 Hello everyone, welcome to NPTEL course on Rural Water Resource Management. This is week 7, lecture 4. In this week we have been looking at the hydrological water balance approach which has been called by different names example mass balance, water balance or hydrological balance equation. It is a very important equation to understand what is the net storage in the system after considering the major inputs and outputs and the storage then can be transferred into a rural water resource. So this can be actually used for agriculture or livelihood options, give the farmers a sustainable irrigation pattern etc. The past lectures in this week we have been looking at how the water budgets can be established, what are the units, what are the different parameters that are key to establishing these equations and also arresting a unit to start. Moving on, we understood that watershed scale basin scale or catchment scale is very important because a watershed by the definition it catches water and sheds water. So water shed, a catchment catches water basin catches water, a watershed would catch the water and shed the water. Shed meaning it will give it off and that is what is very important for the hydrological cycle because the water continuity would happen. If all the water was stored here then how will the water go to the oceans, lakes, plants and then evaporate or evaporate and then go back to atmosphere and again rainfall all those things cycle would be incomplete if the watershed loses the property of shedding the water. So some people do say let the rivers flow and that is the meaning of also incorporated in the watershed approach. Basically all the area within a watershed would capture the water and bring it to one point through channels through drainage networks or streams. So these are like streams that you could see and they are bringing the water through all these channels and to the last point of the watershed where all the water would come from your watershed and get out. There are some storages, groundwater storage, soil moisture storage, some check dams, big dams etc and those should be accounted for. We have known how to account this in the surface water balance but here what we are going to see today is how do you define the unit for your analysis especially the watershed approach. This is an example image from the US government NRCS and we'll be seeing how to use a map basically a topographic map to understand and delineate a watershed. Let's look at the methods. There is need for info on elevation. So the basic criteria for defining a watershed is elevation because it runs on the principle that water flows from high potential to low potential. So in a given area you need to find the highest potential and the highest potential is given or triggered by the height in which the water sits which is eventually the elevation of the ground, the trees. So if you know the elevation of the ground and multiple elevation points along your area then you could establish this high potential to low potential connection. So in an area if everything is flat everything is the same height and I put water here and water here would they flow they will not flow because it's the same potential but if I have a undulating surface a slope and this is the nature it is a slope normally you don't see flat land in an actual system. So if a water falls here and a water falls here will this water let's call it b and this water a will be go to a or a go to b. a will go to b because a is at a higher potential and b is at a lower potential. If you have undulating surface like this still a b c so a will go to b c will go to b b will not go to c. You see the point that you have a high potential low potential and a middle potential c will not go to a but c will go to b because b is the lowest potential and water would want to go. So given this understanding of land and elevation we understood that elevation is the key in driving this potential difference and to understand how water flows which is what is needed inside the watershed. So what do you need in a watershed to delineate a watershed is info on the elevation. So when you get these info from maps, cadastral and topo sheets manual and also it could be available from state and federal survey departments. It is basically a survey department mandatory to have these images or maps and most of them are paper maps and if you go to these offices they will be willing to give you a photocopy of it but not the actual map and there are some other methods that I can share with you in the coming time. So it was manually done so which means very very long time ago they would send surveyors to monitor the elevation from one point to the other. Where is the lowest elevation point? We've had this in the groundwater class we've across many other classes in this lecture series. The lowest elevation point is your sea level. Sea level is at zero am I right? So we have zero so what would a surveyor do? They will go to the zero location because that is where they know and from there they would use a differential scale to actually put points along the trail and then measure the elevation difference. You cannot measure the elevation difference at zero level for example Mumbai coast and Delhi right? You cannot because you cannot see the elevation difference so you have to break it into multiple smaller points that is what a survey is and then go to every point and use these instruments to see the differential elevation and then at the end of the day you will have a map. I'll show you a map how it looks like. So that is the manual method. The automatic method or digital method I would call more is from the digital elevation models which are models driven by satellite and remote sensing data. There are other ways also you can digitize a toposheet. So the maps that have been made using surveyors and manual which are more accurate but spatially you cannot have many points because not every meter or kilometer they would have. So those maps can be digitized which means converted into digital format. I said it is available as a paper but you can convert it to a digital format. The other things that you could use satellites which are remote sensing area satellite payloads which are going above the earth's atmosphere or you can use smaller remote sensing methods like radar like planes. Planes can take elevation using LiDAR technology all those things. So you could understand basically there are two ways. One is your manual toposheet method and your digital elevation models procured by remote sensing data. Either way your goal is to have a map with elevation points. And this is a very interesting website in University of Texas. It houses all the important toposheets for entire India. All you have to do is go to this link and then click which is your area of interest and they have divided India into grids or tiles more importantly and just click on the tile number and scheme. So given an example how to do it okay is it northeast, north, west, etc. So you can have the region here and the number on this sheet along with the sheet number sequence. So this is NE44 this is NENG44. So in the NEG44 which one do you want and then if you click it you can go to the actual map. I'm zooming in to show you another view of it. So these tiles have the same number you could see that oh why am I seeing another two another three which is the same. It's because this is ND44 33 and ND43 3. So there's a big difference between the ND and ND right. So this is ND43, ND44. So use these numbers in particular or if you cannot use the scale just go to this and click it. They have actually linked each box style to the accurate toposheet. I'll show you an example. Before we see that NC43. So NC43 is Calicut, NC43 4 is Eero which is these areas and then you have all the NCZC's and NT's here. So you could see that you have an image of the toposheet by just clicking on the toposheet. And what you see inside is if you zoom in you will see elevation data and the names of the important locations and water bodies. So the blue water bodies are available. These are mapped way long ago. So if you go now and check these water bodies it may not exist. Maybe it's converted to urban setting. Maybe it has been small because of encroachments on the side. But this is a very important map to show how many water bodies we had and in what time. So if you look at the name, our spellings are very different from the conventional use because it is done by US people and they use 1960 to 1929 data. So this is before independence. And so who did these exercises? It was done by the Britishers. So the Britishers did very extensive mapping of these elevations along India's boundaries. And for a particular reason that they wanted to see how the elevation radiators so that they could build railroads, roads etc to take out the processions of India faster through ports and stuff. So if you look at very closely you can see the numbers of the elevation. What is the elevation and what is the unit? Read the information here which is given in this box. It gives you clearly what is the unit used, what is the data that has been used to make these maps and the location of the style is given here. So all the information you need is given and each one is divided into types. So this is one way of getting your data. I'll also show you some remote sensing data but this lecture is mostly to show what you do to delineate a watershed. So basically you need an elevation map. An elevation map is nothing but lines which tell you the elevation of the region. So if you have points or lines which are called contour lines which actually tell you the location and the elevation of that point. So if you have this line for example along the line the elevation is same. So this one is at 400 meters so along this line all the wherever the line goes it is 400 meters that is how you should understand. So they've divided the land into contours and contours would have the same elevation along the same line. The next contour would have a different so you see differences of elevations. So this point what you see on the as a center point is the highest elevation because it's a smaller circle. So if you look at a mountain or a hill it will be like this and the center point is smaller area because see the area is very small that's why the circle is small and then if you come down the mountain you can put concentric rings of equal elevation and that is why you see a concentric ring and then after some time it joins to the next hill because the hills don't stand by itself right they might have a hill range Himalayan range Wesson guards range. So they actually are close together so you'll see multiple hills with different elevation points but at one point they coincide because the elevations are the same for example here the elevations are the same the base of the hill is almost the same. Okay so how do you deline now I've given you a toposheet using a toposheet or a method how do you use manual method I'm not using here satellite data because satellite data there are tools open source that can quickly deline the watershed for you you don't have to give a lot of input but it is always good to understand the methodology so let's get into the methodology draw a circle at the outlet or downstream point of the stream those who are interested you can print this out from the slide and then do the exercise the basic exercise is given in this slide okay this one okay good so or you could take the previous India image and take a slide take a toposheet out and then you can do this exercise the first thing you should do is draw a circle at the outlet or downstream point of the stream interested so every watershed is defined by an outlet so that is where the water would be collected and it will be shed outside your watershed outside the area so that watershed outlet point is very important and that's where you should determine this is my point of interest so that is the information that is needed from your end so you're giving an outlet point okay then it could be it could be above the dam below the dam so if you see here there is a lake forming there's a stream running and a river and it forms lakes or and there's a dam or something because there is more water here right so then the level of water is high and below right below it you have put the outlet so all the water is collected and taken out then what you do is observe the topographic elevations quantum lines on both sides of the circle which means from here you should look up and from here you should look down so both the sides you should look at the topographic elevations or quantum lines because some maps would have a point and the point would have a elevation so here what the book is saying in our CS is saying is just look on the side of your point and then find the highest elevation put a small x at the high points along both sides so I'm putting one x here which is the highest point and then I move up to see where is an isolated high point it should not be a connected line it should be a small circle and a high point okay so then you go on marking so if you see here clearly you see a hill and on the top of it I put the x here also hill hill so isolated points so along the isolated points a small x can be put and then you connect the x via lines from one side of the circle so now I put all the x's okay and I now join the line from one side so I go up go like this to delineate the boundary I cannot go this side because there's no nearest high elevation isolation point it is here so I come down and go here then connect connect connect and connect all these points out of the watershed outlet point so connect x's via lines from one side to the circle note that water flows is perpendicular to the contour points I'll have a figure to show what do I mean perpendicular it cannot flow along because if it flows along a line the elevation is the same I've already told you if the elevation is the same water will not flow from one point to the other point it is the same only when it is elevation is different so this point is a higher elevation this point is a lower elevation high to low so how will this be this will be one contour line and this will be one contour line so it flows perpendicular to the contour line okay note that water flow is perpendicular to the contour line and not parallel or along continue the line until it passes around the head of the watershed which is this this is called the head of the watershed and eventually become connect to the circle which you started so and then it connects here a lot of errors when you do it manually because you might miss a point which is high elevation but you didn't notice it or the data on the paper did not capture so there are some errors in building eating watersheds which is where an automatic method is much much more useful and then once you've connected the line so from here I draw the line connecting all the x's and then come across the boundaries yes then I have made my watershed so this is how the watershed looks for example as I said you started here with the outlet point and then I moved up to see where the nearest high elevation point so I have an a high elevation point here and here which one would I take I would take this point because from here this is the closest okay and then I move on there is a high elevation point here but then this one is much more higher and or within one side of your river so you have to go closer to the river as much as possible don't go away from the river closest to the river which is the highest point because if water falls on the ridge what will happen it will fall 50% on this side and 50% on the other side because it is like this the point as I am saying the highest elevation point is this one and if water falls on this one it has 50% chance to go left and 50% chance to go right okay but all the water inside the watershed will flow to you so all the watershed would do is collect the water from all the area and put it into the drainage which is your stream or river and bring it out to the outlet point so that is why it's important to have an angle from your river so you already know where the river is and from your river you should look up to see or down to see which is the closest high animation isolated point so here it is this one so I put a point and then again I move up and then sideways I found this point 230 and then here a small point here there's no other point just look down because all it is running lines it is not closing you see here it is not closing if the circle is closed then that is an isolated high elevation point okay so I have one point here one point here one point and then come up down and then go through the watershed boundary and come here let's take some key important points this is called the head of the watershed this is called the downstream of the watershed head stream downstream okay the downstream is where most of the water would come because it concentrates all the water and brings it to the outlet point so the flow is always greater at the downstream point compared to the head but the head stream point will give you where the river originates so the river is originating here along the head stream along the head watershed not downstream okay so you would see all these locations where it is and then this watershed also gives you an understanding of where the elevation gradient is and where the land use land covers okay so you can also move it here that totally depends on you where you want to put your outlet point I wanted here because it will be good to see how much water comes into this dam how much water comes into this river if you put it above the dam then the water is mostly from here and coming around and this dam water drainage all these areas would not be captured so let's see these concerning points and why we don't use them okay so the first point we do see a saddle this is called a saddle point and you could see the lines run perpendicular or cut through the contour lines they don't go along the contour line so never go along so I didn't use this point because it will go this way and then come whereas I have a very nearer very nearer to the stream I have a high isolation point why would I not use this point because it's not closed only when it is closed it forms a full circle okay it forms a full circle and hence you can have an x mark but here since the elevation is there it is a high elevation but it is not closed which means water from here can still enter this path and that you need to accommodate why will I not use this point it is a high elevation pick it follows your methodology however from your stream from your stream if I go outwards this is the first elevation point which is isolated so I should stop here and come down this elevation is for the next watershed boundary okay so we saw how we need to use and neglect some of these points and the rules and concerns we did mention earlier and I'll show you how it is available on a contour line so if you have contour lines from the topo sheet remember that the watershed boundary which is red runs perpendicular to it okay watershed boundaries are perpendicular to the topographic elevation why because this one is at a high elevation another elevation lower lower lower and this is the lowest so that's how the watershed boundary is created you cannot have along the watershed boundary to be along the contour line because then it is the same elevation wherein you don't have much difference in water flow boundaries run through the center of closed contours and saddles so you have this high elevation point the boundaries will run through it not along it not around it and also saddle points saddle points are between two high elevation points so there is a high elevation point here and a high elevation point there so between that there will be a saddle okay so this is your saddle so you have a high elevation mountain a high elevation mountain and in between you have a saddle and the watershed boundary would run through it okay so look at how it runs through it stream flow to the opening or a contour these so you see how the stream flow would occur is along the contours the V's towards the opening is the flow direction to the opening of the contour so this is your opening of the contours and water would flow along this side along this direction so this is within your watershed blue is not the watershed boundary it is the stream so once you have the stream network created you also notice that it flows on the downwards so this one is flowing downwards the stream from north to south and all the contours are at opening like opening like this so water is flowing through and why because your high elevation to low elevation so this part is your high elevation and of the V and this part is high elevation this is lower elevation water flows from high to low potential and it cuts through okay this is within there is no high elevation point isolated here because that is at the boundary at the at the boundary of your watershed so the high elevation point isolated is for your boundary of the watershed okay so with this I think we have covered the need for watershed as a unit of analysis we understood how a watershed is delineated using the elevation points okay you have an elevation along your topographic map a topographic map is a manual map where they have the elevations and using that you can manually delineate the watershed by identifying your first your stream of interest and along the stream where do you want an outlet and from the outlet you put the elevation points and do the boundary okay this is a manual task but in nowadays it is very easy to do this exercise within five minutes on your pc using GIS software and the data is also free the next class I'll show you some data and some watershed boundaries that have been created using your open source DEMs and open source GIS software so that it is free open source is free for everyone you are not required to do it by hand you can do it by these softwares because a computer access is getting better nowadays right okay so and this is the starting unit of analysis for your water balance once you do the watershed then within that all the parameters you can take you cannot take outside like a district boundary you cannot take because now you're working at a watershed boundary so watersheds are very important to understand how the water flows and within the watershed all the water would concentrate precipitation into runoff but if you take it from a district boundary it can take from water from outside the discharge point may be outside your district boundary so that is where district boundaries are not used for water balance equations first you do it at a watershed scale and then maybe you can apply it to the district state national scale if needed with this I would like to conclude today's lecture on the watershed analysis unit which is your watershed area delineated from elevation maps and we will see in the next class to wrap up the watershed balance equation lecture series and I will see you in the next one thank you