 Hello everyone, welcome to the next lecture in the course and today we are going to discuss about application of remote sensing in water resources management. So this is like hold together a different field like in the last lecture we discussed about land use, land cover classification and some applications of such using like the classified data and change direction and all. So water resource management when we want to like see the applications then definitely it involves some sort of background knowledge in the field like in the introduction to this application section. I told that whenever we want to do some sort of applications then definitely the domain knowledge is needed for us to effectively apply remote sensing data sets. So this lecture we will try to get like a very broad overview of what water resource management and how different remote sensing data sets can be used in water resources management. So what water resource management is to simply put water resource management is to deal with or is to develop a kind of procedures for efficient and sustainable use of water resources. That is we need water for many things not only us even like the natural systems they need water for their own survival even other living beings plans and everything. So there should be some sort of like mutually agreed use of water as humans we cannot take all the water available on the planet earth right we also need to spare something for the environment. So water resource management is to deal with these kind of problems from efficient management of water resources so that it can be used sustainably for the beneficial use of humans as well as for environmental protection. So this is like one of the definition there are like different definitions available in various textbooks but we try to cover like in a broader sense. So what all the activities that will classify as water resources management. So water resource management will always deal with operational management of existing water resources say we have like a reservoir how to effectively operate it for sustainable water use. Development of new water sources say exploration of groundwater or like kind of like construction of new dams so that water can be provided for agriculture and drinking water purposes for a region all these things. Then planning and maintenance of associated infrastructure water resource management people has to also deal with the infrastructure associated with it say a pipe network. So it deals with water resource management say there is like a reservoir water has to be distributed to a town nearby means it will be distributed in form of like pipe network. So there will be like specialized people dealing with those things installing the infrastructure needed and maintaining it. Early warning and monitoring of natural hazards say drought, drought is a natural hazard. When drought occurs then there will be like lack of water it may affect food production it may affect all living beings. So there should be some sort of activities going on for effectively monitoring such natural hazards or even to forecast it say flood is a natural hazard. So when people predict that a flood region is going to be flooded so the water resource or the infrastructure dealing with water resource say a dam or kind of thing they have to prepare for it say adjust the water level so that the incoming water can be stored and released slowly. So all those plans they have to make. So effectively monitoring as well as prediction of natural hazards comes under the purview of water resource management and also management and reduction of risks. Whenever something is done by human always there will be some sort of risk associated with it say construction of a dam and maintaining a reservoir behind it. So this involves some sort of risk what if the dam breaks or if some say a flood occurs how much people will suffer from the flood how to mitigate the risk involved or how to mitigate the hazard the implications of hazard so all these things. So water resource management is not like a small field it encompasses several divisions and there is like separate master's level program to teach the different aspects of water resource management. So it will be almost impossible for us to cover it in one course rather than even like in one lecture what we are going to see today. So what are all the important variables that we will be requiring for water resource management naturally for effectively managing the water resources it will be beneficial if we have information about all components of water cycle and also how water is being like utilized basically. So water cycle means how water like water keeps on transforming itself from one phase to another or from one place to another. It goes through several parts of a water cycle say from the land or oceans it may evaporate and go to the atmosphere a part of it may come back as rain. So whatever comes back as rain it will be flowing along the hills and land and it will be reaching the rivers the rivers will flow to the oceans a good fraction of it will go to the groundwater system and so on. So this effectively is like a very simple example or simple explanation of like water cycle. But if we can quantify all these different components of the water cycle it will be really beneficial or helpful for us to effectively manage them. Say if we know there is like a severe groundwater shortage in one particular region then the government over the region may not permit development of more groundwater extraction zones like installation of bore wells on both. So there can be some sort of policy restrictions or if we are able to like tell the rainfall is going to be very poor in the next season means ok people will be prepared for it. So they will be like trying to maintain the water they have in the reservoirs reduce the consumption all these things. So effectively if we have information about the different components of water it will definitely be of great use in management. So what all the different components of water that we can get from remote sensing precipitation or rainfall basically evapotranspiration soil moisture stream flow groundwater level and in some places rivers are fed by snow. So the snow extent and snow water equivalents water quality all these things are some of like the important variables that people working in the domain of water source management will require. In addition to this experts in the field will also require information about meteorological variables say temperature, humidity, pressure and all which will feed their models. Normally remote sensing will not go alone in most of the things for water source management. People will try to combine remote sensing data sets with some sort of models and for running those models we will be needing meteorological variables. In addition to that physiographic information such as land use land cover maps, topography that is elevation all these things. Again these can be obtained from remote sensing land use land cover we just saw in the last lecture topography that is DEM we can obtain from optical remote sensing or like stereo photogrammetry that is or radar remote sensing or even LiDAR all these things. So basically several variables have been listed for as needed in water resource management. In this lecture we will not be able to cover everything but we will quickly see how remote sensing is helpful in retrieving two variables. One is evapotranspiration and another thing is soil moisture. First we will see how to model or estimate evapotranspiration from remote sensing. So what evapotranspiration is? Evapotranspiration is the transfer of water from liquid phase or solid phase to vapor phase and getting removed from the surface to the atmosphere. Say you have like a bucket full of water if you keep it open for some time then after maybe like few hours or maybe one or two days the level of water in the bucket would have gone down. So it is the water has escaped from it. So the liquid water has transformed itself into vapor phase and escaped got mixed with the atmosphere and it is kind of lost. Similarly this process is a continuous process it happens everywhere and all the time. Evapotranspiration actually is kind of like a driving cycle of this or driving force of this water cycle. It connects the land and the atmosphere because water has to move from land to the atmosphere and evapotranspiration actually provides a pathway and also it connects energy and water cycles like the earth as a planet and as a system has so many cycles within it water cycle, energy cycle, carbon cycle or other nutrients cycle and so on. So there is always some sort of interconnection between these different cycles for the globe to function as a system. So this evapotranspiration provides a link between the energy cycle and the water cycle because when evapotranspiration occurs then only water will get transferred from land or ocean to the atmosphere then only it can again come back. If evapotranspiration ceases then the cycle is cut off. Not only like water getting directly transferred, plants also will transpire water like when you pour water to plants it takes in water for its functioning and a large fraction of that water goes back to the atmosphere in form of transpiration like plants will open their stomata like tiny holes in the leaves which will allow the water to escape. So it provides cooling to the leaves and it also like part of like the way in which vegetation functions. So this is closely connected again with carbon cycle. How much plants transpire? Very much closely connected with carbon cycle because plants store carbon it is the primary producer. So it is kind of like a complex system in which evapotranspiration is actually like a finding place. So monitoring this evapotranspiration is one of like the crucial elements for understanding earth as a system and also for various applications in water resources, agriculture purposes and so on. So evapotranspiration is basically controlled by meteorological factors that is for evapotranspiration to occur. First thing you need a source of water. There should be some water present maybe like on the land surface it may be wet or that can be healthy vegetation or that can be like a water body anything any source of water must be present. Then some atmospheric forcing means there should be some input energy to the system so that this water can gain energy and move away because it needs some amount of energy for the water to get transferred from liquid form to vapor form and escape. So that energy has to be provided. And when water transfers face like say for example like this is like a water body so the water molecules on the surface will get transferred as vapor. But now let us say this is like the entire air parcels closer to this water body is now saturated with water. Now it has to be moved away unless there is like a gradient say here there is some water vapor in the atmosphere there is like a water body or water surface which acts as a source of water. So this water vapor has to be moved away so that fresh water can evaporate. So there should be continuous transport of this water vapor away from the source which is done by wind. Similarly like temperature, humidity all these things play a major role. So evapotranspiration is controlled both by surface controls like surface parameters say if there is a vegetation, vegetation will not let all the water from it to escape. There will be some sort of like controlled mechanism through which vegetation will transfer water. So evapotranspiration is controlled by meteorological variables and also by surface biophysical controls. So effectively in remote sensing we will try to monitor these different controls and try to use it for estimating evapotranspiration. Because evapotranspiration cannot be measured directly by remote sensing sensors. We can only measure few variables that will provide us clues related to remote sensing say vegetation parameters, surface temperature and all these things these are some of the variables which are closely connected with the process of evapotranspiration and using these variables we have to estimate evapotranspiration. So the remote sensing based models for estimating evapotranspiration can be broadly classified into three categories. One is like vegetation index based models, then Penman-Monteth or Pristley-Taylor formula based models and then surface energy balance based models. There are like other different class many classes are coming but these are some of like the classical three classes of models. Nowadays like even machine learning models are like being used to very large extent but these are the three models we are listing here are like the classical ones. So what these are and how these models can be helpful in retrieving evapotranspiration we will get like a broad overview for it. So the vegetation index based models, this is applicable over like vegetated surfaces basically and this is kind of like an upscaling strategy. Upscaling strategy means say you have a large region over large region. So let us assume it is homogeneous in terms of vegetation and water availability and so on, homogeneous region. Let us say you have some ground measurements of evapotranspiration distributed across this region, some 4 or 5 ground based measurements you have. Along with this ground based measurements, let us say we are measuring meteorological variables such as temperature, humidity and all plus we are measuring the vegetation, maybe we are measuring some sort of vegetation indices using remote sensing. Say I am measuring NDVI or I am measuring EVI some sort of vegetation index. So at this particular small field or like one pixel may be an image, I have a collective information of the actual evapotranspiration that is occurring. From remote sensing dataset I will get NDVI. From any other source either from ground measurement or even from remote sensing again we can get meteorological information. So using this data from the say 5 stations we can develop a relationship between ET and any of this auxiliary variables let us say NDVI or EVI. Say some sort of relationship let us say it is something like this. It can be vary with region to region. Some people have related ET only with vegetation index, index some people have related evapotranspiration with vegetation index and meteorological variables and so on. So effectively what we are doing we are developing a relationship between evapotranspiration and this vegetation index. We have it over those 4 or 5 points in our region where we have ground measurements then we get satellite data. So in satellite data we will have vegetation index for all the pixels. So what we are doing for other pixels over which we do not have ground measurements we will apply this relationship. Say you have one equation ET is equal to some equation in terms of NDVI or EVI and you get this EVI information from remote sensing just apply it in this equation and get it. So this is like a very simplified explanation of vegetation index based models that is why called it as upscaling method. Upscaling means you have few ground measurements how to apply this what to say how to apply the data acquired over those few ground points to the entire region. So this is one of like the simplest methods that was developed but these sort of methods are applicable only over homogeneous regions. If your region if the characteristics of the region changes then the equation that you developed from the ground stations will not be valid. The next class of models is Penman-Montet or Priscilla Taylor based models. So these two equations the Penman-Montet equation and the Priscilla Taylor equation they are like some of the classical equations in hydrology to estimate evapotranspiration. So they were defined to estimate kind of like a maximum evapotranspiration that can occur like people call it as like potential evapotranspiration or in some sense people will also call this reference evapotranspiration. So these are like different aspects but these two equations effectively are like the classical equations. So the Priscilla Taylor equation that is given here basically tells that it is kind of like a simple model which will help us to calculate evapotranspiration as a function of radiation. That is I already told you some sort of energy is required to transfer this water to vapor form and based on this radiation like solar radiation is one of like the major variables there are like other form of radiation that will occur we will see later in the later slides. So how much radiation is available or conceptually how much energy is available and what fraction of the energy goes to the atmosphere as Et. So it is kind of like a simplified model or we can also call it as like energy based equation or radiation based equation. So because the primary connecting factor is energy or radiation. On the other hand the Penman-Montet equation is kind of like a full-fledged equation that relates the energy terms, meteorological variables and also the surface control. So this Rs by Ra is like the surface controls on evapotranspiration these variables E s minus E a, delta, gamma, these are all meteorological variables r and minus g is the radiation control. So it combines all factors that influences evapotranspiration. So the Pricely-Tiler equation is a simplified representation of Penman-Montet equation. The Penman-Montet is like the expanded way Pricely-Tiler is shortened or simplified version of Penman-Montet you can think it in terms of like that. So these equations are effectively developed and tested across different sites in the globe and normally people will run these equations with ground observed data. But later after like year 2001 there were like lot of studies which investigated the potential of remote sensing data sets for getting the variables required for these equations. Say if we somehow get all the variables required by these equations through remote sensing or some sort of like atmospheric reanalysis product or something then we will calculate evapotranspiration. So this is the basic concept behind this Penman-Montet or Pricely-Tiler based equation. So this slide gives us some basic example for this model where here we have listed one model called PT-JPL Pricely-Tiler Jet Propulsion Laboratory model which is one of like the very famous model based on the PT equation Pricely-Tiler equation. So essentially this calculates evapotranspiration using several variables that can be retrieved from remote sensing that is say green canopy fraction, soil moisture, say it uses vegetation indices, it uses some sort of like temperature data, humidity data, all these things comes from reanalysis product and so on. So effectively this model tells us okay these are all the different remote sensing data sets if we combine them together in a certain fashion we will be getting evapotranspiration. So there are again plenty of different models available say Penman-Montet model is the basis for development of ET by MODIS product. MODIS has its own LAN product evapotranspiration level flow product which is based on Penman-Montet equation. So these are several different models are available like each author may improve upon the equations used, how to relate the remotely sensed variables with those very parameters within the Penman-Montet equation or Pricely-Tiler equation. So that itself is kind of huge class of models are available within it. The next class of model is the surface energy balance based models. So these class of models is the one that we can actually feel the connection between evapotranspiration like how it connects the water cycle and the energy cycle. So we will see what surface energy balance is in a brief manner. Let us say we have a surface here, land surface basically. So solar radiation is like the primary driving force it provides most of the energy to us. We all know that the incoming solar radiation primarily composed of energy within say 4 micrometers or 3 micrometers. So most of the energy that is coming from the sun is coming in the wavelength less than 3 micrometers maybe from 0.3 uv radiation to 3 micrometers. We call this as short wave radiation. This comes in form of radiation and we call this wavelength as short wave radiation. Then atmosphere and land surface both by virtue of their own temperature will emit energy that also we have seen in thermal infrared remote sensing right. So land surface has a certain temperature say T surf atmosphere has its own temperature T atmosphere due to which they will have their own emission. So that emission will occur in infrared wavelengths that also we know primary infrared wavelengths but this will also extend to microwave. So essentially the wavelength of emission due to land surface of the atmosphere will typically be greater than 5 micrometer in most of the cases or it will be primarily greater than 8 micrometer. But effectively this is this has a wavelength longer than incoming solar radiation. So we call this emitted radiation as long wave radiation. This is like a generic name given okay. People will not generally discuss about the wavelengths involved. They will just briefly say whether it is short wave radiation or long wave radiation. So these are effectively the driving forces of energy on the surface. In addition to this when this happens let us say a surface is there this is like incoming solar energy. The surface has some sort of reflectance or I will call it as albedo because we are going to measure it in a hemispherical sense. Due to the surface albedo a fraction of it will be reflected back right. So this is RST incoming this is RST outgoing that is incoming energy is what is coming in from the sun outgoing energy is what is being reflected by the surface. We have seen almost all surfaces has some sort of reflectance. So some energy will be reflected back what is coming in from the sun. So what is effectively remaining in the surface the effectively remaining short wave radiation is what that came in minus what is being reflected back. So this will look something like this the radiation that came in minus albedo times the radiation that came in say albedo for surface let us assume it is 0.2. So that means the surface will reflect 20% of incoming energy. So remaining 80% will be there at the surface 20% is reflected back. So this is with respect to short wave radiation. Similarly land surface due to its own temperature will be keep on emitting energy. So we call this as RL upward this will be going upwards from the surface to atmosphere. Atmosphere is there it will be emitting energy on its own due to its temperature that will be RL downwards. So all these things these are the radiation components. So effectively the net radiation available at the surface is given by RS incoming minus RS outgoing plus RL incoming minus RL outgoing. So this is like the net energy available from radiation at the surface. So this is due to solar radiation RL incoming is due to atmospheric emission RL upward is due to earth surface emission. Now this in all this radiation will drive two important processes at the it drives many processes but for our discussions we will talk only about two processes what we call it as turbulent exchange of energy. What will happen let us say land surface is now being heated up like during daytime solar radiation will be more it will be heating up the land surface to like a very good extent. So what will happen the temperature of the land surface has to go up somewhere right there is like air parcel nearby there is land surface. So land surface is getting heated by incoming all these radiation components. What will happen the land surface will now heat up the air molecules near the surface air molecules will go lesser in density it will go up new air molecules will come down colder air molecules they will again get heated up they will move up. So this is kind of like a heat transfer through convection that is air molecules getting repeatedly heated by the land surface they move up and they come down. So it is kind of like a cycle it goes on it is a convective process. This way of energy transfer from the land surface to the atmosphere we call it as sensible heat flux. I write it here sensible heat flux. It is called sensible heat flux because we can sense the heat transfer when you go near like a hot barren surfaces we can feel like the hot air blowing like air is like taking away that heat energy from the land surface to us we can actually feel the heat we that is why it is called it as sensible heat flux. If there is water present on the land surface say it is not a dry surface some sort of water is present then what will happen is this radiation terms whatever is remaining on the earth surface will be used to remove some part of water to the atmosphere as Et. Because water needs energy for itself to get transferred from liquid to vapor form say you have like a water body there. So whatever is the radiation that is present a fraction of that energy will be used up the water to get transferred to vapor form. So it uses some energy on its own that is called latent heat flux or otherwise evapotranspiration. If you call it in terms of hydrological sense we call it as evapotranspiration. If you call it in terms of like energy balance terms we call it as latent heat flux. It is latent heat latent means hidden the energy used by water will not be known to us because when there is like a phase change from liquid form to vapor form the temperature of the water it will not change the energy will be used up by the water for this phase transfer. So the temperature will not raise we may not even know that a phase change is happening energy is being used. So this is like the broad explanation of surface energy balance equation. Say the net energy or the net radiation of the surface is comprised of these four quantities that will be equal to these three H plus E plus Q. H is the sensible heat flux E is the evapotranspiration and Q is what we call it as ground heat flux that is transfer of heat from the surface to the ground surface through conduction process. So in the introductory lecture to thermal remote sensing I told about different processes right different ways of heat transfer conduction conduction radiation. All these things are happening here on the surface incoming energy is primarily through radiation. The transfer of energy happens through convective process again like under the ground the heat gets transferred through conduction all these things comes. So effectively if you think all the radiation terms that is coming to the surface effectively will be equaled out by these three terms H plus E plus Q. So you can see like you if we can calculate this energy terms we can easily calculate the E out of it. So that is like the basic principle of surface energy balance base equation ok. So this is how we will calculate. So in this lecture we will stop here and in the next lecture we will quickly see how remote sensing is helpful for getting E t from this surface energy balance equation. Thank you very much.