 to understand what are the various applications of radar remote sensing when it comes to hydrology and over the course of last few lectures, we have been understanding how precipitation can be measured or water levels in the rivers, how can they be measured using radars on board platforms like spacecraft, okay. So in today's lecture, we will try to understand how soil moisture as well as terrestrial water storage can be measured using radar remote sensing. So let us start with satellite-borne radars for measuring soil moisture. See when you hear about soil moisture, the thoughts that come to your mind is it is a very small part of the water budget, you know it is present as a very teeny tiny part of the water budget and still it is considered immensely important because of the crucial role it plays in agriculture, in weather and climate and sometimes even in the spread of diseases. So soil moisture as such it is present at a very critical zone at the surface of the earth that is it is present at the interface between land and atmosphere, is not it? That is why even though it is a very small part of the water budget, it is very crucial, it plays a very crucial role in terms of agriculture as well as weather and climate. So let us start with the basic definition on what exactly is soil moisture. It is the water stored in soil expressed in terms of volume or weight, please note that the concept of soil moisture, you know it differs from the perspective of a farmer and from the perspective of a meteorologist and soil moisture this variable is highly varying in space as well as in time, it is not homogenous which makes it even more challenging to map the spatiotemporal distribution of soil moisture from radars that are onboard satellites that are kilometers high above the surface of the earth, okay. Again please note that there are different means of measuring soil moisture, we can have in situ probes or sensors that are installed at ground, okay. These ground based measurements, they measure soil moisture at a point, you know it is not an average representative soil moisture of an area but in situ probes or sensors, they measure soil moisture at a particular point, wherever it is inserted it measures the value of soil moisture there and from a remote sensing perspective the soil moisture estimated shall correspond to average values within a footprint, remember when we discussed about radar based footprint which are elliptical, you know it can be the footprint as such can be considered as an ellipse with a major axis and a minor axis and we also discussed how this footprint can be converted to a pixel, isn't it? And a pixel represents a certain area on the surface of the earth, it does not represent a single point. So from remote sensing perspective the soil moisture that is measured by satellites it shall correspond to values that are averaged within a footprint and for a radiometer that operates in passive microwave remote sensing this footprint scale can range from a few hundred meters to even kilometers for a space bond radiometer. Here by scale I am referring to a characteristic length or a characteristic time and in the context of soil moisture measurements. So here the graph shown in front of you, you can see spatial resolution mentioned as the x axis which ranges from centimeter level to kilometer scale and as the y axis you see the temporal sampling interval that is how often are the soil moisture measurements being taken and shown here is the axis from minutes, fine scale to even decades. And I have tried to classify three systems, one is an air bond system which can have a high spatial resolution then comes a satellite imaging system which has a spatial resolution from few centimeters to meters and coming on to satellite monitoring system which has spatial resolution that can extend up to kilometers. Now please remember that I want to make a distinction between the process scale, the measurement scale and the model scale. See process scale is the time scales at which a process takes place. It rains today, what is the value of soil moisture tomorrow, time scale at which a process takes place and when we say measurement scale it can be defined in terms of the spatial and temporal sampling characteristics of a sensor system. How often is it being measured, is it being measured daily by a satellite or you know once a satellite passes over IIT Mumbai, when is the next time that the same satellite sees the same area over IIT Mumbai and what is the value of soil moisture that is measured you know spatial and temporal sampling characteristics. And third is model scale, you know using land surface models or hydrological models what we do is we take data and we try to realistically simulate a process, isn't it? Which means model scale is the mathematical or physical description of a process. So the process scale that is the scale at which a process actually occurs is different from a measurement scale that is the spatial and temporal characteristics, sampling characteristics of a sensor system and the process and measurement scale this again differs with the model scale at which we as hydrologists we try to mathematically or physically describe a process. Now when we talk about several missions which give us satellite based soil moisture we have already discussed about SMOS. Here let us discuss about the SMAP missions, SMAP as an abbreviation, it stands for soil moisture active passive. So this mission has an L band radar operating at VV, HH and HV polarizations remember HH stands for horizontal transmit horizontal receive and when I refer to polarization I mean the electric field vector the direction in which it is oscillating. So VV polarization, HH polarization and HV polarization and the SMAP instrument it also has an L band radiometer it has a radar as well as a radiometer that operates in active mode as well as passive mode and hence the name soil moisture active passive. Again the radar only soil moisture that we get from SMAP mission it is a 3 kilometer soil moisture estimate from a very high resolution radar backscatter data. So by now I am assuming that after module 2 you are clear with these terms that what is backscatter, what is radar backscatter and so on. Again SMAP also provides soil moisture measurements in the top 5 centimeters of the soil column. So let me show you the sample data just for visualization purpose from SMAP. So what you see here is the track of SMAP that is the path that it covers on a particular sample day is chosen that is November 2nd 2021 and here you see the data, the data extracted over parts of India that is whenever a small part of the track is passing over India the same data has been extracted and shown here. So you can see the different colors refer to the soil moisture in meter cube per meter cube. The different circles you see signify different colors which in turn mean different soil moisture values in units of meter cube per meter cube. See sampling considerations are very important for measuring soil moisture wherein the sampling requirements are in turn driven by the high temporal variability of soil moisture. So in the tutorials we shall be learning in detail about how to download a sample soil moisture data file and how to open and extract the relevant variables in Python. Moving on let me introduce you to one more mission that is proposed to be launched in the year 2022. So the NYSAR abbreviated as NYSAR it stands for NASA is Rosynthetic Aperture Radar which shall be the first radar of its kind to use two frequencies the L band microwave frequency as well as the S band microwave frequency to measure changes in the earth surface. So the NYSAR it shall have an orbital altitude of nearly 747 kilometers with a repeat cycle of 12 days and it shall be operating or capturing images in the L band as well as S band. If you are more interested to know about NYSAR I would urge you to visit these websites. So moving on, at this point let me discuss the measurement principles of soil moisture that is how does microwaves measure soil moisture. You see in the optical or mid-infrared region which spans between 0.4 to 0.3 mu meter there are several water absorption bands at 1.4, 1.9 and 2.7 mu meter water absorption bands. And in the thermal infrared that is between 7 to 15 mu meter the indirect assessment of soil moisture is used through its effect on the surface energy balance but then in this course we are talking about microwaves. So for microwaves the change of dielectric properties are used to measure soil moisture and microwaves as such they are highly sensitive to soil water content and we already know that they can penetrate through vegetation and soil with the penetration depth increasing with wavelength. So as part of the first module we discussed that microwaves they do not signify one particular wavelength but they signify a range of wavelengths and we also saw that they are given different names like L band, S band, C band, X band and so on, is not it? And the dipole moment of water molecules as you see in the screen it causes orientational polarization that is a high dielectric constant. They measure the backscattered energy from the surface and for radiometers that operate in passive mode they measure the emission coming from the earth surface. Let me also take the opportunity to introduce the International Soil Moisture Network. So the International Soil Moisture Network you can visit the website here, it establishes and maintains global institute soil moisture database. It is free to access and this database has been used by many researchers to validate as well as to improve the global satellite based soil moisture products. For example you can zoom into any of the locations to see how data is made available. For example I can zoom into one station and you can see the details are being present, the soil moisture depths are present, you can click and view the data as well as download the data. It is made available freely for researchers to use. You can see the time series also made available and this link allows you to view as well as download the data. Now say you want to see the location over India, you can zoom into the particular site and click on the station to get more details about the data. You can see the different soil moisture depths and the time series information that is being made available. So at this instant let me slowly take your attention to a related topic on scatterometer. So till now what we have done is we have tried to assess how radars to be more specific how satellite-borne radars are used to estimate soil moisture that is what we discussed till now. Now let me slowly shift your attention to scatterometers, see scatterometers are the instruments that provide accurate measurements of the radar cross section of the target RCS and scatterometers are flown in aircrafts or satellites and they provide indispensable information about how microwaves interact with the earth surface features. And space-borne scatterometer can be used for measuring the wind or rainfall. When I say wind I mean measuring wind speed as well as direction over water. Several measurements can be made over oceans with different viewing angles. Examples are quitsat and ERS wind scatterometer. Of course there is a need to measure the same region from different azimuth look angles here. For example the ERS wind scatterometer it uses three antennas which produce three fan shaped beams at 45 degrees forward, sideways and backwards, ERS wind scatterometer, quitsat scatterometer it employs a conical scanning and there are two narrow beams that are scanning a different scan radii. So all in all the measurements from both these scatterometers whether it be quitsat or ERS wind scatterometer they have been examined over land regions as well as over ice surfaces and please do note that scatterometers can also be polarimetric though we are not discussing too much into that. And shown here is the satellite sampling just to let you know about the spatial as well as temporal sampling of synthetic aperture radar, scan SAR and scatterometer or a radiometer. What I will do is I will take your attention to another application of radars in hydrology which is in estimating terrestrial water storage TWS using GRACE satellite mission. Now again ground water when we think about ground water it is a major source of fresh water for human sustenance and irrigation in many parts of the world and India in particular has a very high dependency on ground water as 90 percentage of annual ground water draft is utilized for irrigation as per the Central Ground Water Board 2013. And you may ask what is terrestrial water storage? So it is defined as all forms of water that is stored above as well as underneath the surface of the earth terrestrial water storage and the dearth of observations of terrestrial water storage estimates was solved by the launch of gravity recovery and climate experiment mission which is abbreviated as GRACE, GRACE mission which stands for gravity recovery and climate experiment. So as shown in the diagram the GRACE satellites they are not a single satellite but they consist of twin satellite you can see one here and one here. So these twin satellites were launched in the year 2002 to make detailed measurements of the earth's gravity field and it uses K band ranging system, K band another band when it comes to microwaves is K band and GRACE in particular uses the K band ranging system to provide precise measurements of distance between two satellites. Now these twin satellites they fly at about 220 kilometers apart and over land regions the variations of global gravity fields are due to water mass variations which means the distance between both satellites can either increase or decrease due to variations in global gravity fields and this in turn is captured by GRACE satellite and we interpret it to get information about terrestrial water storage that is a summary. So again GRACE is used to study gravity variations to determine ground water storage on land masses and if we compare the current data with that average over a long period of time we get to generate an anomaly map something known as an anomaly map and an anomaly map it shows us where ground water storage has been depleted or increased. For example if you pick research papers that have been published using GRACE data they point at depletion of ground water in the Punjab region. So that is just one case study but there are many studies over India wherein GRACE terrestrial water storage data has been compared with central ground water board well data to get a comparative assessment. Several studies to examine the ability of GRACE satellite to detect water storage changes at various spatial scales and please note that the studies using GRACE they have been conducted globally as well as regionally. So let me present one such study conducted by us. So what you see here is as the black dots they are the well locations provided by the central ground water board CGWB. The CGWB records seasonal institute well hydrograph for more than 30,000 well locations at Pan India scale and it contains information about the median of maximum ground water level during four seasons of a year that is post monsoon Rabi, January to March, the pre monsoon that is April to June, monsoon that is July to September and post monsoon Kharif October to December. And the majority of these monitoring wells they pertain to shallow unconfined aquifers except few regions in northeast and southern India which contain a large number of confined aquifers. So for this particular study what we did is we have filtered the observation wells for temporal consistency that is only those wells are used which are monitored for at least two seasons for the complete study region and further we have tried to employ an interquartile range based approach to remove outliers in the observation wells. So what you see here is the location of wells that are considered for the sample case study after initial quality control. So you see 5, 9, 8, 8 well locations here which were used for the study. So we tried to study the impact of precipitation on ground water storage using data from India meteorological department IMD and further studies were also conducted using Grays-Maskon solution based ground water estimates. So shown here is a sample plot wherein you see the trend in ground water depth that is estimated from CGWB monitoring well. So the blue dots denote significant trend and the other one denotes no significant trend for different time as in from 2010 to 16, 2003 to 9, 2003 to 2016 and so on. This is part of a study and there are many studies available which try to download and use the Grays data and compare it with the Institute data and one such introduction shall be given as part of your tutorials wherein you will learn how to download the Grays data and how to open the data in python extract the relevant variables and to see visualize the data. Okay? So let me hope that you could understand the different facets of applications of radar remote sensing and hydrology through this lecture and I will meet you in the next class. Thank you.