 In the last class, we discussed about the role of ground water in hydrologic cycle. So, today we will continue our discussion. This ground water represents the important, one of the three important systems of hydrologic cycle which can be referred to as a subsurface system of hydrologic cycle. That means all the activities, all the components of hydrologic cycle which are taking place below the ground level. And here, let us go to this one. So, the ground water is stored in the below the ground in water bearing layers which are known as aquifers. So, this storage capacity of these reservoirs, so that is which are commonly referred to as aquifers as well as the small ground water flow because when these aquifers get saturated, so then excess water from these aquifers flow from below the ground on to the ground wherever there are streams and other surface water bodies. And so, this storage capacity in the aquifers that is the ground water reservoirs as well as the small ground water flows, so they ensure a large as well as a distributed source of water supply. So, therefore what happens is the water supply, the spatial extent gets increased as well as the temporal extent also gets increased, so that is a major contribution of ground water. Now let us also see the ground water emerging into the streams, so this helps in sustaining the stream flow over a large space as well as time when the surface runoff is either low or non-existent. Say for example, let me refer to you here the hydrograph wherein the plot of this stream flow Q is shown against the time t, so this is the time t along the horizontal axis and stream flow Q along the vertical axis. And just above that, so there is what is known as the hydrograph, so this is known as the HYE hydrograph which is a plot of rainfall intensity I versus the time t. And here this hydrograph is a plot of the stream flow, this is the hydrograph, so even though the rainfall is discontinuous that means a hydrograph starts at this point of time and ends here. But still we see that in the hydrograph there is continuous flow, continuous stream flow, so here this portion that means this time duration as well as this time duration after the stoppage of rainfall or precipitation, so that is due to the groundwater contribution to the stream flow. Had this been not there, so then this hydrograph would have been just shown with some lag with respect to this hydrograph but actually it does not happen, so therefore so this groundwater flow, groundwater provides the necessary spatial as well as temporal extent and here what happens is, so this one because of the here you can see, so this is the cross section of a stream and the groundwater is entering and such a stream is known as an effluent stream wherein the groundwater is entering from the banks as well as little bit through the beds also may be into the stream through this what is known as the groundwater inflow which is denoted as QGI. So, here the water table is above the water level in the stream and this difference in the water table will result in the flow of groundwater from the banks into the stream, so here so these through these this hydrograph as well as hydrograph as well as the effluent stream it is evident that this groundwater provides the necessary spatial as well as temporal distribution of water supply and also it is well known that the in many places there are wells and these wells represent the sole source of groundwater in many regions of the world especially during summer when there is practically no other source of surface water and so this happens generally in almost all years continuously that means every summer so the surface water which is available in the form of surface bodies like tanks, lakes or reservoirs so that goes dry so then it is the groundwater which is especially in the wells which will serve as a sole water source in these regions. Now let me also let us also discuss about the origin of groundwater which provides this necessary spatial and temporal sustainability or distribution additional distribution so this practically all the groundwater originates as surface water so through infiltration through this lateral groundwater inflow and so this is known as the natural groundwater recharge so this by this process the groundwater gets the supply and this supply is from precipitation it is also from stream flow it is also from the infiltration or seepage from the lakes and reservoirs. Now many times so this natural groundwater recharge is insufficient so in that case we go in for what is known as the artificial groundwater recharge and this artificial groundwater recharge occurs mainly through excess irrigation it also occurs through canal seepage and it also occurs through intentional water application so here so this ground this artificial groundwater recharge will bring about the balance between the groundwater supply and the groundwater demand so wherever the supply is insufficient compared to the groundwater demand compared to the water demand so there we artificially enhance the groundwater supply through what are known as the artificial ground water recharge structures. Many times the sea water also enters through the course into the ground and which may be undesirable but this also represents the groundwater recharge so here you can see this is the fresh water groundwater interface groundwater fresh water interface and this is the sea water level and this is the coast this is below the sea water level this is above the sea water level and here what happens is the sea water flows into the groundwater below the through this QGIS which represents the sea water inflow and this is undesirable because we know that the sea water is brackish or salty and but what we need for most for many of our activities is a fresh water so therefore this is undesirable so in such cases we need to take precautions by extracting the groundwater in from the wells which are not close to this groundwater fresh water interface so therefore in such case if we need to dig a well for groundwater extraction so here so this is the well we need to see that the bottom of the well is much above the interface between groundwater and fresh water only in such cases the we can extract fresh water otherwise what happens is so if the depth of this well is more so that it is very close to this interface of fresh water and groundwater then in addition to extracting the fresh water it will also start extracting the brackish or salt water of the sea which has infiltrated through the interface so therefore we should take precautions so that only the fresh water is extracted through the wells whose depth is much above the interface now let me also refer to you about the groundwater here so this groundwater volume is much smaller compared to the annual circulation so it is said that on an average the groundwater whose residence time is simply the ratio of the volume divided by the flow rate or the annual circulation so the residence time for the shallow groundwater that means shallow means I refer to a depth of say 800 meters below the ground so in this range that means from the ground surface to 800 meters below ground so in this shallow range so the groundwater has a residence time of nearly 200 years so therefore this as you can see so this provides the necessary the water security compared to this the soil moisture which is there from the ground to say 1 meter below the ground it has a residence time of just say 20 percent of the year or say nearly about 70 or 75 days say roughly say 2 to 2 and half months so after that so this soil moisture will not be there so then but this groundwater the shallow groundwater in the which is existing in the groundwater layer between the ground and 800 meters 800 to 1000 meters some people refer to that is depth as 1 kilometer or 1000 meters some people refer to this depth as say 800 meters so therefore this groundwater provides a necessary that is a distribution of extra distribution of water supply over time and it also because we can extract groundwater through many this sources which are such as wells which we can dig in various areas where we expect groundwater in aquifers so therefore so this provides a necessary that is additional distribution of water supply which is spread over time and space so this is how so the groundwater is so important to provide that additional amount of water for additional time in additional areas so now let us go to the next item of the next article which is the the groundwater budget so this is also referred to as the groundwater balance even though it is mentioned here as the groundwater budget essentially it is water balance or water budget in this case basically what we do is we mathematically express the all the water which is available as water as a supply as well as all the water which is consumed through our water demand so through this what is known as the the water balance equation so therefore so this water balance or water budget here you can also I am writing this as the water balance or water budget so here essentially this is so it is a quantitative statement of the balance between water supply water demand and here through mass conservation conservation principle application so here what we do is so in this fluid mechanics there are three conservation principles and water is an important fluid for sustaining life and so these three conservation principles are the conservation of mass the conservation of energy and the conservation of momentum so here so this conservation of mass it is also referred to as the continuity equation so even though we call here it as the groundwater budget or groundwater balance essentially it is water balance or water budget because in the groundwater we cannot simply segregate groundwater completely completely from the surface water for the simple reason that so there is always an interaction between the surface water and groundwater and that is why here what happens is so through various activities so the various quantities of supply as well as demand are estimated okay and here so we can also through mass conservation it is a quantitative statement of the balance between the water supply and water demand through mass conservation principle application so here essentially this groundwater budget equation is the form of that is the total supply that is existing and the total demand that is existing so the difference between the supply and the demand will appear as a change in storage say for example if the supply is more than the demand then there will be an increase in the storage so essentially if we take a system and in the system there will be some inflow into the system there will be some outflow from the system and let us take for example a groundwater system so which is bound spatially and here what happens is so the inflows to this groundwater system so they are essentially from say precipitation or they are also from other water supply such as irrigation or maybe there could be so into this system there may be some lateral inflows so these are all these are all the terms which represents inflow likewise so in terms of the outflow so we can say that is the pumping say for example if I draw this groundwater system and here so these are the inflow so this is the outflow and here let me say represent so this let me this is the groundwater storage so here so this is the system diagram for the groundwater and it will have the inflow from one side of course there may be more many times inflow may be from more than one side also and there is outflow also from many sides and then depending upon this one I can write the expression as sum of inflow minus sum of outflow is equal to that is the change in storage so here so coming to this one so there are so therefore what you can what you need to do so this is essentially one equation so therefore we can at the most determine one unknown okay so here as I said the this equation that is the groundwater budget equation or the water budget equation so can be written as I minus O is equal to delta S by delta T so this is the total groundwater inflow or let us say this is the total inflow and this O represents the total outflow and then this delta S by delta T represents total change storage volume the same thing we can write it as say this is P plus IR is equal to ET so here this P represents let me write all this term P represents the precipitation IR represents the irrigation ET represents the evapotranspiration plus E plus QP plus minus QG plus QS plus minus delta SM plus minus delta SS plus minus delta SG here P represents total precipitation over the area so certain depth so here if P is in terms of certain depth then all other terms also must be in terms of the depth units okay so this may be certain depth say millimeters and then this IR represents the irrigation supply for that area and that is expressed as certain depth over that entire area next it is the ET which is evapotranspiration again expressed as depth from this area as depth in millimeters next is the evaporation so this is a evaporation over that area again expressed as depth so next is this QP so this QP represents so this is a ground water pumping so that is also expressed as certain depth in millimeters and then this QG represents so this is the ground water outflow or inflow if it is outflow it will have a positive sign if it is inflow from the neighboring areas it will have a negative sign next is the QS which represents the direct surface runoff again it is expressed in terms of depth and this delta SM represents change in soil moisture storage again in as depth in millimeters and so this delta SS here I am writing so this is the change in surface water storage okay and lastly this delta SG represents change in ground water storage so essentially here suppose we need to estimate say the that is the suppose we have the surface component surface zone of a crop so here in this case because we are dealing with the surface zone of a crop so it is required it is required for us to estimate this evapotranspiration which takes place through this crop which is in the surface zone so therefore in here we need to make either estimate or actually measure the precipitation the irrigation net irrigation supply the evaporation which takes place in that over that area then the ground water the water which has been pumped which is QP and this ground water outflow or whether it is there in that area then QS is the direct surface runoff from that area then the change in the soil moisture through various instruments such as a least emitter or etc then change in the surface water storage as well as change in the ground water storage if all these terms are estimated then the only unknown in this case will be this evapotranspiration so here this is known this is known this is known this is known this is known this is known this is known this is known so the only unknown is the evapotranspiration which we can estimate provided all our estimates are estimates or measurements are reasonably correct. Similarly, let us say the we need to estimate this deep percolation. So, in this case, so here all other terms, so deep percolation were estimated, we are we need to estimate. So, therefore, we should have the idea about all other terms. So, therefore, here in this case that is the change in the groundwater storage. So, this represents deep percolation. So, here only this delta sg is the unknown whereas, all other terms we need to estimate. So, all other terms, so that is the two, the supply side terms and then seven demand side times terms that is evapotranspiration, evaporation then the qp, qg, qs, delta sm as well as delta ss. If all these either we can estimate or measure with reasonable amount of precision or accuracy, then we can estimate the change in the groundwater storage and in this again if we deduct the change in the groundwater storage in the top aquifers, then we can estimate the deep percolation. So, like this, so this groundwater budget, it helps us in providing the in applying the mass conservation principle and thereby what we do is we make the we estimate the unknown parameter. It may be in case of it may be for a on the surface or it may be above below the surface. So, here, so therefore, this estimation of this change in the storage, the groundwater storage. So, that is one which is very much, so on once we do a reasonable estimate of the change in the groundwater storage, then we can do a realistic estimate of the groundwater supply and based on that we can decide whether this change in the groundwater storage, whether it is increasing over that area over time or whether it is decreasing. So, accordingly we can decide the strategy. If it is increasing, then we need to see that this water, the groundwater storage is within the satisfactory zone. That means, the water table depth is within the satisfactory range of fluctuation and so accordingly we may design proper land drainage system as well as subsurface drainage system and then take out the excess water which is very much required, which is very much the case in case of the systems where there is there is flooding, water logging and things like that. Likewise, suppose due to other reasons other causes, so the in a particular area over a particular period of time, so there is a decrease in the groundwater storage. So, then that needs to be properly estimated and there we need to go for the augmentation measures through artificial groundwater recharge, so that the water table or the piezometric surface depth is maintained within the permissible range. It is not too deep, so that the extraction of groundwater will become infeasible or will become too expensive. So, therefore depending upon this delta Sg, whether that is a change in the groundwater storage in a particular area over a particular period of time, so we need to decide whether we need to have a drainage system there or whether we need to have an artificial groundwater recharge system there. So, this groundwater budget or which is essentially water budget or water balance or groundwater balance equation, so here we are using this continuity principle, continuity equation or the mass conservation principles and thereby by measuring, actually measuring through field test or through realistic estimation of certain terms of water supply or water demand. So, we estimate the change in the groundwater storage in general or we may estimate other things also like the evapotranspiration as I mentioned earlier. So, this completes the groundwater budget and now let us go to this groundwater level fluctuations and as I was mentioning groundwater level fluctuation in neuron mental influence. As I was mentioning this groundwater provides a necessary water security extended over space and time. So, therefore we should see that the level of groundwater in the unconfined aquifer it is known as the water table. So, this groundwater level so this is referred to as so water table in the unconfined aquifer it is referred to as the piezometric surface in the confined aquifer. So, this we should see that the groundwater level fluctuation is within the permissible limits and there are various reasons for this groundwater level fluctuations. So, the reasons for groundwater level fluctuations they are that is the stream flow variation that means the flow in the stream or a river it varies with time and that results in the groundwater level fluctuation and they are meteorological tidal influences and tidal phenomena also urbanization earthquakes external loads. So, here so the stream flow variations as I was mentioning so there will be variation in the rate of flow in a stream and that results in the fluctuations in the groundwater level in the neighbouring areas. Similarly, the meteorological and tidal phenomena suppose there is a rainfall or say flooding or may be the discharge of water from an upstream area or this may be other one there is a surface flooding or even tidal phenomena in the coastal areas. So, there will be due to the sea waves so the what happens is the there will be fluctuation in the sea water level and then this fluctuation in the sea water level will give rise to the tidal phenomena and then therefore there will be variation in the groundwater level. Similarly urbanization so because of this urbanization so people tend to have more tube wells or extract more groundwater through other wells as well as other sources and therefore there is a result in the groundwater level falling down and then similarly earthquakes so these earthquakes what they do is so because of these earthquakes so there will be cracks in the earth surface and due to these cracks what happens is the surface water in a stream or from the bed or from the banks it just carried it infiltrates or percolates easily into the through these cracks developed due to earthquakes and thereby so there it may result in the at some places it may result in the lowering of groundwater table and at some other places it may result in the raise of the groundwater table due to additional due to excessive that is the deposition excessive storage of groundwater which has flown through these cracks developed due to the earthquake and largely these external loads so these external loads so may be due to this say for example if there is a dam or other barrage or such hydraulic structure so there what happens is so due to this water load so there will be the raise in the water table or the groundwater level and so in the downstream of a dam so there may be lowering of the water table so these are some of the examples of groundwater level fluctuations and now let us see little bit about groundwater level fluctuations here we can make we can classify this into two categories the first one is the secular variation the second one is the seasonal variation so this secular variations are variations a number of years at a location similarly the seasonal variations are variations over different seasons within a year any location so when I consider this ground water level fluctuations so we should focus on this the secular variations as well as we should focus on the seasonal variations so in this case say suppose I mentioned so this is the year and then the this is the maximum ground water level in this case and of course let me also represent in this case say here let me say this is say 1950 60 70 80 90 and then say this is 2000 in this case at a particular location the maximum ground water level in a year from this 1950 onwards so it could be it may show a trend like this so initially and so here from this it is evident that say for example from the from very close to the 1980s so there is almost continuous decline in the maximum ground water level during that particular year so these this is just one example of a secular variation and then similarly the seasonal variation so here what to do is within a year we represent the the variation at any location okay and so in this case say if I represent say the months of January March April May June July August September October November and then say December so in this case at a particular location so this is the again let me take it as the maximum ground water level so in this case say suppose for the Indian condition so this is the ground water level so it may reach the minimum in this one and it may reach the maximum sometime there and then again it may slowly decrease so this represents the seasonal variation. Now let me also so just a few minutes back in the influence of ground water in the hydrologic cycle I mentioned type of stream which is known as the effluent stream so this is the effluent stream in this case so there may be contribution from the bed as well as banks into the stream and this is also referred to as a gaining stream similarly there is another type of stream which is known as a losing stream wherein this stream is having a high water level and then so because of this there is infiltration from the stream into the ground and this is the referred to as the water table mound and if I draw the so this is the cross sectional view so this is the here this is the influence stream and this influence stream is also referred to as losing stream so this is the so these are the cross sectional views so this is the cross sectional view and so this these are the top views so in this case so in case of this so this is the flow direction in a stream and so the water will be entering from both the banks on the other hand in case of an influence stream suppose this is the flow direction in this case the ground water the surface water in the stream will be leaving and flowing as ground water through beds and banks creating a water table mound and so these will be the flow lines from this influence stream so a fluent stream having the flow lines converging and the influence stream having the flow line diverging so this so we will continue our discussion on the ground water level fluctuation and the environmental influence in the next lecture thank you bye