Welcome to this lecture number six on zone

of aeration and saturation followed by the aquifers and the characteristics classification

so in the previous previous lecture we discussed about the ground water column and specifically

of course we will briefly dealt about the zone of saturation which is below water table

and zone of aeration which is above the water table and we also discussed about how the

in the zone of aeration how the water is held which is basically in the in the intermediate

zone which is vadose water and here so this water is held by

hydroscopic and capillary action

so here because of this the attraction between the the soil or the rock particles as well

as the water so the water is held the water which is not moving is held by these two actions

and the capillary rise is the we also saw the expression for capillary rise which is

a given by 2 sigma into cos theta divided by r into let it be the specific gravity so

here now let us continue before this one i would like to mention so

essentially so this vadose water water held by hydroscopic and capillary action

and here so in this ground water so water is held by gravitational action okay and continuing

with this the expressions for this capillary rise so here suppose the this is a capillary

tube which is basically a narrow tube this is center line and

the diameter of tube which is given by 2r and

so this is the surface tension force whichis measured as force unit length and this is

the angle theta and then this is the water surface this is meniscus and so this is the

that is the liquid this case it is the water with specific weight gamma and this theta

is the angle of contact which i have mentioned here and say for if we further simply so this

so surface tension which is mentioned as the force per unit length generally it has a value

of say 074 so this is a grams per centimeter ofcourse so this multiplied by the acceleration

due to gravity that is 981 and then this specific weight of water again this is one gram per

cubic centimeter again it has to be multiplied by the so this multiplied by g so this g is

the gravitational acceleration this is 981 meter per second square and this theta approximately

=0 degree for water and clean glass same thing can be assumed in there for water in

soil also same can be assumed for water and soil also therefore the expression for capillary

rise h so it reduces to 5 divided by r okay so as can see from this one the capillary

rise h which determines the height of the capillary zone which inversely proportional

to the size of the capillary pores that is the the radius of the capillary force that

is basically here we are approximating the pores in to the requirement circle or shape

and then taking its radius so this h as a this the equivalent the circular pore radius

gets reduced the height of the capillary rise increases and here this mentioned that say

for different samples that is the say the material grain size in millimeter then the

capillary rise in centimeter so here on the lowest level were the capillary rise is as

low as just 25 centimeters we have this fine gravel whose grain size is that is 05 to 02 ok further as we go so this

is a next is a coarse and of course there are intermediate formations are there so here

coarse and for which it is a one the grain size is one that is a 1 to 5 millimeter and

it has a capillary rise of 135 centimeter and then i am sorry i made a mistake here

so this says this just like correct this one so this is grain size is 2 to 5 and coarse

and the gain size is 5 to 1 let me re write this again so this is the material grain size

in millimeter then capillary rise in centimeter see on the coarse test we have say find gravel

the grain size is said 2 to 5 millimeter when the capillary rise because this is too large

grain size so therefore the pore size is also too large so therefore the capillary rise

is also as say 25 centimeter followed by vary coarse sand the grain size is 1 to 2 millimeter

and the capillary rise is say 65 centimeter still let us go to still smaller this one

that is a coarse sand where the grain size is 5 to 1 millimeter and then the capillary

size is getting reduced and correspondingly the pore size also gets reduced so this is

135 centimeters then the medium sand as a grain size of 2 to 5 and the capillary rise

of say 246 millimeter centimeter am sorry then followed by say fine sand the grain size

is say 1 to 2 and the capillary rise gets further increased to say 428 centimeter then

it is the silk which has a grain size of 05 to 1 millimeter the capillary rise increases

to 1055 centimeter and lastly it is fine slit it has a grain size of say 02 to 205 and the

capillary rise is as high as 200 hundred centimeter so you can imagine so how the grain size the

material gets ahh finer and finer in terms of its grain size so the capillary rise increases

and this has been so sources ahhh study by lachman okay and now let me also represent

here the distribution of water in a coarse sand water distribution in a coarse sand above

water table after draining so here suppose this is the so this is the

moisture so this is the soil moisture content as percentage and this is a height above water

table in centimeter and here say let us say this is say this is 20 and this is 40 and

here let us say this is 20, 40 so here this variation is it will be something like this so this is the so this represents porosity

and here this represents capillary zone and above this this represents the intermediate

zone so in the capillary zone just at the water table level so the moisture content

is exactly equal to porosity and as we go above the water table the moisture content

goes on increasing as we can see here and so when we reach this intermediate zone so

this moisture content it represents only the hydroscopic water so whereas so here we can

say this is the hygroscopic component and then this the moisture content held by capillary

action and this moisture content help a hygroscopic action so therefore so in the intermediate

zone it is entirely the water is held by water is held by hygroscopic action whereas

in the capillary zone water is held by hygroscopic and capillary action so you can see how it

varies and say so we just at the water table level just above the water table so almost

all the pores are filled with this moisture soil moisture and then as we go higher and

higher it is only those force which is only continuous so they will be filled with as

we reach this intermediate zone so it is only the hydroscopic water which is the water held

by the force of attraction between the in the the spaces the void spaces as well as

the air, as well as the rock particle and then the water so so this is how for for in

the zone of saturation the water is held entirely by gravity and that is for all the zone below

the water table and as we go above at the water table so this is held by hydroscopic

as well as capillary action and as we reach the intermediate zone so the number of force

which contains soil moisture gets reduced only those continuous pores ah they first

of all for the capillary action they must have a very small size and then so through

which the capillary rise takes place due to the force of attraction between the soil or

rock particle and water due to surface tension force and above that in the intermediate zone

it is entirely by the molecular the inter molecular attraction between the soil moisture

the air void as well as or rock particles now let us consider the and this water content

which is held by various actions whether it is the hygroscopic action in the intermediate

vadose zone or hygroscopic as well as capillary action in the capillary zone both in the zone

of aeration and further below in the zone of saturation by gravitational actually acceleration

so this moisture can be measured by various methods which is a gravimetric method as well

as the other the tensiometers and so on and the same thing here we can mention here by

a tensiometer basically it made it just pulls so this water which is held in the

that is the measurement of soil moisture so this is by

gravimetric method and which we take the weight measurement and

also say by tensiometer so in this tensiometer suppose this is the soil column and here

so this is the the tensiometer so this is the poros cup and

this is the unsaturated soil and here so this is the suction head which is essentially

water held by the molecular attraction between water air particles as well as soil particle

so now let us come to what is known as the available water or available moisture content

so here suppose i represent so this is the soil moisture

and here so this is 0 so the maximum soil moisture so this is the soil moisture content

which can be which is possible is which is denoted as field capacity so this moisture

content let me represent this as mc so this is a field capacity moisture content so this

field capacity is essentially the maximum possible water which can be held in this zone

of aeration and also the minimum this is amount of soil moisture corresponds to so this is

the permanent wilting point moisture content and of course there is also an intermediate

this is known as the optimum moisture content content or say omc so here essentially so

this field capacity is the amount of water which is held in a soil after wetting and

after drainage as become negligible so generally it is after say two days of drainage so the

after two days of keeping it for draining so all the water which is generally goes out

through drainage that gets drained out and then the moisture content which remains in

that soil sample or soil or rock sample is the one which represents the field capacity

moisture content and the total amount of moisture is the volume for that is the known as field

capacity similarly so this welting point is the ahhhh ahh the moisture content it represents

the moisture content wherein the plants start welting that means say below this welting

point all the water all the moisture it is held by only molecular attraction between

moisture or water particles as well as air and soil or rock particles so it is not given

away by the soil so therefore plants cannot extract any water by capillary action so the

plant welting or drying so this drying plants so further so they die so this difference

between the field capacity moisture content and then the permanent welting so this is

known as available moisture content and similarly the difference between the field capacity

and optimum moisture content which is slightly higher than the permanent welting point so

this is known as the readily available moisture content so essentially so they all the irrigation

so they depend upon readily available moisture which can be easily extracted by the plants

through capillary action so therefore in this irrigation what is done so as soon as this

soil moisture gets depleted to this optimum moisture content level so the one irrigation

supply is given so then the moisture content increase to field capacity moisture content

level and then further again by the due to a plant metabolic activity due to evapo-transporation

as well as evaporation the soil moisture gets gradually decreased and again when it reaches

this the optimum moisture content then the next irrigation is given so like this so this

is the this how the process of irrigation continues now let us come to the moisture

content in the zone of this one so this available moisture so here so this is in the zone of

aeration ok so now let us come to the zone of saturation which is the which is the now

let us come to available water in the zone of saturation

again here so in the zone of saturation so most of the water is held by is this zone

of this is a gravity action of course very small amount is held by hygroscopic action

so here let us define say two terms which is the specific retention

so if we denote this as sr so this is the ratio of the volume of water which is retained

in soil against gravity after saturation divided by total volume so this vr is the the water

volume retained in soil after saturation against gravity

and this v is the total soil or rock volume and let us also define another terminology

here that is the the specific yield which is denoted by sy so here this is equal to

vy divided by v where again v is the total soil or rock volume and this vy is the volume

of water drained and obviously this is by gravity okay and so this n which is the porosity

of course few hours in the previous class i think i represent this by alpha and few

authors they use the notation n so this is equal to the

specific retention plus specific yield so this is a very important relationship between

porosity and specific yield as well as specific retention and here so essentially so if we

if i represent so this is the water table so water retain above water table is known

as the field capacity water here you can say it is moisture retained

in the zone of aeration here you can say let me the maximum water and here so this is the

and below this it is the water retained so this is what i retain corresponding to

the specific retention water sr maximum water retained in the zone of saturation so essentially

this field capacity as well as the water retained so they represent the same water content while

the field capacity represents the water the maximum water retained in the zone of aeration

wherein all the pors are saturated well the water retained represent the maximum water

retained in the zone of saturation and obviously so this is corresponding to sr so this here

we can say this is and both are against gravity so like this so the essentially this field

capacity as well as water retain so they represent the water which is which can be retained ahh

maximum extent in the zone of aeration as well as zone of saturation so now let me also

represent here so the and as this ground water this one so our objective is to harness or

extract ground water as much as possible so therefore this specific yield is more important

to us and here suppose i represent on a scale the specific yield express as percentage so

here from the lowest level say this a 10, 20, 30, 40 and in the highest this one say

44 we have peat which is basically in the vegetative matter which is decomposed and

then the lowest specific yield is observed in clay which is so peat as 44% and this clay

which is as minimum which is 3% and in between we have say the dune sand 38% and we have

say medium sand which is say 28% and we have say coarse gravel 23% and we have sand stone

21% and we have lime stone 14% and we have silk here with generally the this one okay

so these are some of the typical values of the specific yield in different materials

ranging from the minimum 3% in found in clays and maximum of 44% found in around 44% found

in peat so in between we have silk, limestone, sand stone, coarse gravel, medium sand, dune

sand okay and of course few more so this is the variation of specific yield in different

soil or rock furnishing now let us come to the aquifers their classification

characteristics and classification so so this aquifers are essentially there

is a water bearing layers so here you can say in this so typically we can say say this

above this lime stone all this rock or soil formation they represent aquifers below this

limestone all this soil or rock formations they represent either aqui tards, aqui tudes

and aqui fuges which we discussed in the previous class in the previous lectures so here i would

like to draw you attention to a schematic cross section representing the various aquifers

so this are the impervious so this is the impervious strata so this is the recharge

area so this is the ground level and here i would like to represent the water

table or piezometric surface so this is the water table and this is and this is peizometric

surface which represent the basically the energy

and here up to this recharge through different forms of precipitation so this soil moisture

the water moves and this this is known as the confined aquifer which is essentially

confined at top or as well as bottom confining layer so this is the confining layer

and here suppose we drill a well here which penetrates all the way up to confined aquifer

and here this one we may find we may so this is water table

so this well is penetrating through the all the way and here this is the confined aquifer

and then this is the unconfined aquifer also known as water let me write here so this unconfined

aquifer that is water table aquifer so this unconfined

aquifer represents top most aquifer which has only the one confining layer at the bottom

whereas its top surface represents the water which is subject to which is undulating and

it is depending upon the slope the areas of recharge, discharge and pumping and other

factors and in this case so now let us come to this well which has been dug in the which

has been drilled in the in the area and the land area which is below the peziometric surface

and it penetrates all the way up to the confined aquifer and here the total s this peziometric

surface represents the energy the total energy at that level for this depth all the way for

confined aquifer so therefore here the well starts oozing out water that is corresponding

up to peziometric surface and such a well is known as a flowing well

so here the water gushed out of the well on its own so now artificial pumping is required

and on the other hand suppose we drill a well which penetrates only the unconfined aquifer

in this case the surface the water surface corresponding to the water surface at that

location and so therefore this well which penetrates only the unconfined aquifer is

known as the water table well and on the other hand suppose we drill another well

which penetrates through the unconfined as well as unconfined aquifer so here water table

corresponds to this one and the peziometric surface and this well is known as artesian

well so this confined aquifer is also refer to as artesian aquifer which

is also known as pressure aquifer okay so there are three types of well the

flowing well the water table well as well as artesian well when the flowing well the

water is gushes out on this naturally by because of the the ground surface below the peziometric

surface is there whereas in the water table well as well as the artesian well so the water

surface will be below the corresponding to the water table or the peziometric surface

and this artesian well it draws from both the confined as well as unconfined aquifer

at the bottom as well unconfined aquifer at the top so we will stop here and we will continue

further our we will continue discussion in the next lecture thank you

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