 Hello everyone. Welcome to NPTEL course, Rural Water Resource Management, week 2, lecture 3. In the previous lecture we looked upon hydrological parameters number one which is precipitation which is the input to the system. In today's lecture we will look into the evapotranspiration which is one of the key losses to the system. Please understand when I say loss it is a water which is taken away from your watershed or unit of analysis. In our unit of analysis it is the rural watershed or farmland. So imagine this is your farmland on the bottom and you have precipitation which is the incoming water resource and transpiration is the water that the plant takes, uses and then gives it out back to the atmosphere, transpires. So that is transpiration. Evaporation is the loss of water from the surface of the earth, soil, land surface, etc. and open surfaces. So let's look at some fundamentals. Evapotranspiration in short it is called ET is the loss of water from ground to the atmosphere. It is mostly on the ground because plants and open surfaces are on the ground. ET includes two components very important components. One is your evaporation and then your transpiration. Evaporation is the process where water is lost from open sources example urban land, land and water. So urban means for example your roof you can have a rainfall on the roof and it can evaporate when it is hot. Example of an urban setting is also roads. If you go on a highway you see if it is raining still the road is dry because it evaporates faster. Then you have evaporation from the land as I shown in the image below. You have evaporation happening in the land surface because of the sun. Evaporation doesn't happen much in the night. So you have mostly happening with the sun which is the driver of the hydrological cycle. Then you have water evaporating from your water bodies example fresh water you could see the evaporation arrow going up and you can see water evaporating from oceans water is going up. So all this is included in your evaporation term. Transpiration is from living means. So it includes all living organisms from which water is transpired. Let's take examples the biggest transpires are trees because trees take a lot of water and they consume the water and they give it back into the atmosphere. Why plants need water is because it is like a medium of transport of nutrients. So there are a lot of nutrients in the soil water mixes with the nutrients and since plants cannot readily take the nutrients what it does is it takes a solid soluble nutrients in the water so it takes up the water and all the nutrients stay in the plant whereas the water is left out of the system. So that is transpiration. Then you have water in the same process water can be transpired from plants which is smaller form of a tree if you want to look at it the same process and other format is humans for example is part of a human being. So if you run all the water or most of the water in your body is transpired as sweat you could feel the sweat coming out so that is when water in your body is lost to the atmosphere or out of the body. So losing water is by this process is transpiration. So you could see that both evaporation and transpiration is a water loss to the system. Okay moving on let's look at the other components from where evaporation and transpiration can happen. Transpiration can happen from living organisms here we have soil, soil, grass, materials etc etc whereas evaporation can happen from your oceans, lakes, rivers because even flowing river where it's far you could see coming not only stagnant but even some evaporation can happen limited but still some can happen and then water which is falling on the rocks etc can evaporate. So evaporation is from non-living organisms whereas transpiration is from living organisms. Why is evaporation estimation important? Evaporation and transpiration why is it important? It's very important to escape the loss of water from open surfaces. Let's take one by one and in this study looking at why evaporation is very important. Look at the two images that I've taken in our field visit in Maharashtra. You could see that there's a big lake and then there is a field is the first image a field with water applied and in the other hand you have farm ponds where water is being stored in ponds for future use. Maybe they want to use it in the dry season or after a couple of days after the rainfall has stopped. So in both the cases water doesn't stay there because it is stagnant some water is lost to the groundwater okay but most of the water is evaporated. So that is the biggest loss. So if the sun is shining bright and dry and there is less humidity in the air then evaporation process will take up. So there's a lot of loss from the system because of evaporation. So that is why it is very important to estimate the loss of water from open surfaces. There are some projects in India where they wanted to close the canals on the top. So you have a dam and from the dam water is taken to irrigation plot areas field using channels or canals. Okay so both if you have aligned a cemented a can or a channel then water can flow through it doesn't go into the groundwater fine but some what they did is they wanted to arrest the evaporation. So they put solar panels on the top so they arrested the water from evaporating from the channels and canals. So they understood that water is very important in that area and they could not afford to have any losses and one of the key losses as I said is evaporation. So it is important to estimate because the remaining is the water that can be used by others as long as the sun is coming up every day there will be some evaporation. So unless you understand how much evaporation happens it is hard to quantify what is the water remaining for your agriculture or other uses the domestic use etc and this is very important for rural areas because for example here if you have a farm pond without understanding the evaporation you put the money and the budget in time in to put these structures you see these structures to capture water but if you didn't understand that the evaporation is so high that you will lose the water within a day or two then the whole point of the farm pond is lost. So that is where we are trying to say it is important to understand evaporation. Transpiration as I said transpiration is from living organisms why is it important directly tied to carbon sequestration by plants. Why would that be? Because plants when they grow when they want to grow they need nutrients and as I said in the previous slide nutrients are transported from the soil into the plant biomass using water. So what is a vehicle or a transport mechanism? Take like our body we have blood flowing in the body what is a key work of the blood to take oxygen oxygen from one part across the other parts of your body. It's the same thing water takes nutrients from the root to the other parts of the plant some water is remained in the plant as fruits some wetness you know when you crush a leaf you have some wetness some fruit juice etc but most of the water is given off we don't give the blood off but that is the closest analogy you could look at okay so water takes is used by the plant to take up nutrients first soluble nutrients and then goes up so if you understand how much transpiration happens you can understand how much plant growth happens if you know how much plant growth happens you could indirectly measure the carbon sequestered by the plants or how much carbon is kept in the plant or trees so when a plant or tree grows let's take a tree for example the biomass the wood that increases is a good part of carbon sequestration the carbon is brought and kept in your tree so it is very important for Indian government and other working on climate change mitigation adaptation to understand how much carbon a country is capturing and trees are one of the most important and valuable natural resource that can capture carbon very effectively and very cost effective okay so by the rates of transpiration we can understand how carbon sequestration we can understand what a use what is used for transpiration is not available for others as I said if you have plants and trees growing the water is first taken up by them and only the remaining water is available for the other aspects of the soil so for example your groundwater recharge in infiltration percolation all this is after the plant has taken up the water otherwise the plant's potential to take up the water is much much higher the rate is higher than your infiltration and percolation in some in some regions but in some other regions where you have a very slow moving material then it could be opposite but most most regions your transpiration is the water that is lost from the system even though it is a benefit for the plant it is lost it is taken up and given out into the atmosphere so it is very important to understand how much volume we actually transpire humans animals living organisms transpire very less compared to the water budget or compared to the hydrogeical cycle parameters so it's not a big thing to put input in an equation in a water cycle so you didn't see a human transpiring in a water cycle because the plants transpire much much bigger and the domestic transpiration rates are very small compared to that if you know how much water plants are using you can better manage the source ah this is the important part your rural water management course if you know how much plant transpires and how much the soil underneath it evaporates then you can have multiple methods natural to control the evaporation or to reduce the evaporation and transpiration from your field if you do so then you store more water in your system and that is why it is very important to understand how much water the plant is using and the plant to use water is directly related to your transpiration okay so for better managing now you've come up on the different terms for managing rural water you need to know how much water comes in we got the precipitation and how much water is lost which is one of your transpiration or evaporation and the remaining the remaining part would go in as used for other resources so now if I know how much transpiration occurs and how much I get okay so for example I get 100 millimeters of if my plants are going to take up 120 millimeters of water so the 20 millimeters I have to substitute using other resources like groundwater or sand mitigation etc it might be expensive so the measure of transpiration which you could do before the planting or rainfall can help you to better manage the resource some examples are drip irrigation where instead of applying for the entire plant body and other areas of your field you apply only directly to the root zone of the plant for example you could reduce the evaporation because all the soil is not wet only the soil under the plant is wet and that would reduce your evaporation transpiration also can be reduced because you directly apply only a little bit amount of water in known quantities not too much water courage planting in the monsoon knowing your transpiration rate can help in courage planting because if you know your rainfall as I said 100 millimeters however your crop needs 120 millimeters then you would change your crop or reduce the acres to to come back to the volume which is equivalent to your rainfall so your rainfall volume should be more or less equal to your land crop water demand otherwise you'll have to substitute from some other resources it is also important for groundwater irrigation because once you know how much water is taken up by the plant you know how much can be substituted by groundwater as I said not always rainfall is given new water so some of the water you can take from groundwater irrigation so now we have come across these two important terms evaporation and transpiration let's see how we can measure them since it's measured mostly together okay so evaporation can be separate transpiration can be separate however in most rural cases where we are doing farm assessments or field assessments we club it as evapotranspiration since it's measuring both together this kind of complex and there are two major types which is your physical measurements and your empirical models let's look at the physical measurements you can have the FAO prescribed method on the screen and you can see that it is basically a big mass which is being measured before and after applying the water okay I'll show you the cross section so that you could look at it the first method is lysing meters and another meter is sap flux meters for the flow of the class I would explain one method which is the most accurate which is the lysing meter empirical models are models which are statistically made by relationships with other variables and knowing one variable you can estimate the ET okay so knowing a couple of variables you can estimate the ET because the ET is a function of those variables so some of the models are like Penman-Monteath or Penman method the first method was Penman method then people worked on it to convert it to Penman-Monteath taunt-white method etc we would look into not the equation but what goes in and out of the Penman and there are very simpler models like KC method okay so I will go into the KC method by failure let's look at the physical measurement which is your lysing meter so this is a cross section of the lysing meter what do you see here is a farmland okay so first visual a farmland and you're cutting a cross section in the so you're looking at the side view not on the top this is your top you have your crops growing and I'm looking in the side this is your cross section what you see is crops growing so crops growing on the top and you have two places where you have some micro lysing meters we can ignore that for now but it is a piece of land can you see this piece of land is first evacuated out and then a mass balance scale is put in so a big measuring weight measuring device is put in here so there's a weighing facility it's like if you've seen a truck going and standing on a weighing scale truck full of load that's how they estimate how much a load of a truck is so they'll take the truck they stand on the scale they weigh the scale with the truck before or before empty and then they go after empty they come back and then they estimate the load okay so it is a large scale land mass which is taken out look at it it's a weight at 15 tons and the area is 1.5 to 2 meter square and then depth at 2.5 meters so that's 2.5 meters approximately 15 tons of land is evacuated you have to be careful you should not disturb the sample it's evacuated and then you put the mass balance inside once you put your then you put back the land so you this is your land you take a piece out you put your scale underneath your scale to measure the weight then you put back your land and in the land you start growing how do you grow crops the row crops you apply water so when you apply water the mass would be increased because you're adding weight through water and the next day the water would have evaporated or transpired so the water is lost and your weight would decrease so now you have a weight of your water which is being taken up or decreased and from the weight you can estimate the thickness of water that is being taken out so this is how a lysimeter in the real life looks at I've taken this picture from the Parbani University in Maharashtra see that this is the land piece that was taken out and the mass scale which is put in so you have grass growing it's the same crop on both sides so when we apply water the scale here you can see the scale would accurately measure the land with the water and next day I come back and take a measure all the water has been either evaporated or transpired knowing the soil I can estimate how much evaporation happens so most probably you will come back to transpiration but since ET is one term it doesn't matter to give it as moving on so now we have estimated evapotranspiration what about the open bodies water bodies etc there's no crops there there's no land there how do you measure transpiration so transpiration is zero so in open water bodies evapotranspiration is only from evaporation so we can label it as evaporation to measure i and d has an evaporation pan technique which is basically a aluminum pan you can see there are multiple dimensions for it which is given in the figure but most importantly think about visualize a pan so this is a pan with water and a scale a scale to measure the height of the water so the first day at nine o'clock in the morning I would measure the water or six o'clock before sunrise or at sunrise I would measure the water level and then the next day I would come back and measure six o'clock or in the evening after the sun has set I can measure the value so what has happened in the whole day is when there is no rainfall and these are done only when there is no rainfall when there's no rainfall the water level would decrease because of evaporation from the top and that decrease is the rate of evaporation per day so if you're doing it per day analysis per day okay so it is very important to not have data taken on rainy rainy days so most probably it is done on a wet not wet day or dry day when there's no rainfall so you would estimate your water loss as a thickness which directly goes into your water budget so this is a IMD evaporation pan technique so now we've looked into the major physical methods by measuring evaporation for evaporation and transportation or ET together let's look at regions where if you don't have these how do you estimate we have empirical models empirical models will are developed by the understanding between variables and how ET is a function let's take an example the crop coefficient approach is the simplest and most widely used because it is promoted by the FAO you could see that the simple equation is ETC which is evapotranspiration cropped area C is equal to KC which is your crop coefficient times ET naught ET naught is the reference ET so if you have the definitions here and what is your KC is it is a function of your plant okay and it is a function of your crop height albedo reflectance of the crop soil surface so it is both the function of the soil and the crop of that particular location KC canopy resistance how much the leaf area resist the water loss and evaporation from soil all this is a function which is already built and given by FAO and different KC values are given for different crop types and different regions for equatorial regions tropical regions human semi-human all those things but if you could go to this website I've given in the bottom you could easily get KC and what is ET naught ET naught is a reference crop evapotranspiration for that particular area so both if you see a units of millimeters per day and if you're going to report it your hydrological cycle as per day then the per day unit goes off it is just millimeters right and then your ET naught is a reference crop most probably it is grass alfalfa crop or any other reference crop for that particular region and all the ET value ET naught values are given in FAO we use alfalfa crop okay for our region also so ET naught would be a higher value whereas KC would be a smaller or much more or less closer value okay so your ET C is a multiplication of this so it's a function value because ET C is related to ET naught and KC could be the proportional to constant so reference ET naught is defined in Calgary using FAO-Binman-Monteeth equation as I said it is a function of multiple variables wind speed temperature radiation coming and radiation going on so very complex equation which the FAO has done for you so you can take the ET naught from chapter 4 and you can estimate ET C by knowing the KC and the KC is also given to you by FAO let's look at some KC values so these are the KC values and as I said the plant grows and then the initial period is KC initial and then the mid range is when the plant almost matures and then the end is when the plant is dying off or ready for harvest okay so you could see that initial stage the plant doesn't take consume much water okay because it is having less area of leaf or other aspects the KC mid is when it matures so that is the max you can see all the max values are in the KC mid and KC end is when it is ready for harvest it dies down etc so if you look at these for a particular crop height and a particular crop the FAO has given you the data for KC let's take cauliflower carrots so you have 1.05 0.95 there's no KC initial because it's under the ground and KC mid is there but for small vegetables like you have your brinjal etc you have some KC values okay eggplant here so you do have some 0.6 1.05 0.8 etc so from this exercise you could calculate your KC from the previous ET0 from chapter 4 you can take your ET0 and you can estimate your evapotranspiration because you can see how it is a very mathematically empirical based model okay so these are the two methods widely used one is physical method and then you have your empirical method on top of that we also have satellite remote sensing derived products so satellites which are on the top we have already said that it has been used for measuring precipitation it can also measure indirectly your evapotranspiration so they have some capturing devices image capturing devices which are related to your evapotranspiration an amount of water vapor that accumulates in the atmosphere so you could see here an image taken by nasa modus platform it is open source anyone can take it and you could use it widely and many of the indian data products you can find in this robo and also derived from satellite products the one issue is the scale the scale might not be as a small village or even a plot scale so that is hard to get that values but for a village or a district boundary you can still get good ET values from these satellite products and more importantly it is open source free to use and cost effective so these are multiple methods so we have discussed today about evaporation transpiration what are the process you combine them together as evapotranspiration we've looked at why it is important in the ideological cycle we've looked at what are the methods to measure physically and breaking methods and if these two methods are kind of costly and not available readily you could get into your satellite products for this we conclude the evaporation part of the seminar or lecture let's meet in the next stage