Lecture 26: Irrigation Wells

Lecture 26: Irrigation Wells


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Ah. This is lecture number 26. ah In this lecture we are going to ah learn
the ah Irrigation Wells ah. So, this week and the following week we will
be concentrating on ah I mean some ah information from ah irrigation wells as well as ah irrigation
pumps ok. So, because these are ah important, when you
are talking about ah extracting water or a I mean ah water to the field. So, ah we have seen ah how to you know ah
get water from canals? So, now, in the local if you have ah some
water sources like in ah irrigation wells. So, still ah we need to understand, how to
ah get the water from irrigation wells for the irrigation purpose? So here so, in order to understand that ah
we need to know the soil formations and and what are the different soil water zones just
to understand, ah and what is an aquifer, ah what is ah ah not a good aquifer right
and the aquifer properties? Ok. So, in this ah lecture we will be focusing
on the aquifers and their properties ok. So, now, see here the soil water zones. So, if you if you start digging the soil from
the surface right. So, you can observe different zones ok. So, the first zone here if you see. So, the first where ah the crops are grown
so, this zone is called soil water zone right. So, this is as for as crop production is ah
concerned this is important this zone. And, then so, this is the soil water zone
ok. So, then below that there is intermediate
ah zone and then fringe water zone ok and the groundwater and internal water . So, if
you if you take a profile like this ok. So, here generally so, this a groundwater
where the soil is completely saturated. So, if you if you dig hole in this particular
point right and definitely you get water ok. So, and if you make half of this for example
so, this is the water ah groundwater and above this this is the surface ok, this is the plant. And, the first zone here you see the soil
water zone right, and the second is intermediate zone, and this is a capillary zone because
the groundwater can ah move upwards due to the capillarity ok .
So, ah so, these are the main zones the first soil water zone, intermediate zone, ah capillary
fringe zone, groundwater and this is the internal water. So, this is not accessible for usage ok. So, then again so, this is called belt of
soil water and this is intermediate belt and this is a capillary belt. So, and so this whole thing from here to here
so, since it is ah I mean the soil pores are not filled with water and I mean every time. So, that is called suspended water right,
suspended water or zone of aeration. So, this is called zone of aeration, because
the pores will have ah air in it ok both air and water in it. Whereas, there is a another zone it is called
zone of saturation. So, where you you see the pores are completely
filled with water. So, that is the ah zone of saturation ok. So, ah and from here to the zone of aeration,
this is zone of aeration it is also called the Vadose zone ok. So, vadose zone is unsaturated zone. So, which start from you know land surface
to the water table ok. So, that is the complete zone of aeration
ok vadose zone water. So, then ah so, this is called ah the zone
of rock fracture. So, where the water is available and the zone
of rock flowing. So, this water will not be available for utility
ok and ah ok. So, this way at least we understand ah the
what kind of water, which is present in the the soil? So, the first is soil water, intermediate
zone, and capillary fringe, and groundwater and then ah this is unavailable deep water
deep groundwater zone ok. . So, ah so, here the top of the saturated
zone or groundwater zone is called the water table. So, water table is the top of the ah groundwater
zone. So, that is called water table. And, this water table is influenced by atmospheric
pressure. So, directly you can have the influence of
atmospheric pressure, because this soil is not completely filled with water. So, there is a atmosphere interaction to the
zone. So, the pressure at the water table is equal
to the atmospheric pressure ok. . So, again the groundwater if you see so,
ah the volume of Vadose zone below the soil moisture zone is estimated to be ah around
10,000 ah mile cube. So, which is ah a potential groundwater recharge
ok so, which is a potential groundwater recharge. So, here what we are expecting up to this
whatever water, which is stored in the vadose zone will be contributed to the groundwater
recharge and that will be estimated ah as 10,000 mile cube ok. And then so, the next if you see this Soil
Water ah Zones further, the zone of aeration ah is acts like a filter. So, if you remember this we have like this
is zone of aeration and zone of saturation. So, zone of aeration which acts like so, ah
a filter for the rainfall, rainfall ah infiltrates down and this is a filter meter this is zone
of aeration and recharge to the groundwater ok .
So, ah from the water table if you go below 0.5 miles. So, that accounts about 1 million mile cube
of groundwater ok. And, that will be around 3000 times greater
than the volume of water in all rivers. So, it is a huge amount of water, which is
lying as groundwater ok, which is below 0.5 miles from the water table. And, the groundwater below 0.5 to 5 miles
is not recoverable. So, this is simply lying over there and it
is not accessible or not recoverable. So, ok that is and the groundwater accounts
nearly two-third of the fresh water, groundwater flows very slow. So, flows like 10 to 15 meter a day. So; that means, if you have 10 meter ah distance
or 10 meter let us say room having 10 meter you know length. So, it takes you know a day ah ah to flow
from one end to the other end. And, the great depth the water may takes you
know 1000 years, suppose if you have a great if you go deep deep deeper see the 1000 years,
it will take and that that passes through aquifer and sometimes completely stagnant
ok. And next is so, this is a simple example to
see ah how the water availability will be used for the existing population? So, assume that worlds the recoverable groundwater
is this much ok. And, how long will this water be able to meet
the needs of world’s population say 5 billion, we have 5 billion ah population. And so, how long, ah this amount will be useful
for the 5 billion people. So, the 5 million if per capita daily consumption
is taken as 4000 litre ok, per capita daily consumption is taken 4000 litre . So, let
us world population is 5 into 10 power 9, that is the ah 5 billion and per capita daily
consumption is 4000 litre and total world daily consumption is population and per head. So, to into 10 power 13 litre per day, ah
say total world water consumption per day. And total ground water supply is 4 into 10
power 6 kilometre cube you convert that into litres you get this ok. So, now, you divide right divide ah this by
this you get 2 into 10 power 5 days, you convert that it takes 548 years to to deplete or or
or or utilise the entire ah water reserve for the 5 billion population ok. So, next is a ah what are the type of geological
formations? So, if you if you ah go down because water
is there ok. So, the water is there does not mean that
it is available always ok, it all depends on the media ok the porous media. Suppose, if you have porous media in the sense
this is soil type we are we are talking about. So, if you if you have sand, if you have clay,
if you have silt loam. So, if you have rock fractions or if you have
you know solid rocks. So, all this things based on the type of particular
a geological formation with the water availability will be ah defined. So, for example, ah the types of geological
formations are very important in order to understand, whether we we able to get the
water from the particular location ok. So, here we have an aquifer. So, what what is exactly the aquifer? So, each logical formation has a particular
property like. So, it will. So, for that it is kind of a for example,
assume that it is a sponge ok. Sponge is a porous material ok porous media. So, you add water what happens. So, it absorbs water at the same time if all
pores are filled. So, the water is going to drain due to gravity
ok. So; that means, the sponge can absorb as well
as transmits ok. So, that is the particular property. The similarly, the soil or sands or other
rock ah material; they will have a particular property. So, we are going to see whether that property
can be ah I mean favourable for water availability or not. So, here aquifer so; that means, a geological
formation, which absorbs water and transmits water. So, it has both characteristics ok. So, if you have a geological formation . So,
it absorbs water and transmits water this is very important, because where you can extract
water right. So, aquifer is very important in case of extracting
water source or water reservoir. Generally, the soils ah like sands or gravel. So, there you have a definite pore ah structure. So, that will be a good aquifer. So, if you observe ah like ah ah the drillers,
who drills you know the the tube I mean wells. So, they will be keep digging digging digging
as long as. So, every time they will they started checking
the material which is coming out from the whole ok. So, then when when the sea sand, when the
sea sand is coming out from the whole then they think that ok there is the aquifer and
they stop digging it and that is it so, there. So, because the sand is good aquifer material
where, it can absorb ah or it can ah it can take water as well as transmit water ok .
. So, here the similarly aquifuge aquifuge is a geological formation ok. So, ah it neither absorb neither transmit. So, if you say ah suppose this is a material
right. So, it it cannot absorb as well as once it
is not absorbing there is no question of getting water from this material. So, this is an called aquifuge; aquifuge are
solid granite. So, if you have a solid granite right. So, since there is no definite ah pores. So, water cannot penetrate in; that means,
it cannot absorb and it cannot transmit ok. So, these these these are aquifuge ah bad
aquifers you do not get any water in that formations ok. And, aquiclude the geological formation ah
that it ah porous aquifuge ah sorry aquiclude aquiclude is a porous ok. So, it only can absorb, but it cannot transmit
ok. So, contains water, but it is not capable
to transmit water . So, that that is aquicludes; for example, clays. So, clays absorb right and it swells. So, once it it has water it starts swelling
hence very very I mean ah ah a small quantity can lose out, but ah it is really not transmitting
ok anything. So, this is aquiclude and aquitard aquitard
the the name indicates aqui tard is kind of a retard. So, aquifer got retarded so; that means, a
good aquifer after you know long period of time ah. So, the sand is good aquifer. So, having clay particles into a welcoming
clay clay particles into the system. So, that can block the pores ok. So, that acts as aquitard ok. So, this aquitard what happen? So, ah it it it also absorbs water, but the
thing is it is not ah going to transmit water, but it is definitely influencing the neighbour
ah neighbouring ah aquifers ok. So, so this is important. So, geological hydraulic conductivity is too
small ok. And, ah permit development of a well or spring,
but sufficiently large to influence the hydraulics aquifer adjacent to it like sandy clay etcetera. So these these 4 are the important geological
formations, we need to ah consider here . So the next is the ah the ah confined aquifer. So, now, we are talking about aquifers aquifer
that is a good ah which can absorb and transmit water ok. So, the confined aquifer, so in this case
what happen if you see this picture? So, they are ah if the soil layer is present. So, this is a soil layer or reservoir which
is present in between 2 confined ah layers. So, it is generally impervious layer. So, this is an impervious layer; that means,
water will not pass or water will not enter, water will not pass, water will not enter. So; that means, it is ah ah confined between
the 2 impervious layers ok. So, this is ah called a confined aquifer so;
that means, the water or the saturated stratum is the under pressure basically. So, definitely if we drill hole into that,
if you drill hole into that; that means, if you dig a hole or if you put a well into this. So, definitely water will ah enters through
this right like a jet ok. Similarly, if you see this so, this is one
impervious surface this is another impervious layer right. So, this is geological formation. So, when you make hole it is going to come
out ok. So, this kind of a wells are called artisian
wells or flowing wells. So, that; that means, this continuously flows
ok. So, they continuously flows, suppose if these
head the pressure head, which is ah more than the level of you know the well the outlet
at the well definitely it is going to flow. If, the pressure head which is less than the
the length of the well right so, this will be staying here constantly ok this will be
staying constantly here. So, that is called these are in this case
this is called flowing well whereas, in this case this will be artisian well ok. All right so, ah yeah here artisian well and
flowing wells are there rise and fall of water in well results due to the hydrostatic pressure
in the stratum. So, so raising falling it all depends on the
the pressure, which is build up inside the ah the aquifer this is confined aquifer ok. So, the next is ah ah unconfined aquifer. So, here if you see the unconfined aquifer
case so, the one part is ah is impervious surface then other part is the open to the
atmosphere. So, suppose this is the ground level and there
is a impervious layer and this is a soil stratum; that means, it is completely saturated right
completely saturated . So; that means, this in this case the pressure
which is acting on the stratum is atmospheric pressure right. This is not confined between 2 2 ah impervious
layers, but since this is open to the atmosphere now right. So, the pressure which is acting on this ah
ah this is ah atmospheric pressure. So, in case of unconfined aquifer, so the
soil stratum a geological formation will be sitting on the impervious layer is sitting
on impervious layer only one one side ok . And, the top of the geological formation is
ah known as the water table here. And whereas, the top of geological formation
in case of confined aquifer it is called a piezometric surface or a potentiometric surface
ok. So, here also the rise and fall of the water
table results primarily form the changes in the volume of storage in the structure. So, when the piezometric surface fall below
the bottom of the upper confined stratum the confined aquifer becomes unconfined aquifer
ok. So, in case of ah ah confined aquifer. So, this is the pressure right. So, this is the ah pervious impervious layer
layer. And, this is a soil stratum in case of confined
aquifer. Suppose, if this pressure falls below the
ah stratum so; that means, this is not ah right now it is a full and that there is hydro
enough hydrostatic pressure. And, then it is definitely influenced by the
atmospheric pressure. So, then ah the the pressure equipotential
line if you join that will be equal to the water table not the ah not the ah piezometric
surface in case of confined aquifer. So, when the confined aquifer can act as unconfined
aquifer. So, that is because when there you are not
getting enough pressure in the unconfined ah confined ah geological formation. So, that becomes unconfined even if it if
it even if it is sitting between 2 impervious surfaces .
Next is the perched water table, this is a localized ah condition if you see ah. So, for there are there are clay you know
layers, which are formed in locally. So, since it is a clay layer for example,
this is a clay layer. And so, the water which which will be stored
here just like your unconfined aquifer ok just like unconfined aquifer. So, since it is a locally you know ah formed. So, this is called perched water the perched
aquifer and this is called perched water table, this is perched water table because the water
table is influenced by atmospheric pressure here ok. So, the here if you see. So, this is ah unconfined aquifer, this is
the unconfined aquifer and this is water table whereas, this one is the local ah unconfined
aquifer or perched water perched aquifer and it has also have ah water table ok . So, if
you drill hole here. So, water will be available up to this if
you drill hole here. So, this is connected to the unconfined aquifer
and since it is a water table. So, water will be seen at this point ok. So, this is basically formed due to clay or
any sediment deposits ok . So, the next is leaky aquifer ok. So, the other one is leaky aquifer. So, here ah in case of confined aquifer, if
you see there is a impervious surface ah this is also impervious surface. So, in between this is a soil stratum. So, sometimes what happen . So, here impervious
I mean layer could be this is aquiclude this is called aquiclude where it cannot transmit
or absorb water, this is also aquiclude ok or to be aquifuge right if you aquiclude ok. Sometimes, ah you have aquitards right. So, aquitards are kind of a semi ok. So, in that case what happens? So, if there is leakage ok say simply there
is a leakage so; that means, it is not perfectly impervious layer and ah this is so, this kind
of aquifer is called the leaky aquifer ok. So, leaky aquifer case with your leaky ah
like a pervious could be on top or it could be on bottom ok. The pervious surface could be bottom or top
in both cases, it will be leaky aquifer. . So, if you observe this. So, here and also the leaky aquifer could
be a confined leaky or a unconfined leaky. So, in this case leaky confined aquifer. So, impervious and impervious, but still you
see the leakage from the bottom to top. So, here unconfined aquifer you can still
see the leakiness here. So, this is leaky unconfined aquifer ok .
So, the next is if you ah over all see the all all aquifers sp this is the picture. So, let us say ah at this point there is a
recharge is taking place. Since, this is open to the atmosphere. So, here this level is ah called water table. Then from there the entire water will be flowing
through the 2 impervious layers. So, then this is a confined aquifer and if
you drill hole at this point right, the water which is ah under hydrostatic pressure is
going to flow right. So, here also if you drill hole to this and
you get a water level here right. So, whatever this line we are showing . So,
that will be that that will give the the pressure head at this point and pressure pressure. So, the pressure present in in this confined
aquifer right. So, that is this is also called ah the ah
piezometric surface or potentiometric surface ok. . So, since since this piezometric surface
is above the ah top of the well. So, it starts flowing continuously ok. So, here since the the top of the well is
ah more than the piezometric surface the water level will be staying in the ah in the well
ok ok. So, that is one thing and above that since
these 2 are impervious surfaces this is a confined aquifer. But, above that so, there is again geological
formation, which has this is impervious surface bottom and the top there is no impervious
surface this is open to the atmosphere. So, this and you can see the the top of this
ah geological formation called water table, because this is atmosphere ah intervention. So, if you drill hole at this point. So, water level which is ah seen ah in this
water ah is called water table well, this is water table well or phreatic well this
is also called phreatic well, this is the artesian well since it is connected to the
confined aquifer, this is ah phreatic well ok. Since, it is connected to the ah unconfined
aquifer ok. So, there is also you can also see the local
locally ah formed ah you know aquifer that is the perched aquifer and if you may if you
have connected to the connected to this perched aquifer is perched water table value ok right. So, this will show the ah aquifers and the
wells and then 2 surfaces water table and piezometric surfaces .
So the next is ah the same the piezometric surface is already explained. So, this is an imaginary surface, which ah
connects the all points the pressure in the aquifer ah confined aquifer this is the confined
aquifer ok . And, the elevation of surface at a given point defined by water level is
a penetrating confined aquifer ok. So, similarly if you see ah the water table
I already explained. So, the water table basically connects the
unconfined aquifer and the water table the pressure at the water table is equal to the
atmospheric pressure. So, the recharge area here if you see, ah
which is the the way the region, which supplying water to the ground water for example, here
now ah this is the recharge point where it supplies the water to the ground water. So, this is a research area and ground water
basin. So, this is a physiographic unit containing
one or more aquifers. So, aquifers the the whole ah; that means,
topography can have you know one aquifer or more than one aquifers. So, that is called as a ground water basin
and the aquifers can be connected or interrelated they can be interrelated ok. . So, the next is the ah the springs ok. So, the springs ah ah so, when water water
flow naturally from the aquifer to ground surface is called a spring suppose so, there
there is a ah this kind of things. So, there is a water which is infiltrating
down, but you have you have a impervious surface here. So, definitely that is ah going to pass through
these and this forms the spring ok you can see so, this this forms a springs. So, there are different kinds of springs ah
the component of hydrosphere, if you see this is groundwater flow right in confined aquifer
and this is unconfined aquifer. So, the recharge which is going here right
this is impervious surface due to impervious surface. So, the water some of water will flow through
this and some water will flow through this and there there. So, this since it is a impervious surface
you can see continuously the water flowing right this called a spring. So, the next the types of springs could be
like a depression spring. So, the depression spring where ah ah where
here the water which is infiltrating down ah will be this is a depression here right. So, the depression is connected to the water
table right. So, the local water table since there is a
water connected to water table and it is going to flow automatically right because there
is a depression right. And, here ah there is a contact spring the
contact springs are like ah the High K. So, more ah water will ah infiltrate and at
the bottom there is a Low K right and again High K. So, what happen here the water which is ah
I mean infiltrating down ah cannot infiltrate in further layer. So, and that will be formed as a spring ok. Because, the ah and again there is a ah another
layer which is High K ok. So, when the Low K is ah you know sandwiched
with ah 2 High K s. So, then you can have this contact spring
. So, in the fourth one is the artesian spring,
the similar to ah the artesian spring is suppose here it is a High K right and here also this
is a Low K the Low K and High K. So, water which due to High K is a water is
going to ah pass because if the Low K it cannot penetrate down ok, and then here from here
this is High K. So, water from here and here ah will be oozed
out here from here because it cannot enter through this right . So, this is ah so, the
water ah I mean here the resulted from the water under pressure from confined aquifer
either at the outcrop of the aquifer or through the opening of the confining bed ok. And, the tabular of fracture spring so, sometimes
what I am there is a fractures if you see right. So, the water or factures are connect[ed]-
interlinked, the water from this fracture this facture this facture. And finally, if there is a ah way out right
all fracture water will be oozed out as spring ok. So, this is called a fracture spring the same
thing here . So, the other ah these are the aquifer properties
if you see. The porosity ah is one of the properties,
because the soil media and void ratio another ah ah property. So, these properties are already explained
in lecture number ah 3 of week number 1. So, you can go through that and other other
was an effective porosity. So, the effective porosity is the out of 100
percent of porosity. So, how much is really participated in the
flow process groundwater flow process so, or ah what is the percentage of porosity,
which is ah ah which is effectively you know used in groundwater flow? Ok. . So, the portion of void space in the porous
material through which fluid can flow so, that is the thing and if the portion that
experiences the flow process. So, that will experience the flow process. The other process may not be ok may not be
experience the flow process. So, for example, if you have a pore here right
this is a pore and soil particle ah sorry. So, so this is the ah the soil particle and
this is another soil particle and you have pore right. So, ah some portion of the some portion of
the pore will have fluid that will be you know flowing the other you know stagnant. So, in that case the porosity is not 100 percent
and effective porosity is 100 percent, but effective porosity is not 100 percent ok. . So, aquifer type if you see lime stone the
effective porosity will be 0 to 20 percent and sand 25 to 50 percent clays 40 to 70 percent
right. And, so, next property is permeability. So, the permeability is basically the media
property the sand property ah whatever it may be so, not the fluid property. So, a permeability is the measure of medias
ability to transmit fluid under hydro potential gradient. So, if you have some gradient of flow right. So, the fluid will flow from one place to
another place. So, and and then that is that is ah due to
the property of the ah a material a property of the material ok. So, that is called the permeability. So, the approximately the proportional ah
to the square of the mean grain diameter suppose the permeability is expressed ah is equals
C d square. So, C is the dimension less coefficient and
the mean diameter d. So, it is expressed in Darcy. So, 1 Darcy is equal to 9.87 into 10 power
minus 13 meter square and it is the property of the of course, soil or not the water property
ok. And the next is the hydraulic conductivity. So, here the hydraulic conductivity ah it
is again fluid as well ah it is the property of soil as well as the water ok. So, it contains both fluid and soil property. So, let it is defined as the volume of water
that moves through a porous medium in unit time under a unit hydraulic gradient, through
a unit area measured at right angle to the direction of flow, if you have the flow measured
you know ah here right q. So, the k the hydraulic conductivity which
is equal to the q right, when it is measured through a 1 meter square or 1 ah 1 unit area
of cross section and then 1 unit area of cross section ok. So, then ah and the unit gradient, suppose
here the pressure and pressure the difference the pressure gradient is unit then the flow
is equal to the hydraulic conductivity ok .
And ah so, this is first given the equation is given by Darcy. So, Q is equal to minus K dh by dl into A.
So, here the negative sign indicates the pressure gradient, which is the the gradient of pressure
I mean pressure is decreasing ah towards the flow direction if you see here here the pressure. So, this is the setup the Darcy setup let
us say the soil sand column. So, input what is ah input in here. So, water is initially this is completely
saturated. So, water pass through this and collected
here. So, this is q. So, since it is equilibrium. So, the q which is going in will be same as
q coming here, but the pressures are different here. So, that pressure h 1 here and h 2 here the
difference in pressure. So, that is ah ah delta h and the gradient
is l delta h by l that is the gradient. So, ah since the pres[sure]- ah the flow rate. So, this is h 2 minus h 1. So, h 2 is less h 1 is more. So, that is why this negative sign is required
in order to get the positive q ok. . So, ah so, that way this this equation will
be used to estimate and knowing the q and knowing the ah area of cross section and the
gradient you can estimate what is K? Ok. .
Yeah. So, and then the K value here if you observe
clearly. So, the K value . So, as I said it has both
the fluid as well as ah solid property. So, K this is permeability and rho by mu. So, these are the fluid properties ok. So, K has both fluid as ah solid properties. So, mu dynamic viscosity and unit weight of
water k is intrinsic permeability. So, for ah hydraulic conductivity K the ah
is clay for Clays 10 power minus 8 to 10 power minus 2. So, it is a big range right. So, sand 20 to 100, Gravel 100 to ah you know
1000, Sand stone 0.001 to 1 ok. So, hydraulic conductivity ah will be more
in case of gravel and sand and less ah very less in case of clay right. So, that water is very difficult to transfer
or transmit in case of clay soils ok . So, with this this is the ah in this lecture
we mainly focused on some basic stuff on ah you know groundwater. So, what is geological formation? What is an aquifer? What is a aquifuse, aquitard right and aquiclude? So, these are different kinds of geological
formations which are present underneath. ah And, also the different ah soil waters
ok and then the properties of aquifers and we also studied, ah what is the ah aquifer
types like confined aquifer, unconfined aquifer, ah perched aquifer and then leaky aquifer
ok. And, then ah we also studied the water table
and ah I mean water table and peizometric surface and also different wells ah based
on their ah connection to the different ah you know aquifers. So, the valleys connecting to connecting to
confined aquifers is called artesian well, if the valleys connecting to ah your confined
ah unconfined aquifer it is called phreatic well or water table well. If the valleys connecting to the ah perched
ah aquifer that is called perched water ah well ok so. Thank you.

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