 Hello everyone. Welcome to rural water resource management NPTEL course. This is week five lecture two. We have been looking at groundwater hydrology and groundwater components. The stress is more on groundwater in these weeks because we understand that for rural water resources, groundwater is a key resource. And it is very important, especially now to conserve it. So let's go into the lecture for this today. In the past lecture, we looked at porosity. We define the special temporal variations, how it can be estimated. In today's lecture, we will look at specific yield is defined as a ratio of the volume that the total volume of the rock. So let's visualize it to see how we can understand it better. So we have a material, a soil material where we want to understand the groundwater. And H1 is the initial water saturated layer of a water table. And after some time, the water table falls down to H2 and that is because of gravity. So we have a saturated layer and after gravity. So gravity is working on the layer. So you water level will drop down. Q is the volume of water which has been discharged because of gravity to reach to H2, the new level or the new stable level. So specific yield is defined as Q, the volume of the water that has been drained due to gravity by the volume of the rock, which is H1 minus H2 by the cross section. You have to understand that all the rock is wet but unsaturated above the water table. And this takes a long time because gravity acts very slow and it has to overcome the sediment or the attraction of the rock material on water. There are different names given, specific yields, storage and drainable porosity, all are the same. So different papers would label it differently and it depends on how they would like to term it. Like specific storage is the remaining water that is in the soil profile and drainable porosity which is drainable because of gravity is given. So specific yield is the same and there are different names given. Let's look at specific yield in a gravel. Gravel is big big stones with less sediment in it and the porosity space is also large. So if you have large volume of water coming, the volume of water discharging because of gravity is also fine sand would have a moderate drainage, moderate specific yield because some water would be stored in the water spaces because of the interactions between the sediment. Whereas clay have very high potential to hold on to water. And given the same water applied to all gravel, fine sand and clay or solid rock. The specific yield is very low because of the slow or no drainage. In some regions, the clay layer is termed to be an impervious layer which means it stops the water from going through it. So it is kind of an impervious layer because water gets absorbed in the clay and it actually prevents it from moving further down. Which is where gravity acts and puts down the water. So specific yield is a very important term to understand the nature of your aquifer to discharge water ready. If it is very slow, then the groundwater potential is also very less. Let's look at another figure. Well, A is your volume of rock saturated with water. So full of water is now gravity is acting on it. So you've watered the field, you have a saturated layer and now gravity is pulling water. After the drainage, one unit volume of the rock has been dewatered corresponding level of the saturation. So you have lowered the level of the saturation by taking out one unit of water due to gravity. Specific yield is the ratio of the volume of the water that drained from the rock going to gravity to the total rock volume. So now Q is the unit that has come out due to gravity and that divided by the total rock volume gives you the specific yield. Another important parameter is specific retention or SR of a rock or soil. And it is defined as the ratio of the volume of water, a rock can retain against gravity. So this is kind of opposite to your specific yield. So how much water can be retained against gravity, specifically in this with gravity, how much volume can come out. That is the ratio you take over the total volume. Whereas here it is the water that is retained in the rock to the total volume of the rock. Since the specific yield represents the volume of the water that a rock will yield by gravity, drainage with specific retention, the remainder, the sum of the total is equal to porosity, which is n is equals to Sy plus SR. So let's think about it. n is your volume of void by the total volume of the solid. So we have BV by VT. Whereas Sy is your volume of water that has been dewater that has been come out of the system by your total volume of the solid. Whereas the specific retention is the water remaining in the profile by the total volume of the rock. So it is actually the sum. So if you sum both Sy and SR, it is zero porosity because it is a totally saturated system and which means there's no air, only all the volume of the voids are filled with water. And in specific yield some water is released due to gravity and in specific retention the remaining water is held down. So the total water is now combined together as porosity. Let's look at a graph to better understand the relation between porosity, specific yield and specific retention. So you have total porosity as n, which is the summation of specific yield and specific N is equals to SR plus Sy. And we have the grain size of your soil profile rock profile here. Placid sand gravel compost percentage on this order, porosity or specific yield or specific retention. So what you see here is in a well sorted aquifer, which means it is sorted almost same size. There is high porosity in clay, silt, but when it comes to sand, which is very small, it comes down. So you have lesser and lesser porosity or percentage when it comes down, specific retention. The total porosity is almost 60% to 30% so it is also coming down, but your specific retention comes down very fast in gravels and cobbles because the gravels and cobbles cannot retain that much water. It may have a high porosity, but all the water would fall down due to gravity and that we saw in the earlier slide we discussed about specific yield. So in clay, the specific yield is very less, not a lot of water comes out. In silt as the size of the grain increases, you have more water easily available for gravity and it is well sorted aquifer. So you have a very high amount of specific yield, but normally well sorted aquifers are not available so you have a specific yield tapering off in sand, gravel and cobbles. So the worst is the clay, the specific yield gravity cannot pull down water, whereas gravel and cobbles it is easier to pull down whereas sand, it is much more easier. So when you play in the beach, you see you dig a sand castle or you can see water coming on the shore of the sand. And after the wave goes back, you could see that the water just flushes down. So it quickly comes down because of gravity. And that is why because the specific yield is very high in sands. So how much water is retained, it is the opposite. So specific attention if we look at in clay it is very high, whatever water the clay has it will not let it go, it will hold on to it tight and long. And it's the same procedure for plant available water also. So it doesn't mean that clay soils are good for plants, no, because if gravity cannot pull, the same pull might be exerted by the plants. And some plants with higher pulling capacity can grow in clay soils. The good part is clay can fight gravity for you and keep the water up, but the force to take the water should also be high for plants. So only some plants can grow like for example cotton goes well in clay soils. So the specific retention is very high, and then it slowly comes down, slowly comes down as your size increases. Okay, so clay cell will have a higher retention, but when it comes to sand, the water just flushes through gravel, it doesn't stay. The specific yield is very high in these higher grain size materials. And that is why your specific retention is also very small. It's the opposite, right? So in a well sorted aquifer, when it is fully sorted, it just goes to zero. The specific retention goes to zero, which means all the water is drained out. So this graph clearly explains the relationship between your specific yield and specific retention and the total porosity. We've also discussed the specifics of when these specific yield can be high and specific retention can be high. Think about a cricket pitch or a golf course. You don't want water to stay there, right? When there's a rain, you don't want water to stay there. So what material will they use? They would use a material with high specific yield. For example, you have sand, gravel and cobbles under the course, under the golf course, or under the pitch, so that when water falls, quickly it goes down and then through the gravity, you can extract the water out. So only on the grass surface, there are some water which they use some techniques to remove it. But in a rural setting, it's the opposite, right? You don't want water to flush down totally due to gravity. You want water to stay as long as possible so that your plants can survive. So in that perspective, your retention is high in silt, which is your combination of sand, silt and clay. Good loamy soil is good for plant growth. The base is a fine size sand, but clay is much, much finer. So much, much attraction on the water particles. And sand, again, as you move up in grain size, you would lose more water to gravity. So specifically at the side, but the retention is very, very low. So somewhere you need to balance it out. If you want good groundwater potential, you would go to gravels and cobbles. Think about where you would use it. In a rainwater harvest structure, for example, most of us have rainwater harvesting structures. What would they do? They would have gravels, cobbles, sand, so that water can flush fast into the ground. And sand and silt can act as a filtering material, right? And then it goes to the groundwater aquifers. So it is very important to have such a strata to capture the water and also quick specific in so that water drains so that more water can be put in. If you have clay soil, then you cannot have rainwater harvesting. You'll have to put in gravels and cobbles to push water faster into the groundwater aquifers. So these are how you could visualize the porosity concept in your rural water management, urban water management, and especially for groundwater management. Moving on, let's look at a particular field sites sample. Specific yield or sediments taken from the Humboldt River Valley of Nevada from the book as a function of the median grade sites. So what we could see here is the specific yield is very, very low for clay. And it starts to increase as the size of the grain increases. So it starts to increase the silt, very fine sand. And then medium sand, coarse sand almost the same. And then it comes down, very coarse sand, very fine gravel. The specific yield comes down. So the best to drain the water will be sandy materials, sandy soils. But then if you have gravel in between, then you can arrest the water. But the worst is clay. So clay is almost at zero. So please understand that specific yield differs at a different region. And that's why there's always a range. And most importantly, specific yield is acted upon gravity. The other thing that can also induce specific yield is when you have a cone of depression. So we've talked about cone of depression in our groundwater class. When you pump too much, then you artificially induce water to go through your pump. And that can also push water because of specific yield. That can also push water to bring down the level of the water table. So that is how we estimate specific yield. We have a water table and the water table comes down. And how much water is deep water, you take it out and put a ratio to the volume of the rock or the material. Moving on, let's look at some ranges. It is always a range. It is not one value. So I'm using multiple sources. You could see I will use freeze and cherries book, which is mostly trusted for these values. I've also used some case studies and other books from India also to look at the variations in the values. So specific yield, clay can have maximum of five, minimum zero and an average of two. So most probably, as I said, we can take the maximum and then take an average to get at where it stands. And most probably it will be on the higher end, but sometimes you'll just strike it in between. So coming back, the average is normally used in a lot of literature and clay has a good range from zero to five. Sandy clay, 12 to three, the range you can see it expanding. Silt can have 19 to three, fine sand, 20, 10, medium sand, 32 to 15 and so on. I won't be reading all these values. But for coarse gravel, it is 26 to 12. So what do you understand is there is some difference, some range is created. Why is there a range? Let's pause for a second and think. Why such materials will have a range and not one value? Because the climate and the use of the land is not the same across. So let's visualize. We have a clay field. I might have a tractor, so I have tilled the land. It is clay, but I've tilled the land. I've done a lot of work to stabilize the soil on the sites. And also I have put in plants where it can infiltrate deeper into the clay. I've picked specific plants. So what happens is through the pathways, through these pathways, the porosity has increased. And specific yield is a function of porosity. So you can have clay, but you can do things to increase the porosity. You can do management to increase the specific yield. So water can go in. So if you have more pore spaces, water can go in. Yes, clay holds on to the water, but if there is excess porosity or excess pore space, water gets stored and flushes out due to gravity. Remember, since it is very small, the size of the grain is very small. I'm going back to this slide. The size here, you have it very small, which means it has more surface area to attract water. There are many, many clay particles. The volume is the same. If you take one kilogram of clay, silt and gravel. The volume is the same or the area or the mass is the same. But the surface area of connection to the water is much, much bigger in clay, silt and sand. Because the size is small. So you have a circumference and you have a more surface area in connection with the water. So now how do you overcome it? By increasing the porosity. So by increasing the porosity, what you do is you create more void space for the water or air to come in. And right now we're talking about specific yield. So we're going to saturate the soil by applying full water and water can be easily drained. Compared to a non-agriculturated land or a non-agricultural managed land. So tilling is one thing that can introduce porosity into your soil and materials. And you have a mixing of various aspects in clay when you did. So that is one thing that we can do. But the more natural thing is to let the vegetation take care of porosity. So if you have a good tree cover, if you have good vegetation, native vegetation with deep roots that can go in and break the soil structure. Then you have more porosity. So porosity is an aspect that can be changed. Suppose I have clay. I put on one side, I put crops. So you have root zones developing and more porosity. On the other side, I have clay and I'm putting a road on top of it. Okay. How do you put a road? You put gravels, etc. But then you put a tar and then you have a road roller which goes on, which means you're compacting the surface. So when you compact, you have already clay with very less pore spaces. When you compact, it gets more pushed. The weight of the soil material or clay material doesn't change. The weight is same, but the volume has changed. You have pushed it down. Same thing, you can take an experiment at home. You can take a beaker. You can take a soil, break it down and then shake it and put it in a container. You can see the volume is high. Now take half of the volume of that container and put it in another equal container. You have equal volumes. In the other one, you can push it just by some force. You can push it and you can see that the soil material will come down. This is the same thing which would happen in a field. If you compact it too much, it could be compaction by tractors. It could be compaction by people walking on it or even grazing of animals. Then you compact the soil, which means your porosity is reduced. So no more water can go in. And your drainable porosity, your specific heat, all of these components come down. So land management is related to these key parameters of porosity, specific heat, specific retention as much as your water availability. So when we say water management issues, it is not just that I'm not getting water from rainfall. Climate change is happening, monsoons are shifting. It also includes how you manage your land. How well are you managing your land? Are you having the correct species of plants and trees to increase the porosity, to increase the water retention in your soil? And if you overdo it by non-native species, what happens? All the water is taken out. And then your clay cracks. So clay is a very, very tough soil system to work with compared to the other things. So alone is a combination of sand, silt and clay. We will not get into soil structures a lot because this course would be on water management with some more specifics of the network. So always there is a range. Understand there is a range because of hydroclimatic changes because of the size of the clay also. Clay doesn't have one size, it is a range. So depending on what clay size you have, you have a specific heat. But most important, depending on how the land is used. You have a difference in specific heat. So this understanding would help us better understand the ranges and also better understand how we can conserve groundwater. Thank you. We will see you in the next class.