 Hello everyone, welcome to the next lecture in the course remote sensing principles and applications. We are discussing the spectral reflectance properties of few commonly occurring earth surface features. Today, we are going to start with the topic of discussing about the reflectance properties of water. So, when we take water, the reflectance that we observe from water bodies can take several different paths. Let us look at this particular slide. So, if you take the radiance, let us assume the sensor is located here. So, the radiance reaching the sensor from a body, from a water body can come from different different paths. First thing can be just the pure path radiance from the atmosphere, we are not interested in this. Second component can be from the surface reflected part of water that is whenever like EMR travels from one medium to another medium, like this is air, this is water. So, when the medium changes at the surface, some reflection will happen, we have seen this. So, a part of the radiance reaching the sensor will be from the surface component of water. Then water is little bit transparent we know that is water is not completely opaque, it will allow light or other wavelengths of EMR to pass through. So, some portion of EMR will be passing through the water body and there will be different different elements either dissolved or suspended, different materials can be present there and the EMR that is penetrating through the water can interact with these dissolved or suspended materials and it can come out of water again and can reach the sensor. This we call it as volume scattering like or the reflection due to the presence of any materials in the volume of the water body. Then if the water body is shallow, then some portion of the EMR can penetrate water deep enough, it will interact with whatever is present in the bottom of the water body and it can again come back and reach the sensor. So, this is reflection from the bottom of the water body. So, essentially the radiance reaching the sensor from a water body can take any of these four parts this is cannot be neglected atmosphere always be there, surface will always be there and most likely volume will also be there. If the water body is shallow, effect of bottom of the water body will be there or most of the or the EMR may not reach the bottom of the water body at all depending on the depth. So, four components we should consider or if we remove this at least three components we should consider which includes the signal about the water body and based on the materials present the signal will vary and that is what we are going to see in this particular lecture. Before moving on to understanding about what happens to these portions LB or LB the volume scattering part or the bottom part first we will make it clear that the reflection from the surface of water body will most likely be specular because for a clear or still water if the water body is like free of any sort of waves or any sort of disturbance or something it will be like fairly smooth. So, whatever reflection happens from the surface will most likely be specular in nature. On the other hand once the EMR penetrates through the water body and when it interacts with whatever is present inside the water that particular radiation when it comes out it will be diffusing nature that is component 3 and 4 here whatever you have written they will be most likely diffuse in nature because there are high chances that during the interaction with molecules present inside the water it can be like scattered in different directions or there can be multiple reflections say one molecule may reflect towards this this may reflect here and so on. So the EMR signal that we are going to receive from the volume component or the bottoms of bottom of the water body is more likely to be diffuse in nature but whatever reflection that happens from the surface is more likely to be specular in nature. So, this particular slide tells us how water absorbs or scatters in general for different different wavelengths. So, essentially now we know a large like certain portion of EMR will penetrate through the water and for water reflectance is pretty low mostly like the reflectance will be in order of like less than say 10% or even less than 5% in most of the cases. Hence a large component of EMR which is incident on the water body will most likely penetrate through the water body or get transmitted through the water body. So once EMR gets transmitted through the water body then a certain fraction of it will be scattered and certain fraction of it will be absorbed only remaining fraction will be allowed to pass through or come out of it. So in the earlier classes when we discussed about the properties of EMR we came across few terms what is known as absorption coefficient, scattering coefficient and attenuation coefficient. Three different terms we defined in the class. What are they? So the total attenuation coefficient is it is inverse of the attenuation length that is 1 over attenuation length is the attenuation coefficient what it represents if certain amount of EMR enters the water body after travelling through certain length that EMR energy will become 1 by e times of the original quantity which entered the water body. So, what distance it has to travel for its power or energy to become 1 by e times. So, that distance we take we make it 1 over the distance we calculate as a attenuation coefficient and the total attenuation what we study has two components. One is scattering another one is absorption these two combined together gives the total attenuation. So, if we study about the laws in power only due to scattering we call the coefficient as scattering coefficient. If the loss is only due to absorption we call it as absorption coefficient or if we take both of them together we call it as attenuation coefficient. So, essentially attenuation coefficient Ke is equal to Ks plus Ka that is this is total attenuation coefficient this is scattering coefficient and this is absorption coefficient. So, if you look at here these three coefficients are actually experimentally got observed and it is given for different wavelengths. So, for let us leave ultraviolet portion because most likely we will not receive the radiation on the earth surface. We will start from blue wavelength from 0.4 micrometers. You can see that the absorption and scattering is fairly low for blue wavelengths slowly you can see like it is in the same order the value does not change much for blue wavelengths. Then when you encounter the green absorption slightly increases and when we move on to say yellow or red portion the absorption further increases and when we move to NIR portion you can see the absorption has increased manifold it is like so high. So, what this means water body or water in general is fairly transparent to blue wavelengths and green wavelengths. So, blue water body is fairly transparent to blue then comes green wavelengths but water absorbs a good portion of NIR and even red the absorption is pretty high in comparison with blue or green wavelengths. So, this suggests that water is a very good absorber of NIR wavelength then it absorbs red to the next maximum extent but it allows blue or green to pass through that is its absorption is quite less. So, when absorption is quite less and when it is passing through there are high chances that it can fairly interact with whatever being present in the water body. That is say we have NIR this is like a water body let us say NIR is incident over it NIR wavelength it will be absorbed there would not be any chance for this NIR to interact with whatever is being present within the water body. But let us say blue wavelength is being incident on the water then because it is less absorbed it will be transmitted through and if something else is present in the water there may be some soil particle there may be something got dissolved whatever this passing this blue wavelength that is passing through will most likely we will interact with it and this particular signal will be recorded by the sensor. So, if we want to study the what is being present inside the water body or if we want to study about the bottom surface properties of the water body then we should use blue wavelengths or green wavelength which is less absorbed by water. On the other hand NIR will be strongly absorbed by water and its penetration depth in the water is extremely low. So, the total attenuation that happens that is combination of both scattering and absorption the total attenuation that happens actually is very low for blue wavelengths then comes green and for like red and NIR the total attenuation coefficient is fairly high extremely high actually for NIR portions. So, this particular slide gives us in a more clear way you can see like the reflectance of water is pretty low even in like blue wavelength or blue or green wavelength the reflectance is quite low water is actually like a very poor reflector. Reflector is like less than 5% in most of the cases, but you see like the transmittance the transmittance is increasing from here the axis is marked from here from 0 to 100% you can see the transmittance is extremely high for blue and green wavelength. So, it is almost 90%. So, water allows blue or green wavelength to pass through with very less absorptance and reflectance. So, reflectance is quite low. Once we cross the green wavelength the transmittance decreases very rapidly like you can see like the transmittance goes towards 0 very fast and once we reach this something around this 0.8 micrometer the transmittance is almost 0. So, most of the red and NIR wavelengths will be absorbed whereas the blue and green wavelengths will be allowed to pass through, but in general the reflectance from water is very low in whatever the wavelength the reflectance curve you can see it is like very low pretty somewhat higher reflectance in blue and green wavelengths this is actually the reflectance curve that I am highlighting somewhat higher reflectance in blue and green wavelength, but very low reflectance in red and NIR after NIR it becomes almost 0. So, water the reflectance is quite low most probably less than 5% but transmittance is high in blue and green wavelengths and absorption is high in red and NIR wavelengths and it is very high in NIR wavelengths. So, what is the implication of these things? Since the transmittance is extremely high for blue and green then because of this EMR in these wavelengths will penetrate the water body and it has very high chances of interacting with whatever is being present in the volume of the water body or the bottom surface of the water body. So, in the previous slide I told you like LB component and LB component. So, the LB component and LB component will be predominantly observed in blue and green wavelength because of transmittance. On the other hand in NIR wavelength most of the thing most of the EMR will be absorbed by water itself. So, nothing will be coming out to forest and red can penetrate to some extent because the transmittance in around say less than 0.65 micrometers is still around like 60% so it can penetrate to some extent but due to increased absorption it will have very less chance of coming out of it. So, essentially what will happen is let us say a mix of EMR is going towards the water and based on the wavelength the penetration depth will vary. Say blue wavelength can penetrate deep into water body, green can penetrate with less depth, red is further less depth and NIRS has very low penetration. So, if some other object is being encountered by this wavelength it will be scattered or reflected by that particular object. Let us say some soil particles is suspended in the water body. So, blue which penetrates deep can come out of it, can get reflected by that object and can come out of water again. Green also can do that but red or NIR will not be able to do that. So, if we look at it, if we get any particular signals from water body. So, for in blue wavelength the signals from the water can come even from like very deep surfaces say 15, 20 meters and there are reports telling that for very clear calm water bodies signals has been obtained even from a depth of up to 50 meters in blue wavelength. Then in green it may become something around say around like 5 meters like the depth from which the signal can come because only till then EMR can pass through and escape otherwise it will be absorbed completely by water. As more and more as EMR progresses more and more into the water it will be continuously being absorbed. So, its energy will be keep on reducing at certain point it will become 0. So, after a certain depth any wavelength will be totally absorbed. So, most likely blue can penetrate much deeper say something around 15, 20 meters. Green can be around say 5 meters red can be 1 or 2 meters NIR is still lower even like a thin layer of water say like even half a meter or something can absorb NIR if there is no bottom effect or something if the water is like really clear then NIR can be absorbed very rapidly. So, now we will see one very good example of this penetration capacity and what it can provide us for different, different wavelengths. So, in this particular slide we have a portion of land and water sea Caribbean sea map together. So, this is the figure A is observed in green band, figure B is observed in red band and this is figure C is observed in NIR band. So, if you take the NIR band in NIR all land surface features have very high reflectance very high in the sense like much higher than water. So, naturally whatever be the land surface feature be it soil, be it vegetation, be it urban surface whatever will have relatively higher reflectance. So, land will be appearing bright in NIR portion. On the other hand water body with its very high absorption in NIR will appear pitch dark there would not be any reflectance from NIR or any reflectance from water body in the NIR portion of electromagnetic spectrum. This suggests that NIR band is very suitable or highly suitable for delineating water bodies like if we want to map the boundaries of water bodies or separate water body boundaries from land surface NIR band can be highly suitable for mapping this because NIR is strongly absorbed by water bodies. This is one thing. Then we will come to green band. Green band green has relatively higher transmission. So, what happens green band gets transmitted inside and immediately after like land surface we will have like lot of this shallow land surfaces like we will have like what is known as continental shelf, continental slope all these things. So, immediately the depth will not increase immediately there will be some amount of land surface or soil present underneath the water. So, the EMR that is penetrating into the water is now getting reflected by the bottom features whatever is present there and that is what we are now seeing in this particular green band and here there is a coral reef called the palanka reef that signal is also being observed. This is because as EMR gets travel through water if something else is present inside like this is like the bottom surface this is the reef. So, this can penetrate through get reflected from this particular feature and this signal we are now getting back and getting recorded at the sensor. On the other hand EMR in NIR band as soon as it hits the water body it penetrates and it gets absorbed it does not have any chance to reach this spot. Even before reaching this spot it will get absorbed near the surface itself. So, as the penetration capacity of different wavelength changes our ability to see the objects present underneath the water surface will increase. So, if we want to study about anything present inside the water it is always better to use blue or green wavelengths. But the only problem with blue is it will be highly scattered by atmosphere so we may not get good signals but green we can use for sensing underneath the water surfaces. If we want to map the water and land boundaries then NIR can be like a most suitable band for doing it. So, now we have seen a general property of reflectance how water reflects how water transmits. So, in general in the infrared portions water is a very good absorber it absorbs most of the things. In the visible portion water is equally like a poor reflector as well as like poor absorber relatively poor absorber. So, it allows most of the EMR to pass through it. So, if we combine all these things then we can define certain factors which will affect the spectral fact spectral nature of the wavelength of the EMR that interacts with the water body. Some of the factors are first thing how pure the waters concentration of pure water presence of what to say vegetation like organisms we call phytoplankton or presence of chlorophyll in general if vegetation is present in water or not. Then the presence of dissolved or suspended organic matter like sometimes like organic matter like vegetation may die and decompose it may get dissolved in water or something it can happen. Similarly inorganic substances like say soil particles it may be washed out from land it can get mixed with water body. So, they will be suspended in the water body. So, presence or absence of such dissolved or suspended inorganic materials that can change the reflectance. So, all these things can change the reflectance of water body. In addition to this the depth of the water column how deep it is if it is really deep then EMR will not have a chance to interact with the bottom surface at all. But if it is relatively shallower then some signals from the bottom features whatever is present on the bottom surface will come up. So, how deep the water body is may play a role. Similarly for large water bodies such as oceans or seas presence of waves wind or basically the surface roughness can influence the reflectance reaching the sensor. So, now we will see little bit in detail about how these factors will affect the reflectance. The first factor we are going to see is the presence of vegetation or what is commonly known as commonly referred as phytoplankton that is if we take large bodies of water such as oceans then tiny organisms of vegetation very tiny particles of vegetation can live they are called phytoplanktons which are like will be seen in like very large groups. So, hence it will be visible even from satellites but they are like highly tiny organisms. So, they are really important in the global carbon cycle itself because what will happen they will be extremely abundant in the ocean surfaces in certain seasons and they will absorb carbon dioxide from the atmosphere and do photosynthesis. When they die they will like sink to the bottom of the ocean that is all the carbon that got assimilated within these particular organisms will sink to the bottom of ocean and it will settle there. So, essentially the phytoplankton enables the ocean to act as like a carbon sink this is one of the factors we know like oceans act one of like a carbon sink. So, the presence of phytoplankton is the major reason how ocean is acting as a carbon sink the phytoplankton absorbs water sorry absorbs CO2 it goes inside the water body. So, monitoring this phytoplankton in water bodies is an extremely important task for understanding global carbon cycles. Similarly, certain types of algae may be there in water bodies we might have observed lot of like algae in lakes or ponds or whatever. Some types of algae may be toxic in nature like most of the cities or most of the living livelihoods places in the around the world depends on freshwater sources for drinking water purposes like lakes or ponds or whatever. If such algae develops on such water bodies then the water may become unfit for human consumption or consumption by any other living forms. So, it is really important to monitor them. So, monitoring vegetation in water bodies is actually a very important application of remote sensing people are doing it on an operational basis for various purposes in understanding global carbon cycle for monitoring water quality for all these applications monitoring of vegetation is really important. So, how the presence of vegetation will change the reflectance the example is given in this particular slide. So, for clear water body we will talk mostly up to NAR range because after NAR there is no reflectance everything is mostly absorbed. So, for like a clear water body there will be like higher reflectance in blue then the reflectance will slowly decrease in green further decrease in red and in NAR it will be very low like the reflectance is almost close to 0 for clear water bodies. However, when vegetation is present or when algae or phytoplankton is present in the water body then the characteristic vegetation kind of reflectance will come up because we have seen for vegetation reflectance in blue will be pretty high in green reflectance will be quite sorry in blue reflectance will be very low in green reflectance will be relatively higher in red again there will be absorption and in NAR there will be increased reflection right same thing will happen. So, in blue wavelengths reflectance will go down because of presence of vegetation in green wavelength reflectance will increase in red wavelength reflectance will go down again in NAR reflectance will increase. So, the presence of phytoplankton or algae will influence the reflectance and we will see a characteristic vegetation like curve vegetation like spectrum reflectance curve rather than seeing pure rather than seeing the reflectance of pure water body. So, decreased reflectance in blue increased reflectance in green increased reflectance in NAR these things will become more prominent as vegetation either algae or phytoplankton or whatever vegetation settles in water bodies. This again gives another example like normally in open ocean water like deep ocean water bodies water can be like relatively clean and relatively free from any other impurities. So, essentially we will observe like the curve for clear water surfaces, but near the coast where other materials may be present like vegetation may be there, suspended sediments may be there due to which a change in reflectance may occur. So, here you can see there is slightly decreased reflectance in blue slightly increased reflectance in green portion similarly increased reflectance in red and NAR. So, all these things happen because of presence of vegetation or other particles near the coast like if this is the land surface near the land surface we can observe all these things while in deep oceans where water is fairly clean we can observe the curve for like a clean open ocean water. Say this is again for increasing concentration of vegetation or chlorophyll concentration. So, here the concentration increases from curve A to F. So, this axis this direction represents increasing concentration of chlorophyll we can see that as the chlorophyll concentration increases this dotted line is for clear water. As the chlorophyll concentration increases the reflectance in blue goes down drastically and it increases fairly in NAR you can see like near the NAR portion increased reflectance is seen whereas for clear water the reflectance becomes close to 0 even in like near the red itself. So, this suggests that the presence or absence of vegetation will clearly change the reflectance and the concentration also will change. So, observing in visible and NAR wavelength will enable us to understand the presence or absence of vegetation this is like monitoring of like ocean color like how in what color like ocean is looking is kind of really important to know. If the water is like extremely clear then this thing like EMR will penetrate through especially I already told you blue will be like penetrating a lot. So, you take like a clear open ocean water very clear nothing else is present. So, what will be there ocean water we know it contains lot of dissolved salts within it salt is nothing but it will be dissolved in form of like molecules salt molecules will be there. And we have studied in earlier classes if the molecular sizes much lower than the wavelength involved then those molecules can cause Rayleigh scattering. So, vegetation sorry not vegetation the clear open ocean water bodies when wavelength penetrates through it blue wavelength will be scattered by the salt molecules because still clear open oceans still has salt molecules within it. So, it will be undergoing Rayleigh scattering. So, all the blue wavelengths will be scattered by this and will be allowed to spread throughout the water column that is why clear water bodies appear blue to our eyes. Similar to the reason of why sky appears blue the same concept whatever molecules present within the water be it salt or be it the water molecules itself they will scatter blue and the scattered blue wavelength will be spread inside the water body itself that is the main reason water appears blue to our eyes clear water. But if something else is present then that will interact and rather than seeing a blue water we will be seeing like kind of greenish water or something else. So, monitoring this ocean color is extremely important for us to understand what is being present in the water bodies. So, as a summary for this particular lecture we have seen like a general introduction about the spectral reflectance property of water and what are the different factors that influence the spectral reflectance property of water bodies. We discussed about the influence of vegetation on the reflectance part. So, in the next class we will further discuss the other factors which influences the spectral reflectance nature of water. Thank you very much.