4. Shortage of power supply.

• To improve the functioning of the co-operatives a Water User's Association Act (WUA) has been established by the government of Maharashtra.