 Welcome to this lecture number 3 which is in which I am going to continue from where I stopped in the previous lecture that is on ground water level fluctuations and environmental influence and here we know that it should be our endeavor to maintain the ground water level which may be water table in case of an unconfined aquifer or it is known as the piezometric surface in case of a confined aquifer. So this ground water level should be maintained at the optimum range. It should neither be too shallow nor should it be too deep and to maintain this optimum level of ground water during all the times. So we need to study the reasons based on which the ground water level fluctuates. So some of the reasons are listed here. The first one is stream flow variations followed by meteorological and tidal phenomena and urbanization is also one of the parameter which influences the ground water level fluctuation followed by earthquakes, external loads and so here land subsidence. Here let me also write land subsidence etc. So these are some of the reasons or causes for the ground water level fluctuation and let me also continue. So in the previous class we are discussing about the secular as well as seasonal variations. While the secular variations represent the variation in the ground water level over a period of more than a year, seasonal variations they represent the variation in the ground water level within one year over different seasons and there is also third type of variation which can list it as the short term variation. So in this case so it is basically daily or diurnal variations which is very commonly observed in municipal water supply wells. So essentially here you can say this short term variation is a daily represents daily or diurnal variation, seasonal variation represents the average annual variation, average variation within an year and secular variation represents the variation in the ground water level over a period more than a year. So we need to consider all the three and during the all the periods right from period ranging from less than a day to a period which is say around a year to a period which is greater than that may be a number of years could be 5 or 8 or 10. So all these things we need to consider. Now let us consider firstly the stream flow and its impact on the ground water levels. So let us consider in this figure a stream with bed shown along the sloping bed shown here and then there is an impermeable strata let this impermeable strata consist of a hump as shown here in the middle and because of this hump in the impermeable strata. So the water streamlines the ground water streamlines will show an upward trend and especially in the hump so where the height of the impermeable strata is at its highest. So the sum of the stream the ground water streamlines they will be above the ground so they become surface water streamlines and so in this portion so that is say from here to here. So this water table will be above the ground level whereas to the left of this the water table is below the ground as well as to the right of this section the water table is below the ground level or in this case it is a stream bed level. So here what happens is so this portion that means from the hump to the portion where the ground water streamlines have transformed themselves into surface water streamlines. So it will be acting like a effluent stream or a gaining stream because here what happens is so the all the ground water will contribute as effluent or as will contribute to the stream flow. Similarly the downstream of the hump what happens is because the it is the downstream part of the hump so here what happens is the surface streamlines which were appearing here they again develop a downward orientation and then they get transformed into ground water streamlines. So therefore from this point onwards downstream the water table is again below the ground level and this portion between the hump and the location where the surface streamlines again get converted into ground water streamlines. So that will behave like a losing stream or an influent stream so in this case what happens is because of the water table higher than the water table in the adjacent as one so the stream will lose so through infiltration as well as the through infiltration occurring at the bed as well as the banks so therefore it will be acting like a losing stream or an influent stream. And in this in the middle portion so it is the rising water in a stream channel from the emerging ground water flow so this is how the stream flow will affect the ground water level. So many times this stream flow the effect of the stream flow on the ground water level is also visible especially in case of floods the flood wave. Let us consider in this case a flood stage hydrograph which represents a variation of stage which is essentially the water level in the stream at a particular location with respect to a standardized datum such as the mean sea level and its variation with time. And in this case the time values are represented in terms of flood periods so flood period is the period basically it starts from the duration from the time instant when the stream flow or the discharge goes on increasing abruptly above the base flow which is basically the ground water contribution. And then again after reaching the peak flood during which time there will be peak stage so this is the peak stage here again it starts the flood wave starts receding so the flood level as well as the flood discharge gets reducing. And finally it reaches the normal level so this period from the increase from the time instant when the flood wave started increasing to the time instant when the flood wave started decreasing so this is known as flood period. So here it is indicated as the time instant between 0 and 1 so here this is the flood stage hydrograph and the difference between the initial water level which is here in Indian terminology it is known as the no flood level or the danger level and also the highest flood level that is the final water level that is the water level during the flood peak. And this one this initial stage of flood is also the final stage because after the flood wave recedes it again goes back to this one and here this is the ground surface which is slightly above the flood stage flood peak stage. And in this case now let us consider how the bank storage volume as well as the recharge and discharge vary in case of a flooding stream and so now let us consider the bank storage. So essentially this bank storage volume is the volume of flood water which gets stored in the banks of course which includes beds also. And here what happens is so this as the flood stage goes on increasing from the initial level to the flood peak stage. So the bank storage volume also goes on increasing and when the flood stage is at its peak the bank storage volume is also at its peak and as the floods stage goes on decreasing the bank storage goes on also goes on decreasing but more gradually not as abruptly as the flood wave. So here in this case if the time for the receding flood stage hydrograph is may be of the order of say 40 to 50 percent of the flood period whereas in this case the time for the reduction in the bank storage volume it may be almost say 2 and half at least 2 and half times the flood period. So it is more gradual here. And now let us consider the recharge and discharge volume which occur due to the water entering the bank storage and the water leaving the bank storage. So here initially when the flood wave starts and the flood stage goes on increasing from the initial level to the flood peak stage so the recharge goes on increasing and it reaches the maximum and again so this recharge then the recharge goes on decreasing and at this time instant when the flood stage is at its peak and the bank storage volume is also at its peak. So here so the recharge volume which essentially is represented by the volume bound by this curve so this recharge volume is also the maximum. So here this is the peak recharge intensity and this recharge intensity goes on decreasing and then the recharge volume so this is the area here so that is maximum. And once the recharge is maximum then what happens is the water starts leaving the bank storage and so that will appear as a discharge. So this discharge which starts from the time instant of the flood peak stage it starts increasing initially reaches peak somewhere between the flood peak stage and the first time flood time period. So somewhere in between so this the discharge volume is this the discharge intensity is maximum and then so like the bank storage volume so then onwards so the water leaving the bank storage will also be gradual. So essentially this area which goes on increasing represents the that is the water leaving the bank storage and again so in about 2 and half to 3 times as 1 so it shows the it again comes back to its original value. So this is the impact of stream flow so previously in the previous case we studied the we discussed about the ground water level in case of a river which is having an impermeable bed with a central raised portion or a hump and in this case number 2 we studied the variation of the flood volume especially the bank storage volume as well as the water entering the bank storage during the recharge process and water leaving the bank storage during the discharge process. So these are some of the cases of stream flow influencing the ground water level. Now let us come to what is known as the base flow and this base flow as a name itself says so it is the flow which is existing in a stream due to an extended contribution of ground water. So obviously the stream flow it originates from the ground water flow so that this base flow is a stream flow portion which originates from the ground water flow. Now let us consider a flood hydrograph as I was mentioning here so this is the total hydrograph which we can also call it a flood hydrograph say somewhere here this during this period there is a precipitation or rainfall then because of that so there is surface runoff increase and then it starts the flood hydrograph and it reaches the peak. So this is the peak discharge or the peak flood discharge and after that it goes on decreasing and again it will reach the total discharge will be equal to the base flow wherein entirely it is the base flow which contributes to the total flow from this time and beyond. Then now here let us study the variation of the base flow as we discussed in the previous case so because during this period recharge is taking place the ground water recharge is taking place and unless the ground water gets saturated so there cannot be any discharge. So therefore this is essentially the dotted line from here to here this represents the recharge phase and then there is no this one and if we from this point onwards the discharge starts so this discharge it goes on increasing reaches a peak and then it starts decreasing and so finally it starts gradually decreasing and it reaches from this point onwards it is only the base flow which is contributing this to the stream flow there is no other contribution to the stream flow beyond this point. But if we consider the if we neglect the bank storage then what happens is so the base flow hydrograph which is essentially the plot of the base flow with time it shows less fluctuation so because so here what happens is so this base flow hydrograph in this we are neglecting the bank storage and because of this one so it is showing the minimum base flow does not reach 0 because there will be always some but this is not the actual case so this is just the base flow hydrograph when we neglect the bank storage. So here the minimum base flow is also less is also slightly more than the 0 which is the actual minimum base flow and the maximum base flow is slightly less than the actual maximum base flow which occurs somewhere here so like this so it is the base flow which essentially is the one which provides the this one that is which provides the extended ground water distribution over space and time and it is essentially this base flow in the in hydrology it is represented by the various infiltration indices like the phi index the w index so which represent essentially the infiltration part and then so especially this w index the initially abstraction also so essentially so the base flow represents the minimum total flow which exist during the period wherein there is no flood flooding. Now so this represents the base flow variation during a flood hydrograph the actual variation shown by the dotted line and the base flow hydrograph neglecting the bank storage shown by this line which shows a higher minimum and a lower maximum. Now let us consider the other factors such as the impact of evapotranspiration on the ground water fluctuation so in the previous class we mentioned that even though ground water it is below the ground so always there is some small portion of ground water which is closer to the surface which is very close to the surface level and it is affected it is subjected to evaporation and so therefore what happens is so because of this small that is the portion of this ground flow which is directly subjected to evaporation near the surface so there in the surface near the surface so the evaporation expressed as a percentage of pan evaporation is very high as you can see here so this is say some in the range between say the surface and say 0.5 meters or say maybe 1.45 meter which is one and half feet so around this range in this top layer so the ground water evaporation which will be almost 25 to it is more than 25% and even it reaches as high as say 50% of the pan evaporation rate there but as you go deeper and deeper so this the evaporation effect it will be less and less visible on the ground water fluctuation because so what happens is so this is the ground water is at a depth below this 0.5 or say 0.45 meters and therefore what happens is the ground water evaporation of ground water expressed as a percentage of pan evaporation it goes on decreasing and even it may be so at deeper depths it may be practically negligible it may be of the order of say 5% or even less than that so this is how the ground water fluctuates due to the effect of evaporation so the effect is significant in the ground water layer which is at the top that means from the surface to say 0.45 to 0.5 meter below the ground and below that the fluctuation of ground water level due to evaporation so that will be significantly less. Now let us also consider the effect of transpiration because as you know evapotranspiration consists of evaporation which is occurring at the open surface or the open water bodies or the bare soil the soil moisture gets evaporated there and the transpiration is basically the water which gets which transforms into the vapor form due to the metabolic activity of the plants so the plants they during the process of metabolism so through this stomata openings so they give out water in the form of water vapor which is known as transpiration and here say let us consider a day in few days in summer and the depth of water table and as you can see here so this day 1 represents a 24 hour period similarly day 2, day 3, day 4 all of them represent 24 hour period and as we all know so the temperature is maximum sometime immediately after the midday so therefore during that period the transpiration rate will also be maximum so therefore the depth of water table will be the highest that means the water table will be the deepest at that and again as the evening approaches and then as the at the midnight and then so beyond that so what happens is so due to the reduction in the transpiration activity so the depth of water table it shows a slight increase so this may be of the order say few centimeters may be something like less than definitely less than 10 centimeters generally of the order of say around 5 centimeters then again the next day it happens the same thing what happens is so here somewhere immediately after the midday the transpiration is a maximum the rate of transpiration is a maximum so the water table depth is minimum so this kind of daily variation of the water table depth due to transpiration it follows a you can say this is a sinusoidal wave now let us consider the effect of the vegetation evapotranspiration essentially because it is extremely difficult to measure evaporation or transpiration separately because when we plant when plants grow on the ground so there are some areas where there are plants which are planted artificially or plants which grow out naturally whereas in the neighboring areas so there may not be any plants so essentially so in such case so there will be loss of water from evaporation in the bare soil or the soil or the area where water is the plants are watered by say flooding and other similar processes so here what happens is so therefore it is a well known practice to express this total loss due to evaporation in transpiration together as evapotranspiration so here see in case of a bare soil so the evapotranspiration is the depth to water table the plot of say depth to water table versus evapotranspiration it follows this curve so therefore so at this one so at this depth so there is zero evapotranspiration so this could be say a few meters may be 2 or 3 2 to 5 meters also depending upon the type of soil and the presence of the impervious strata below and on the other hand say wherever there is a shallow rooted vegetation so these consist of plants whose roots they do not spread say more than say 1 meter from the ground surface so in such case what happens is so the at the ground surface the evapotranspiration is the maximum and then as we go deeper and obviously this depth the depth where in the evapotranspiration will be practically 0 so this depth will be higher than the depth due to the depth observed in case of a bare soil so here so the variation of evapotranspiration and the depth to water table in areas having shallow rooted plants or crops or vegetation it follows this one obviously here this the and here as you can say this is a bare soil so this evapotranspiration is entirely due to evaporation whereas in this case so if you can say this is due to evaporation and this is due to transpiration so and at this depth which may be say something like say 1 to 2 meters so here so the at this depth and beyond so the evapotranspiration becomes practically 0 and now let us consider deep rooted vegetation like such as the large trees so for example I can give this banyan tree so this banyan tree that is one of the where in the roots so they also grow and they also reach the ground then they grow bigger and bigger and thereby giving additional stability to the banyan tree and in this case the roots may go even few meters deep also maybe sometimes even maybe tens of meters deep rarely so here what happens is this the plot of evapotranspiration and the depth to the water table so it will follow a curve which is even which is having a at the ground level the evapotranspiration is even more because it is a deep rooted vegetation and the when it is deep rooted vegetation it also has a large number of leaves as well as stem through which this transpiration takes place so therefore the evapotranspiration at the ground level is larger and also so up to this the this is basically the it represents the root zone depth which may be of the order of say few meters say 5 to 10 meters so only beyond that the evapotranspiration becomes 0 so therefore so this is the this diagram shows the variation of evapotranspiration with the depth to ground with the depth to water table for bare soil shallow rooted vegetation so where in we can say the vegetation is the root zone depth is say less below less than 1 meter or so less than 1 meter whereas deep rooted vegetation where in the root zone depth is even up to say 8 to 10 meters or so so in this case so this is the variation so now let us come to the variation or the fluctuations in the ground water table due to other meteorological phenomena such as the atmospheric pressure and we know that so it is the atmospheric pressure which is the prerequisite for a low atmospheric pressure is a prerequisite for precipitation when there are wherever there is a low pressure formation so there what happens is the water the atmospheric moisture which is as well as the moisture which is in the clouds it has a tendency to precipitate and this precipitated the clouds with water atmospheric moisture condense in them becomes heavy and then there the precipitation starts in various forms of precipitation so here what happens is so the as the atmospheric pressure decreases so here so the these peaks because the atmospheric pressure it is shown to be in the graduation it is the higher atmospheric pressure is shown in the downward direction so therefore this as the atmospheric pressure increases so there is no possibility of precipitation so therefore the ground water the well water level which essentially represents the ground water in unconfined aquifer it will be at its lowest level and as the atmospheric pressure decreases so it represents so these points so here so the there is a possibility of precipitation and then due to precipitation the well water level which represents which also represents the ground water level in an unconfined aquifer increases so like that so this is the effect of atmospheric pressure on the ground water fluctuation in case in this case we have considered only the unconfined aquifer. Now let us come to the the effect of precipitation in general and say rainfall in particular on the ground water level fluctuation so here let us consider the weakly hytograph which represents the amount of precipitation which has taken place during that particular week so in this case this rectangle represents the precipitation depth in the week number one similarly this represents the precipitation depth in week number two so like that so this is basically a stacked bar graph with the width of each of the stacked bars equal to the duration of one week and in this case now in the in red colour what the average water table elevation in the wells which represents the ground water level in an unconfined aquifer is represented in this case as you can see here so this is basically here we can say this is we can consider this to be the monsoon season in a region say in India where the monsoon is significant and this one so here what happens is as long as there is some amount of average weakly precipitation so this results and the average weakly evapotranspiration is somewhat less than this so this the ground water the average water table elevation will go on increasing and there are some places where the average water table elevation is almost stagnant so those they those represent the weeks wherein the total weakly precipitation as well as a totally total weakly evapotranspiration is more or less same or slightly less in that case it also shows like decreasing trend and in this case so here say this during week number 17 and 18 there is absolutely no precipitation so in this case the precipitation amount the weakly precipitation amount is 0 whereas the weakly evapotranspiration amount is significant so therefore it shows the water table shows a lowering trend so that is shown by this and then again on the 19th week so there is some precipitation and because of that the water table again and as long as this the depth of precipitation is slightly more than the average weakly evapotranspiration so this water table goes on shows a slight increase so like this the various meteorological phenomena will impact the ground water level fluctuation now let us consider the tidal range the effect of tides on the ground water level in this case we know that so then in English there is say that time and tide wait for none so these tides essentially are daily tides which occur with the time period of say 12 hours I am sorry 24 hours and 50 minutes there are also the spring tides and so in a duration of say 24 hour 50 minutes so point on earth will be closest to another similar point on moon so essentially these tides occur due to the attraction between the earth sun attraction earth sun earth moon as well as sun and moon so this obviously of course sun and moon it would not have any effect on the earth but at least the attraction between earth and sun as well as attraction between earth and moon it earth and moon so this attraction results in the daily tides so which will so in a period of say 24 hour 50 minutes there will be 2 spring tides there will be 2 high tides and there will be 2 low tides and then similarly during summer and then during winter spring as well as autumn there will be so tides so here let us consider so the tidal effect on the ground water level and in the left hand side in the figure we are considering a confined aquifer so here this is the ground water level so here although the ground slope will not be as steep like this generally so for simplicity so the interface between land and ocean is represented by this vertical line and so this represents a confined aquifer and here so this tidal range is represented at 2 H 0 where H 0 is the height of the water surface above the mean sea level at that particular tidal location or coastal location and then at a general distance x measured from the coast so this is the coast here which has been approximated as a vertical line so at a general distance x the tidal height is equal to 2 H so here what happens is because of the tidal effect so the when the water level in the sea is maximum so obviously the ground water level at that level will also reach that will also be reaching that maximum and of course as you move away from the coast so this variation will also be decreasing and eventually at some distance upstream from the coast so this tidal variation will be practically negligible and so this is in case of a confined aquifer and because of this so there will also be the this represents the variation of the piezometric surface in case of a confined aquifer now on the left hand side let us consider the tidal effect on the ground water level in an unconfined aquifer so here so there is an unconfined aquifer which essentially is spreading from the ground level here so this is the ground level all the way up to the impervious strata so this is the impervious layer and similarly this is the impervious layer in case of a confined aquifer and of course this is also the impervious layer the top confining layer which is also an impervious layer and in this case what happens is the same phenomena is observed and the when the tide reaches its maximum range that is its maximum level so the ground water level will also be fluctuating and it will show the maximum level which is h 0 times above the mean sea level and here in this case this represents the mean sea level and at any distance x upstream of the coast so this ground water table will be having an increase in the depth in elevation given by h which is less than h 0 and at some distance upstream of the ground water level so it will be 0 the ground water fluctuation is 0 so this is the impact of the tidal water level on the ground water level fluctuation. Now let us come to the effect of urbanization on the ground water level in this case the we all know that in case of urbanized localities so there will be this large amount of households with say practically impervious surface or finished surface and so which result in significant increase in the surface and now at the same time significant decrease in the total ground water recharge unless and until so there are some specific artificial ground water recharge structures are designed and installed so the ground water recharge will be highly insufficient so therefore so this reduced ground water recharge will result in gradual lowering of the ground water level unless until we take measures to have appropriate artificial ground water recharge measures so therefore the effect of urbanization in general in the areas is very what I should say is the reduced ground water discharge possibly so here this is leading to unsustained ground water levels that means eventually the ground water level will be so deep that extraction of ground water will become unfeasible and will not be feasible anymore now so this is the effect of the urbanization now let us also consider the effect of earthquake that is eq on ground water levels so here what happens is in case of earthquakes there will be a variety of effects as I was mentioning in my previous class the earth shakes and due to this shaking of earth and there will be various in the fault zones as well as in the various so there will be significant amount of damage to the earth and especially in case of structures such as buildings and other engineering structures and so because of this cracks which develop on the in the buildings which also continue along the ground so here what happens is so there will be sudden rise or fall of water level in the wells so if there is a sudden rise in the water level in a well so may lead to sudden rise or fall in well water levels if there is a sudden rise in the well water level immediately after earthquake then we should realize that there is possibly some additional very high ground water recharge taking place due to the changes in the subsurface soil layers due to earthquake and similarly if there is a sudden fall in the well water level then it is an indication that so the water from that particular well is flowing into say neighboring locations, neighboring areas so thereby there is a sudden fall in the water level of the well so these are some of the impacts of this one and now let us also consider say the impact of the land subsidence as well as external loads so these external loads as well as the impact of land subsidence on ground water levels so in this case both these land subsidence as well as external loads they result in a significant variation in the ground water levels so here what happens is wherever there is an external load applied so this load may be due to the load may be due to a particular hydraulic structure such as it may be a hydrostatic load or it may be some other load so here what happens is due to the application of load the ground shows certain amount of deflection the ground the soil above the aquifer shows certain amount of deflection and when this load is removed so this deflection does not really get removed for the simple reason that soil is not perfectly elastic and so there is a lot of plasticity in all in the soil and because of this so there will be fluctuations there will be impact on the ground water level so and then similarly this land subsidence and many places this wherever there is a external this one external wherever there is a say oversaturation of ground water so there the land becomes the soil its unit weight will decrease from gamma sat to gamma sub so in this case in case of oversaturated soils so this is gamma sat changes to gamma sat minus gamma sub gamma w which is the unit weight of water and this gamma sat minus gamma w is the submerged unit weight of water so therefore when the just below just when the unit weight of soil is at or slightly below the gamma sat so it will have the optimum moisture content and it will have the optimum strength and as the its unit weight falls so its strength also falls so this strength the fall in the strength is many times so significant so that there will be land subsidence and so this land subsidence so maybe this landslide is an example of that and so because of that so this what happens is the ground water level fluctuates so these are some of the impact of the earthquake external loads as well as land subsidence and so this one will now in this so this is how so the ground water level fluctuates practically due to various causes various reasons and so this fluctuation will result in the ground water level which is either too shallow or too deep so that wherein the extraction of ground water level or harnessing of ground water level becomes unsustainable becomes costly so therefore we should see that so by adopting appropriate measures we should by analyzing the ground water fluctuation we should ensure that the water table in case of the unconfined aquifer or the piezometric surface in case of the confined aquifer it is maintained at the optimum depth and so in the so we will stop here and we will continue although I mentioned in the title of this one the literature data sources as well as internet sources in this lecture so we will continue it in the next class. Thank you.