 Welcome to the NPTEL course on Rural Water Resource Management. This is week seven, lecture two. In this week, we are looking at the hydrological water balance, how to construct it, what are the issues, and what are the meanings of each parameter. In the previous lecture, we had an introduction to the hydrological water balance. In this week, in this today's lecture two, let's look at what each parameter means. For that, let's take the same example that we use to introduce you to the water balance equation. Again, there's different names for this. It could be a water budget, hydrological balance, water mass balance, etc. And it follows the continuity equation, which is mass in minus mass out is the storage. Here, the del S is your change in storage, or the actual storage is equal to P, which is precipitation, plus Q in minus Q out, minus ET, plus G in minus G out. Today, we will look at each parameter and how it applies to the overall goal of this lecture series, which is rural water resource management. Before that, some caution notes. These equations can be constructed for any land use land cover, which means it can also be done for urban. It could also be done for your small community area. But because this lecture is on rural water resource management, we will be looking at rural examples and what these parameters mean for rural water resource management. I also want to give another answer of which is the most important part for your water resource management, which is the goal of this lecture is del S, which is change in storage. How can you augment? How can you increase change your storage in the positive is the key for rural water resource management because your precipitation cannot be at once increased or jumped. It is a very slow process. For example, if you want to plant trees, specific trees, and then condense, etc. Or the seeding, cloud seeding is a very costlier affair. We are not going to talk about cloud seeding and increasing your precipitation. So the most important part in your rural resource water management course is your del S. How are you going to preserve, conserve or manage your storage for better water management is the key. Your precipitation is the first term in your water balance equation. It is given as P. And remember that before I jump into the first variable, each variable can be differently used in different books, papers and by different authors. So it is your goal, even if you want to start writing one to have an index, a key for what each variable means. You will also forget it if you don't write it. So P could be anything from precipitation. And R is also called as rainfall or recharge or runoff. You see how one term can be used different ways depending on the user. And there is no universal rule that P should be precipitation R should be runoff. So it is your goal. It is your impetus that you need to give the reader full understanding of what it means. Good. Moving on, precipitation is the major input to the system because it is the one which is giving you rainfall, which is the big component for rural water resource management. It is the input to the system, not all lands get water from your dams, from your canal irrigation. So precipitation is key. It is important for curry season. Before we discuss this point, it is important to understand what is curry season. In India, there are two major growing seasons. One is curry and the other is rubbish. Curry is the monsoon season where crops are rain fed, which means the crop do not get irrigation from groundwater, your dam water, etc., but it looks at the sky for water. So it wants to get more water from your precipitation. So it is a precipitation-only based irrigation system. There are colloquial terms for this also in various parts of India. And also, as I say, various parts of India, the monsoon season also differs. So what is in Maharashtra, for example, your monsoon season starts in June and ends around September or October, whereas your southern states, let's take one for Tamil Nadu, for example, your monsoon will just start in September or October. So you see how the peak monsoon season differs and also the peak monsoon cropping pattern differs. So it is important to understand where you are located and what is your season. Before we also continue expanding the call, understand what unit you're going to use. I have mentioned this in the previous lecture when we defined the unit of analysis, which is watershed, basin, plot, etc. For our study, for this lecture, we will look at watershed as the base unit for analysis. It could also be a plot scale if you're doing a plot, wherein some variables will not come into the picture. So this equation is very important and precipitation gives the important input for curry season. It is also important for the rabbi season. So rabbi season is the non-monsoon cropping, which means it is a fully irrigated crop. Irrigation means you have to apply water. Is it manual through technology, groundwater, drip irrigation, anything. All these constitute irrigation and rabbi is the non-monsoon pack. So sometimes you have curry, winter and rabbi. So rabbi is kind of before your peak summer. So that is also based on rainfall, how people can ask me. There's no rainfall in your rabbi season. So for example, in your curry season, your P is big number, that is the precipitation coming in. And because of that, others are driven. In your rabbi season, P is zero. There's no input into your watershed from rainfall. However, your P would have influenced your delas, which is your storage. It could have filled up the dam. It could have filled up your groundwater. It could have filled up the soil moisture as I put it here. So it is very, very important when you do an annual water balance to have that picture in mind. So your delas, you can understand how much it is. Moving on, we have Q in, which is your discharge coming in or flow coming in. So this gives a total water input to the system from other resources, other resources, as in other water bodies, other watersheds, etc. So this is a water that has been taken from another watershed. And it has been funneled into your watershed through different means. It could be through a river discharge. So the previous watershed would have taken the water, caught the water and then pushed it into your watershed. It could be through a pipe, like for example, Pune supplies some water to Mumbai. So through big, big pipes and water is coming in. So that is the total water inflow into your watershed from outside your basin. And it is a very, very important component, especially in the rugby season, because when the dams are built, mostly the watershed area is above. And the canal system extends down to the farmers and gives you water. So I've given an example as example, canal water, dam release, etc. So this is Q in coming into your watershed. Then you have, it is important to understand the excess water coming into the system, because sometimes your excess could be above your precipitation. And so it is important to document it. Otherwise, your delas might be increasing or your ET might be high than your P, which is not possible. So you need to understand the major, major inputs into your watershed. And Q in is coming from a different watershed. Please understand that if you do not have a barrier in your watershed and all the water is coming in the river, then it would eventually flow out. Let's take the example of the Ganges. The Ganges originates somewhere and it is picking all the tribute trees, water and coming in flowing in. One of the biggest tribute trees is the Koshi river, which originates in Tibet, flows through Nepal and then to Bihar and joins the Ganges. It is one of the biggest contributors of water to the Ganges. Suppose they say, no, I'm not going to give this Q out, which is my Q in, which is water coming from Tibet and Nepal. If they say, no, I'm going to hold my water. I'm not going to give it as Q in into the Ganges in Bihar. Then the majority of your water is going to be reduced. So my Q in, so my Q in is someone's Q out. So think if your watershed is getting water from somewhere else, it is someone else's Q out. So this inflow component is very important also to understand how much you can give others. Let's continue. So we've discussed the positive side means the input into your water. So all positives in the Dallas term, if you have Dallas on one side and the other side has the equation, the positives mean it is an input to your watershed and the negatives mean it is a loss to the watershed or you're giving it to someone. And so the basic idea as per the continuity is, is the positive minus the negative enough to have storage still remaining. Moving on. Now in this lecture side, we will be looking at the negatives. Q out is the flow out. How much water is going out of my watershed? Through runoff, through channels and through other means like you're taking it from a tank, you put water into a big container or a tank or a river and then from there you're taking tankers and taking out. This is a very, very important Q out in terms of rural urban periphery that area. So for example, if you're in Bangalore or Chennai, they'll take a lot of water from outside of your urban settlements, mostly in the rural areas through tankers and it is very, very difficult to quantify it because not all tankers are equal in size. They do not have a log of how many times they go up and down and supply water and we don't know the exact volume in the tankers because most of the time when the tankers run it loses water. Okay. So Q flow out is the flow that is coming from your watershed to the other's watershed. So you're losing the water. So it gives a total water output released from the system and it's also important not all waters you could store because it will be flooding them. If you're saying, no, no, I want to dam in every watershed, every water basin, then what would happen? All the water would be holding on and you will not also get water from the previous watershed. The other point to think about is the downstream communities. The farmers downstream also are dependent on the upstream water. So it is very important to understand how much water is released. Runoff is included, which is you have a land precipitation falls and then runoff happens after infiltration on water recharge. So your runoff is also included along with the pipes that come in and the watershed previous how much water is giving in into your water. So all this is channelized in the river channels or pipe network. So pipe water is counted, groundwater extraction and water extraction in tankers is also contributing. The groundwater can also pull some of your Q. Let's not discuss too much about it, but think about this. Okay. For example, you are having a river flowing and right next to the river, you're pulling groundwater. Is it only groundwater you're going to pull? You may also pull the river water. So that is what I'm meaning by this Q out. PT, evapotranspiration. This gives the total water lost due to living organisms which are transpiring, giving out from the skins, giving out from the leaves as water used for photosynthesis and evaporation from open surfaces. We have discussed this a lot in the evapotranspiration slide, but I'm just going to stick on for another couple of minutes to explain that it is a big negative in the system in your watershed. But when it comes to rural water resource management, can you make it zero and ETB zero? If it is zero, then there's no plant growing. You can minimize your evaporation. Let's say I'm having no open surfaces, water is all stored in the groundwater or whatever water I get, I'm going to put it on to the plants through drip irrigation, so no soil evaporation. But transpiration has to occur for plant life to grow. So for example, rice, sugarcane, whatever crops or food crops that India grows mostly, you cannot control the evapotranspiration to become zero, but you can lessen it. By reducing it, you can increase your delus. So that is the idea. How can I reduce the negatives so that I increase my delus for the next cropping season? I talked about the next cropping season from curry to monsoon to non-monsoon, etc. But I'm not supporting or not approving that every month the land has to grow. So the idea is because this course is a rural water resource management course. We assume that the fertility of the land is sustainable and we're maximizing the water so that the farmers can grow. Because discussing on soil fertility and what process are there to increase the fertility is by itself a lecture series. Is it by itself? Of course. So we will not get into that angle. We're only going to focus on, is my land having enough water? If not, how can we increase it? And why are we not having enough water? If we have enough water, we'll use it for the next season. That's all. So evapotranspiration is the collective loss. It's the collective loss with evaporation, which is from open surfaces, land, water, road, etc. And your transpiration is from living organisms. We had a good lecture series on that. So I'm moving on to the next parameter. So the next parameter is a delus, which is the hero of this equation as per the course is concerned. A delus is change in water storage, gives the total water storage change. What do I mean as total is because this component includes your soil moisture storage. It includes your surface storage like dams, etc. It also includes your groundwater storage because all these other storages are just not as a storage within the system. Because when you do a minus g in, minus g out, how much water is going, you remain as a storage. And where does the storage go to this equation? Same here, we have river coming in and river going out. If my river coming in is lesser than the river going out, then I'm storing some water and that could be here. It could be a check dam. It could be a large dam, etc. So delus, I'm not breaking it yet, is the total water storage in your field, in your watershed. And it has multiple components. We'll get into that. So for our course, this is the parameter that we need to maximize. We need to control. We need to manage because the others are very, very difficult. So precipitation, you cannot big manage it, but you can manage how much water comes in going out, which eventually goes to delus. The ET, as I said, most farmers are stuck with a particular crop. They would like to only grow that crop sugar cane, for example. We can give alternate hybrid varieties, hybrid stuff, but are they willing to grow it is a question. Let's take, for example, ET of the previous traditional food that we used to eat like millets and other things. Those had very less ET value, but now almost everywhere price is the important staple food, which consumes much, much more water than millets. So are we ready to shift that is a question. So ET also is very hard to control right now. You can control your evaporation pretty much, drip irrigation, etc. Transpiration is totally dependent on the plan. You can have a hybrid plan which consumes less water. Same with groundwater in, groundwater out. Your groundwater out is very hard to control if it goes to base flow, but your pumping can be controlled so that you store it. So del S is the key for this course. The next parameter is G in, which is your groundwater in. How much groundwater is coming in to the system? It could be coming in from another watershed because groundwater does not operate with a watershed boundary. It operates as the slope, the land, the rainfall occurring, etc. Some water comes from 100 kilometers. There are a lot of studies and news articles on this. Please feel free to go through it and you would be amazed that how much water can come from where. And depends on your pumping. If you have tremendous high power pumps, you can pump really water beyond the state also. So in the US there are lawsuits on groundwater being pumped from different states within the United States. So coming back, groundwater recharged the system from another watershed because your precipitation is already contributing. But from the other watershed, how much is coming in? It can also be different from different bases, so it is complex to get at. You do not know where it's coming. See, unlike surface runoff, which you can visualize. I know this watershed is here and this is the next watershed. I know the rainfall in this watershed. I know the land use. So I know how much runoff is coming. But underneath what is happening is the groundwater. You do not know where this basin is coming, getting the water. So it is a very, very complex part to get at. And since it is very near and dear to me, groundwater because I'm a groundwater hydrologist focusing more. And the other reason it's near to me is because India is the highest groundwater extractor. And it extracts more water than the next two countries, which is US and China put together. So the combined usage of groundwater by the next two countries, which is China and US, is much, much bigger than India. So India's use is much bigger than these two countries use. So we have to be very careful in managing groundwater. So coming in groundwater recharge to the system. Some water can take more than 100 years. How do you complex? It is very complex to quantify that. How do you know how much water is coming in? So you need to be very careful when you use this equation. G out is the groundwater out of your basin. That is mostly by pumping. So that you could estimate how much water you take out. But the other part, which is very, very difficult to estimate is how much water goes out as base flow into the other watershed. So same as I said, you may get groundwater from this watershed, if this is your watershed, but you're going to want to control to this watershed. So it is always difficult to estimate where the boundaries are because aquifers are wide and spread and sometimes very random. You do not know why they stop. It's a purely geological process. So it gives the total groundwater used. Pumping for agriculture is also included. Industries, domestic use, deep rooted trees. So all the trees and plants that are in your watershed should be accounted for. For example, if I'm having a lake, that is my delas, the lake water is being stored. I'm happy. But around the lakes, if I'm having a very thirsty tree, which is just sucking all the water out into transpiration, and that is your groundwater out because your groundwater is pulled, your lake water is pulled, and then you evaporate the lake. You finish off the lake. So these are some of the reasons that you see in Bangalore now. Most of the lakes have specific trees which can pull water out of the system. So groundwater out is a complex, complex part. So what do you see in most of these equations is, if it comes to groundwater, people say, okay, if I have three watersheds, one, two, three. And if I know that groundwater is coming from this watershed to my watershed, and this is my watershed, and then groundwater can flow from my watershed to the others, what they say is normally, okay, g in minus g out is zero. How much water groundwater is coming in is going to go out. So that assumption you would see, and that is why some of the times you do not see this parameter. And your g out through the pumping for agriculture, etc., are counted in the ET. So make sure you don't double calculate it. Sometimes your pumping water can go to some other place. So be very careful if you have your ET where the water is being taken. Okay, so let's look at some more important points in this hydrological water balance. The first point, very important point is the way the variable is written might be different in different books. There is no universal law that R should be rainfall not runoff. So even you can construct a watershed balance, water balance. So it is very important to, before you break down the equation, go to the author's notes on what each one means. Normally, they'll give a keynote at the bottom of the equation, or they will give an index at the end. So be very careful on what it means. Along with it, the other important point is the units. Are all the units consistent? Because your rainfall could be in inches per hour, whereas your infiltration could be millimeters per second. So make sure all are normalized to one unit, to one rate, rate as in is it per second per day per minute. All of them should be one unit time. Normally, these are done as per day, and then they multiply it by 365 to get the annual rates. Each component may have further equations. So as I said, if you do not know groundwater complexities, some people will say groundwater, then mass groundwater out of the zero, to move the term. But most of the cases, you would have to have the objective clear to rule that out. So each component may have further equations. Your del S, as I said, is a storage. Storage of water where? Is it the groundwater? In the groundwater aquifer? Is it your dam water in your watershed? Or is it your rainwater harvesting on top of your terrace? So you have to be careful that there is a possibility for each parameter to be expanded. How much you want to expand is totally your objective and study area. It may lead to further complex water balance equations. So the equation can go on elongating both sides, which means the del S might be a combination of other delas. Delas soil, delas groundwater, delas the rooftop, etc., etc. Whereas this side also, you can have multiple, multiple equations. Presentation, runoff, rainfall, snow, snow, water, ice, melt, etc., etc. So we'll have an example to show how it can be different. Let's take a small example from the book. The input items into a basin are essentially precipitation for this equation and your surface inflow. Subsurface inflow is the water that comes into the ground and then out into your rivers and discharge area. So P plus GI is the input to the system while losses are E, which is your evaporation, evaporation from open surfaces, evapotranspiration from your plants and the land where the plant grows because you have the land, the soil and the soil can evaporate. So evapotranspiration is a term that could be used only for your plot, whereas the evaporation can be from the lake, open soils, etc. So normally it is combined together as ET, but some people would differentiate as evaporation and transpiration or evaporation and ET for plot. And then your subsurface outflow. As I said, as your groundwater is moving into the land through discharge as subsurface flow, you can also get your water lost out of the system as subsurface out, which is O. So the balance goes to record and recharge the groundwater GR, okay GR, increase the soil moisture SMA, soil moisture A and as surface runoff stream flow R. So you see here P plus GI is the input to the watershed, which is being converted as evaporation, evapotranspiration, G out, which is your subsurface out. In the previous example, we saw G as groundwater. Here it is subsurface outflow. Then you have your groundwater recharge G again coming and then your R as runoff. So all these are your losses and these are your positives. So you need to understand the positives negatives and then if you bring it to this side, you bring all this to the side, the net, if it is 0, then your del S is 0, the storage is 0. So this equation says there's no storage except the SMA. So the SMA could be positive or negative depending on where you put it in your equation. Moving on, you can become very complex or very easy. I'm going to give you an example of how it is very, very simple and this example is taken from the groundwater resource estimation book. The GEC 1997 has given this groundwater estimation committee and it's still used because it is based on fundamental physics because the fundamentals don't change. So you can have the equation says this. Let's take the first example equation given i is equals to P minus R. P is your precipitation rainfall minus your runoff. I'll have this equation like this. So this is your land. You have your rainfall coming in, which is P and all of it can go as runoff with some water infiltrating. So the infiltration is nothing but P minus runoff. So P happens, some infiltration happens, but most of the water goes off as runoff. So there's a three variables out of which two if you know, you can estimate the i, but then let's make it more complex. So here in the example, let's visualize it, it could be a road. A road is there. Some water goes into the road on the corner as infiltration, but most of the water is going as runoff. It just goes into the stream, etc. The next one is E. So evaporation is happening. There's no, in the top one, there is no water body to evaporate, but here i is equals to P minus R minus E. So some of the precipitation goes into your lake. So the lake can evaporate. So that is here. The next equation now I'm throwing in another variable which is T, transpiration. So now the lake, around the lake, I have some crops. So the crops are transpiring. So some part of the precipitation is lost because of water going to crops. Some part of water is lost because it goes into your lake. And then the next one is i is equals to P minus R minus E minus T minus ice. So some of the water can become ice, if it is the cold region. So here we are, we are including and taking more and more complex terms. So another one I'll just finish with this one is snow r, which is your snow water melt, which is a positive. So it can add to your precipitation. So you could see here how an infiltration, a small component, can be explained by simple terms or a complex of terms. So is this feasible for regional scale assessment is your question, but it may not be important for your field area depending on the site that you choose. Moving on, there are various water balance equations. Now we have understood from the previous example that it can be simple, it can be complex. Let's take a watershed approach and solve one. So this is your watershed and you have the area, the whole area is a recharge area and a small area along the stream. So this is your stream as the discharge area for groundwater. So the important component is precipitation is happening and evaporation is happening. And some of the water is going into the groundwater as recharge and the recharge can also give water to the river as base flow, which is discharge. So the first equation, the simplest one is P is equal to Q plus E. So your precipitation is equals to Q plus your evaporation finish. I'm not going to look into all these complexes. But now I say no, no, there is different storages there. So let's look at the storages. So now P is equals to Q plus E, which is the evaporation plus del SS, which is the water stored in the soil and del SG, which is the water stored in the groundwater. Let's look at the steady state hydrological budget example. So this is your recharge area where on which your precipitation happens and there is evaporation on the recharge area. So some water is lost. And the recharge area means water is recharging into the groundwater. So it goes into the groundwater component, gets stored here, which is a del SG, gets stored here on the ground surface, which is your del SS, your soil. And some of the groundwater gets into the discharge area along the stream because the stream can get water from the groundwater. We saw this in the base flow example in the previous lectures. So that can go into the discharge area here. And your ED is the evaporation along the discharge area, which is a loss. So the net water, all these waters would combine and come out of the water stress here. So you can see here how a simple equation can be explaining your watershed behavior. Or if you know the details, you can throw in more other variables to make it more complex. Is it necessary for complex equations? Is it necessary to have all these equations and all these indexes to explain what it is, etc.? It depends. Such level of details is necessary for the objectives of the study, which aimed at looking at soil moisture, water deficiency, which needs to be managed for optimal use of agricultural land and water. So for this example, let's say P plus GI, this is groundwater subsurface inflow, is equal to E plus ET plus GO, the water lost out of the surface plus SMA plus GR plus R. We've discussed all these terms in the previous slide. But what is this equation going to give me? If I know SMA, if I know how much ET is lost, then I can get at SMD as given here, which is your soil moisture deficiency. There's nothing but soil moisture deficient millimeters per month is equals to EPT minus ET. So EPT is the potential ET of the particular plant, which means how much maximum the plant can consume if it has unlimited water supply and if it is a healthy plant, minus the actual ET. So how much is it actually growing? Because I'm giving only less water. I'm not giving unlimited water, it's less water. And that is SMD. SMD is zero during your monsoon because rainfall can give all the water. SMD is a very important concept during your early season because you're applying water. Only when the plant suffers slightly, the farmers would rush and say, okay, I'm going to irrigate. Why would they irrigate if the plant is happy with the water? Or they know prepared that, okay, every three days that plant will get Thursday, I will give water on the third day. So this is the beauty of this equation. It can be as simple as possible, depending on your study area objectives and you can become as complex as possible. If you have the data and you know the objectives. The idea here is if you know that the land has groundwater recharge, for example, but you do not have groundwater recharge data, then you can put it as a limitations or concern for your study. Please don't say that there's no groundwater recharge. If that is the case, it's fine. But if you know that these are the physical principles that are happening, but you do not have data, give the equation for what it is, and then write a description saying, due to limitations of data, I do not have these for supporting my equation. So I'm not going to consider it. However, the assumption is it is kind of supported in the delas or precipitation or ET, whatever estimate you have. So it is always good to be upfront about the limitations of your study and or your objectives have to be clear to understand what the parameters go into your water balance equation. With this, I hope you understood the water balance equation for complexities and the ease of using it and what variables to it. I will see you in the next class. Thank you.