 Hello everyone, welcome to NPTEL Moodle Water Resource Management course. We are today on week five, lecture one. In the previous lectures, the past two weeks, we looked at groundwater hydrology and this will be the last lecture series on groundwater. The more specifics about the parameters and how they are helping to understand the groundwater hydrology. Let's look at the recap of week four and then how it is linked to week five, which is the current week. In the week four, we introduced the groundwater hydrology components and then we looked at which bodies are monitoring and measuring the groundwater, what are the issues and concerns, we looked at it. We looked at the data and what are the different methods to collect data. In week five, we would look into the specific groundwater hydrology components like porosity, specific yield and specific retention, permeability and hydraulic conductivity and we also looked at the three dimensions of hydraulic conductivity. Then what is a hydraulic head, potentiometric surface, etc. If you remember in the groundwater hydrology class, we looked at a cross section, a groundwater hydrology cross section and we showed how different wells are connected to each other by a potentiometric surface. So how that is determined, all those things would be taught in this week. So let's move on. The first topic for today would be porosity. So what is porosity? It is the fraction of volume of pore void space in soil. So if you have a soil and you have solid materials in the soil, it is not fully with solid materials. So for example, you're taking a sample using a cup or a container and if you take it out, if you put soil and take it out and take a soil sample out, it's not fully soiled because there is a little bit of space for air and water and that is initially a void space which is an empty space. Then as and when water or air occupies it, it becomes filled with fluid. So fraction of the volume of pore to the void space in soil is given as porosity. So theta here, the porosity is given as volume of the voids or the empty spaces by the volume of the total solids, which is a total soil. So you take a container that is the total volume and out of this, how much is the volume of the voids? So types of porosity, as you could see, the first one is a well sorted sedimentary deposit having high porosity. So well sorted means it is with size, it is of a similar size and a sedimentary deposit, it has round shape, which means it has been transported for a long distance, good weatherment. So it has been weathered properly and so all the edges are almost are gone. So it is all smooth spheres. So that is a well sorted and it is not irregularly arranged, it is touching each other, which means it is sorted and there is a high porosity because of the size and the sortment, you have good spaces here for air or water to occupy. Then you have a poorly sorted sedimentary deposit having low porosity. So what is a poorly sorted? It is the same thing, almost the same solids, but you have the smaller solids, the size is not the same. They are almost similar size, but here you have a mixture of big, small and very tiny particles. So what happens is the tiny particles can occupy the void space and that is why the porosity is decreased. So here the porosity is decreased. Then you come to see well sorted sedimentary deposit consisting of pebbles that are themselves for us. So the whole deposit has a very high porosity. So in this material, you have well sorted, same as well sorted. It is sedimentary, but consisting of pebbles. So a type of rock. So sedimentary is the process in which the rock or the material is being deposited. So sediments. That is what sedimentary means. But there is a nature of the solid also. So that nature here in C has holes in it. So if you take a sample and you can see, within the solid, you can have void space. You can have porous space, but on the sample itself. So here, right here, you don't have space, but here you have holes. So like the sponge, I gave an example in the previous class. So here water can go in, air can go in and get stored. This helps in also attracting more water and that is why it has a very high porosity. So along with the spaces, almost similar spaces in between the sediments, but in the sediments also you have space. So all this together is being used. Then your well sorted sedimentary deposit whose porosity has been reduced by the deposition of mineral matter, cementation in the interstices. So here you do have a similar as A. So here all we're trying to say is from A to B is of same similar sedimentary deposition. So the deposition is the same, which is through water or something. It is getting deposited. But how the pore spaces and the void spaces are filled is different. So here you have organic matter or mineral matter, cement, cementation, etc. happening within the pores. So it actually drives away the air and water and has very, very less porosity. So you could see how gradually we are changing in the void space between the solids. E is soluble rock made for us by solution. So this is a typical rock structure in limestone. So if you go to caves and stuff, you see water can cut through rock and then flow. Okay, so that is a soluble rock. Limestone is soluble rock where water by going by passing through, it can cut away a path and then go through. So that can be a soluble rock interface. And while it's soluble, please understand that the minerals are also transported and this is where the salt content comes in. Salt is not in salty as the food but the properties of the rock. So while it is being dissolved, it is soluble. So it takes the salt from the rock and then goes along. So the porous solution is there, which is the water in the pores and that interacts with the rock, it dissolves it and then takes it along. So we have a good flow path. So if you could see, it just flows across laterally, et cetera, all are connected well. So it is having a high porosity. That is why you see a river underneath the cave. So that is kind of a groundwater which goes in and comes out cutting down, cutting through the rocks, et cetera, et cetera. So it's just dissolving all the rocks. Then you have crystalline rock made porous by fracturing. So there are some rocks that have fractures in there and because of the fractures, the water can come in. The two ways, also the water goes in and creates a fracture because water expands in the cold weather or when the water temperature drops down, it expands. You put water in a cup and put it in the freezer. It expands and the size increases. So that is where people say do not put a cold drink in the freezer because the water will burst. So it is that same property that water, when it goes into the cracks of the rock, let's say it goes in the morning, whether the temperature is not as cold, but in the night times, the cold temperature can kick in. And in groundwater also, we have temperature changes. So if the water pulls down slowly and then it expands, so when it expands, it causes a fracture and that fracture again will lead to more water being stored and movement of water. So now we have seen that how porosity could change within a sediment by the nature of how it's sorted and by the nature of how the sediment particle is available and also by the process of flowing water. So water can flow through and cut through the rocks and water can go in and cross fracturing. Soils, rocks and sediments on the subsurface consist of matrix of solid mineral grains and pore spaces that can be occupied with groundwater. So this is the part where you have inside void spaces and if the void spaces are filled with air, slowly water can infiltrate and push the air out. Okay, so that is where you have water being converted into non-water, so infiltrated and underwater. So this is the starting point, as we mentioned in the previous classes also for fluid flow. Fluid is a mixture of two phases through porous media, as per Darcy's law. And in this week, we'll also look at what is Darcy's law, why is it used effectively? So the natural porosity of your soil material or your rock material can be looking like this. You have voids where you have air or water or just empty spaces called void and you have your solids. In a partially saturated soil, you could have air part of their void can be filled with air, but most of it water and solids. So I'm just taking a sample, this sample where solids are the same. Okay, and this is your void space. In the void space, the partially saturated will have some air and water. Fully saturated soil will have all the pore spaces with water. And then you have a dry soil where air is full. So if you ask me, are there not any water in the solids? Yes, there are. So same here, are there not any small, tiny fractions in the solids? Yes, but those are negligible. It's not like it will contribute to, you know, groundwater this part, or even here, the air is enough to push water out. No, it's a very small part, which as I said earlier, it cannot be taken out. So there are some amount of water or air in the soil, which can never be taken out. So even by plants, it just remains there because of the property of the soil. So that is why if you want to get accurate estimates, you have to take the sample, crush it, and then put it in an oven overnight or even for a day at 100 degrees. Think about cooking or baking the soil for 100 degrees for at least one day. So that's how much energy and time you need to spend to drive out the air into vapor. So only at that point, the soil will give off the air and water. So it is very important to understand that that is the extreme cases, and it doesn't contribute to groundwater much. Only these two phases contribute, which is you have a wide space and part of it has air, part of it has water, which is saturated partially. If it is a fully saturated soil, all the space is with water. Assessing field porosity, how do you determine porosity for every field site? Is it possible to determine at every field? For example, you have a village and 30 to 40 acres that you would need to go and conduct a water resource assessment for which porosity is important to understand groundwater. Is it possible to go to every location and take a sample? No, it is costly and time consuming. So but you know that by the process of understanding the solid material, which is your science, etc., you can back calculate the porosity. It's not an assumption. Someone else has done it for you through a lot of time in the lab and literature based. So we can use that. So soil geological maps can help. So let's start with understanding the geology of a location. Let's take the village I gave for example, 20, 30 acres. If I know the geology, the first base, the bedrock is the geology, then the soil. So if I know the geological map, I can estimate what type of soil would have been formed because your soil formation is nothing but your geology which has been weathered. So you have the weathered, unweathered geology. On top of it, you have a soil and this geology maps are available. Same way, based on this there are soil maps in India. Then using the literature which I'm giving now, you would estimate what is the range of porosity. Let's have a look. So the fluvial deposits or alluvium where we saw earlier along the Ganges belt along the Kaveri, Ramakutra, Krishna basins, you do have a high porosity 0.05 to 0.35. Please look at the range. It could be anywhere from 0.05 to 0.35. The 0.05 is because some of the intermediate voids can have fine, fine sediments that can go in, which we saw earlier in the class like the last slide. You can have a sediment depositor, but if it is also having a non well sorted sediment, you can have small particles inside and that happens in the Ganges, etc. because sedimentation is occurring almost every day. Every day you have sediments flowing depositing. So by that process, what happens is the fine particles can go inside and deposit. Then you have glacial deposits on the Himalayas which have very, very high porosity because it is big boulders, big rocks and it is pretty well sorted. So it is sorted like this and inside you have big voids peaks. Then we have sandstone, shale, mudstone, dolomite, etc. So here comes the almost smaller, smaller ones which are found mostly in your central India, where you have example factured 0.01 to 0.02. The factured hard rock aquifers we saw last week in class that most of India has hard rock, semi hard rock, unconsolidated aquifers. All these would have a very, very low porosity, volcanic tufts, basaltic lava, all these. So most of the rock materials are metamorphic, unfractured and fractured. Unfractured means not weathered properly. Fractured means water has gone in and broken. So all those things can happen. So this is pretty common in India. Secondary chalk is common. Shales are very common and then your fluvial deposits, glacial deposits on the Himalayas. So I know now from the geology I can estimate the porosity and then if it is a well weathered soil, then you can look at the soil map. So as I said, the geology maps are available. You can go to Geological Survey of India, you can download these maps and then based on this given geology type, you can come back to this slide or the book and then take the values for your thing. But it is a range. So how do you understand where it ranges? It is depending on your field visit. You can take some samples and then say, okay, the porosity is this much. But ballpark you can get. Normally what people do is if you have two extremes, they take the average or a midpoint in it. So range is big, but you can take a midpoint. And if you know, as I said, if it is a young fluvial deposit, then you can take a 0.05. If it is a well structured, well sorted fluvial deposit, you can take 0.35. So use the previous slide where we looked at the sorting of the rock material, the sediments and then come back here to see where the range can fit in. Okay, so here is your time on the x axis and mean moisture content of soil on the y axis. So it is a temporal change in mean moisture content of soil. So your soil content is not going to be constant throughout the year. Okay, it is because your porosity is constant. So you have a good porosity. Okay, but water can go in or go out depending on the use. So if precipitation water can go in, if you bump water can come out or freeze evapotranspiration can actually drive the water out. So soil moisture recharge, let's start with rainfall season. Okay, so you have your soil moisture recharge. So the soil moisture is filling up. Let's first take a step back and look at the three capacities that in the soil moisture. If it is below a particular point I told earlier, the plant cannot take the water out. It is too hard for the plant to take the water out, it will die. So that is a winning point. So that is very, very less soil moisture. Then you have field capacity, which is the best condition for the plant to take water. Because it is well saturated and it is easy for the plant to take. It is not full and dripping because it cannot suffocate the roots. So that is field capacity. On top of that is the maximum porosity. So this is the maximum water that can be stored because that is the maximum porosity. In between that you have the field capacity and then you have the take water. Field capacity is after the saturated water has come down. After gravity has taken up, the water stays in the soil because the soil is properties. And that is the easiest water for the plant to take out. So what happens is when there is soil recharged from down, the soil gets recharged. So your moisture is coming up. It is in the best capacity of the field capacity. So the plants can grow well during this part, this part of the months. And then you have soil moisture above field capacity and groundwater recharge from melting snow cover and spring rains, etc. This is spring season when you're after your monsoon, after the winter, snowmen is already there, it starts to melt. And then you have a peak summer in this part. So what happens is your water comes down from the porous space because plants have already taken it up and your water is being depleted by evaporation. So soil moisture depletion as evaporator transpiration increases. The plants have taken up and the evaporation has driven the water out. So it comes down. What happens is it comes down below the field capacity. Now the plant is suffering. It needs water. You need to irrigate it. So soil moisture recharge from heavy summer rain. So suddenly you have a small peak. Small peak because you do have summer rains. In summer odd days, you get good rain. And so suddenly there's a good peak and then comes back down. Then you have soil moisture depletion during period of maximum evapotranspiration. So this is again another big summer driven event. So a big drought or a big hot weather can take more water out and it goes down your wilting point. Once it hits wilting point, you need to give water. If you do not give water, the plant stays. So that is what irrigation is. So here is the peak irrigation time. Here's your rainfall and snow melt occurs. You're in a happy situation for plant life, water management in the village. But then when field capacity is there and goes below, you need to be cautious. And then some intermittent rainfall can happen. But then after this summer, you have your monsoon. So your monsoon picks up September, October, November. So soil moisture recharge after killing frost have reduced transportation. So this is an example from the US, but it can be applied to anywhere. It's the same thing. If you don't have frost, you have a rainfall season. So you have a good rainfall, then recharge, then a good summer, which actually drives all the water out and then comes out. So this would be your period of irrigation. This is what you need to plan if you want to drive crops up. So if it's a good farmer who says, no, I'm okay only with this crop, which is the curry crop and little bit of subsistence farming, like farming for vegetable fruits for the house, that is fine. You don't have to have good water resources for the irrigation. But if you're going to put rice and paddy here, and then another big hungry crop, cotton and other things, then you need to actually put in an irrigation structure. So let's move on to look at the 3D visual, a well-sorted sand. As I said, a well-sorted means all the small particles are taken away. Almost similar size, you have good water storage, very good water storage across the domain you have it. And this is your alluvial aquifer experience. So this example can be taken for those solids, soils and rock material, which are well-sorted. And then the fractures in granite, which is mostly your hard rock aquifers, Central India, this is your alluvial gangers plane. This could be looked at as your Central India, South India, those kind of hard rock aquifers, where you have fractures in granite. And in the fractures, you have water. So if you put a pump here, you take this water. If you take on the sides, you don't have water. So what is driving it? It is the presence of the fractures which is driving it. And then you have caverns in limestone. So this is the limestone in your mountains or underground where you have a rock that can be dissolved by water. So water flows and it can cut and dissect and then go through. So these are limestone materials and the water can be stored along the cracks and crevices where it flows. Moving on, so focus on modals and free attic zone is going to be the important parts in the next more classes because what we have here is we have a focus on vedo zone and free attic zone. Vedos would be your under saturated zone and your free attic would be your saturated. So what happens here is you have atmospheric water vapor, precipitation occurs, land surface depressions, etc. Infiltration of the water occurs into the vedo zone, the first zone which is your unsaturated zone. Then you have your free attic zone or zone of saturation. So through gravity, water moves down. Part of it goes as base flow. Part of it goes as inter flow and at the land surface you have surface runoff. We've discussed this all well enough in the hydrology class. So then you have a precipitation and evapotranspiration is the losses from lakes, water bodies, ponds and then it can also go to seawater. The same thing can happen in seawater. And this is the magmatic water under very deep conditions. You do have the magma water driven by your lava, etc. And it can also come to the ocean. So ground water can exist from here, the vedo zone until the magmatic water. The vedo zone is still having less water because it is not fully saturated and same here the magmatic water is very, very less in volume. But this is the biggest water resource for groundwater. And here is where you go and see that after some time it does go to the seawater. So fresh water is pushed into the saline water and also into lakes, ponds and rivers. So groundwater is linked into various components. If it is an urban city where you have all this land as cement and roads then water falls and then goes here. So these two paths are not available. So you can see visually where the water is starting, how it goes into groundwater and then how it is managed. Please also understand that porosity ranges are different as per different literature. So always use when you want to do a fieldwork and you're trying to get literature for it. Use literature which is based out of Indian studies or some Indian studies have used it. So all the books I'm referring to you would find Indian studies using those literature values. So well sorted sand or gravel around 25 to 50%. If you put the percentage it is that 0.25 to 0.50 sand and gravel mixed would be 0.2 to 0.35, glacial till which is all the Himalayas, etc. 0.1 to 0.2 and then your silt would be 0.35, 0.5 and clay would be 0.32, 0.6%. So all these are taken from very, very old studies. You can see their dates but again the porosity doesn't change for a particular rock or material because it is the nature of the rock. So there's not much change by new studies. If only if maybe you have new technology to find better results it is fine but again as I said it has been crushed, put in the oven, heated, evaporated. So all those values are pretty accurate. So through these lectures we have seen how groundwater enters into the viatic and your vedal zone through the porosity. We looked at porosity in detail. We've defined it and seen how it changes spatially through the layers and also sediment and etc. And also we looked at the temporal change in soil moisture. So this soil moisture is very, very important to understand the plant water requirement for agriculture. So all the water which is in the soil can only be going into the groundwater recharge. You cannot have zero soil moisture in groundwater. It may be deep aquifer, yes it can come from far away but in your location you have to have a good soil moisture to have a good shallow groundwater aquifer. And that is where we would stop and go to the next component of the groundwater hydrology in its class. Thank you.