 seen that how we can effectively use the offset to take care of many situations in which you can have much better efficiencies by using this method which is only a method of application and changing your layout. One thing which I would like to point out here that by the use of this offset it can also make lot of difference in terms of your parameter selections or in terms of the design of the system. You can get much more flexibility of the design 6 meters, so in sprinkler efficient design you must have at least 6 meters of overlap in terms of the overlap in the vector diameter. In the vector diameter you must have for the design to be acceptable along the main line, you must have at least overlap of 6 meters but by utilizing the offset when you use the offset this limit can reduce to 3 meters, this overlap, overlap in the case of offsets can reduce to 3 meters. Now this gives you lot of flexibility in terms of if you have a situation where your DW requirement is coming to be very large now by using the offset you can take a DW which is or you can make the selection of the nozzles in such a manner that a DW which is lower than that level can also be made use of. So from that angle you will get lot of flexibilities by using the offset procedure and it also the use of offset also enhances uniformity that we have seen in the last lectures we had taken up the situation where we had seen that how the uniformities can be enhanced by the way we make the applications by creating the offset and locating the position of the nature. With that we will go to the next element which is the irrigation interval. Now irrigation interval is a very important element in the design, what do we mean by the irrigation interval first of all? The irrigation interval pertains to the time which is available for applying the irrigation. If you have an area let me take any one area, if you have an area now you are irrigating the total area through some system you have different options available as a designer you have different options available. You might run a mail line and then try to irrigate the area through two different settings half the area in one setting, half the area in another setting. You also might try to divide this into three components, now whether I divide it in one component or the two components or three components this decision makes lot of difference in terms of the total equipment requirement. If I want to irrigate the total area in one go I will need more equipment, it is not just the equipment also I might need different discharges I might need different set of all those parameters which can take care of this combination but that is the question is something different right now we are trying to look at the irrigation interval, from the irrigation interval point of view. If I have the total area divided into three compartments or three separate sub areas I first I irrigate this area then at number two setting I irrigate this area, number three setting I irrigate this area and then again I come back to the previous area. Now how much time I have taken in irrigating each of the area and how much time has lapsed in total between the first irrigation and the last irrigation? We have to look at that lapsed time, is it a time which cannot be this area cannot wait that long by the time I come back to this area, if that is the situation then your irrigation interval is much larger than what is the desirable interval, your irrigation interval cannot be of such a length where the soil moisture reaches below a specified or a desirable level. So if your irrigation interval is going beyond that that means you have to do something to ensure that you are irrigating the whole area in such a manner that you are able to come back to the first area where you started with within a period which is the desirable period which you can delay the irrigation or you can use the second irrigation after that much period and that will be a function of the climate, what is the rate at which the water is getting lost from that soil zone, so all those things we have to look into it and decide accordingly how the parameters have to be set. So when we say irrigation interval we are what we mean is that what is that the duration of the time which we can use for actual application of the water okay and the irrigation interval arrive at the desirable irrigation interval you will like to find out what is the total allowable depletion will be a function of total allowable depletion which in turn will be a function of these quantities, what these quantities are, TAD we have dealt with earlier is the allowable depletion, let us say this is expressed in millimeters, TAM is the total available moisture, total available moisture is the depth per unit depth of the soil, so millimeters per meter depth of the soil and the MAD is the management allowed, this is something which is a prerogative of either the management or the farmer and this is expressed in fraction whether you will deplete 50 percent of the available moisture or up to 60 percent or up to 40 percent that is what and DR is the depth of soil zone or effective root zone and meters, knowing the total available moisture which is a function of the soil type, knowing the management allowed depletion which is a function of again it depends on the crop type, how much, how sensitive the crop is we have seen all these things and depth of the root zone we also know, so knowing these individual quantities you can find out what is the total allowable depletion, how much deficit can be created in a particular soil with respect to the crop which we have, the TAM normally the total available moisture is the difference between the field capacity and CEW which is the limit of crop extractable water, so CEW you will see is the crop extractable water millimeters per meter, this is also the field capacity per meter depth, the TAM as we have just said that is a function of the soil type soil it is sandy soils, TAM is around 80 millimeters per meter sorry and for medium soils all this is available literature these values are standard values might change when you have more elaborate soil classification, similarly the MAD values again as we have just mentioned that depends is a function of what type of crop is in question, if you have high value crop is shallow rooted MAD around 33 percent, so the level of extraction is much lower when you have more sensitive crops, when you have medium value crops, medium rooted you can go up to 50 percent and for low value crops normally they are rooted and depletion can be as high as 67 percent, these are some of the values which are order of magnitudes not little values they can change, can change from situation to situation but in general you will find that the deficit which you can create which you can afford to have will be much less if your value of the crop is quite high and it can go up if the value of the crop is low because it is attached to the yields, how much the yields will be affected because of these higher deficits because if you create higher deficit there will be more stress in the crop, whether the crop can take that stress or not that is the question. Now the irrigation interval is basically now if we say that T i is the irrigation interval is nothing but total allowable deficit by what is the rate at which this deficit is being created or if you say that this is the availability of moisture because that is what you will like to replenish and then at this rate this is going to be lost, this 80 crop is the peak period crop water requirement, this is peak period crop water requirement millimetres per day this quantity is in millimetres you will have T i the irrigation interval in base. So this basically shows that how long you can wait for the next irrigation and that is dependent on at what rate the crop is utilizing the water and how much water is available. T i is the we are basically saying the total allowable depletion so this is the moisture which can be extracted which can be allowed to be extracted and that moisture is depleting at this rate so you can get a value of the irrigation interval which is which is permissible. Now it is not necessary that you have to complete the irrigation in that period you might decide that is again a management decision that is a decision which is which is dependent on you as a irrigator or as a farmer, how much time you want to utilize for the irrigation to be completed but it cannot increase this period the irrigation interval period. You must complete the irrigation within this interval it can be much lesser than this but when you try to reduce it drastically then to apply that water quickly you will have you will incur other problems. So you might not be able to for the design to be better for the design to be economical you will find that you might try to be quite close to this irrigation towel which is available to you because of the various other reasons which we will just look at as we go further. Then comes the aspect of nozzle selection criteria. Now this is a very important aspect in the sprinkler irrigation design because this is the main thing which is deciding most of the other parameters because the selection of this will influence almost all the other parameters whether is the rate of application or is the discharge or is the pressure or is the wetted diameter all those things are interdependent and even the spacings because all those the spacing also is a function or is dependent on which sizes you are using and what will be the distribution pattern what will be the wetted diameter all those things are interrelated. So if you look at this the background of this the nozzle selection let me give you some of the statements which are which will give you the interconnection which are there or the dependences but if we have a nozzle operating at low pressure this will result in operating cost the pressures are low the operating cost will also be lower is quite understandable. But if you have a lower pressure nozzle what is the impact of this selecting a lower pressure nozzle the impact is that you will have smaller wetted diameter as well as lower discharge rate after taking this this step you have landed yourself into this result that by selecting a lower pressure you have reduced the wetted diameter you have reduced the discharge rate also. Similarly if you select a particular type of the nozzle the nozzles can be they can be sprinkler head with a single nozzle they can be sprinkler heads with double nozzle so let us say that if you select a single nozzle what is the result may be that it might it will have a lower uniform to coefficient by the same time under the wind conditions it might be quite effective so it will be it might perform or it will perform better under wind conditions in comparison to the double nozzle sprinklers whereas the double nozzle sprinklers are uniform to normal conditions. Now this high uniformity is because of the increased discharges so in this case the discharges will also be higher in the case of double nozzle so they are when you choose one thing there will be some plus points there will be some minus points and that is where is is important to understand that the nozzle selection procedure is a very iterative procedure it has pros and cons so you have to you have to make a selection and then see for example the various indicators the various constraints whether they fulfill all the constraints or not and accordingly make the choice you might have to come back to the various levels make a selection if it is not satisfying your criteria make another selection and then ultimately select one which satisfies most of your conditions. So is a is a is a trial and error is a balancing approach that is important to understand but the general criteria you must look into make the nozzle selections these criteria these are the discharge should be gross application rate. You know that one of the gross application rates we have even looked at a table which we had formulated where we had given the gross application rates which are desirable rates under that the specific conditions so you have you should should intend to choose those nozzle sizes which have the discharges which are high enough to meet this criteria of having the gross application rate satisfied. The net application rate because gross application rate takes into account the the evaporation and the drift losses also that means the amount of water which is coming out of the nozzle should be at least taken care of by that selected nozzle but when you come to the other side the net application rate you have a criteria which is just diagonally opposite. The net application rate should not exceed the the intercrate of the soil that means you have a constraint on both the sides on one side you are saying that the gross rate must be sufficient to to take care of the application and the gross application rates. On the other side it should not be very high so that the net application rate becomes such that it produces the surface turn off. So you have a you have a close range to play within and that is where the nozzle selection becomes a very very important aspect. Similarly the selection of the nozzle in terms of the rate of application or the discharge which is prevailing discharge it should be let me say q should be sufficient to apply net irrigation. Now comes the the question of how much time you are taking to make the same application which is related to your irrigation interval. Q should be sufficient to apply net irrigation within the operating schedule. You cannot select a nozzle size which has such a low discharge that when you talk in terms of the time taken it should exceed the total setting or the total irrigation in the total area under your interest you exceed the irrigation interval. So that is another constraint put on the nozzle size because the discharge will also be a function of the nozzle size along with the other things like the pressure and those things of course. Then you have another requirement that the wetted diameter must be compatible with the spacing the main line spacing and the lateral spacing the lateral spacing and the main line spacing. So again how much wetted diameter you are going to have is a function of the nozzle selection is dependent on the nozzle and the spacing you have to have a compatibility between the spacing and the wetted diameter because we have said that there should be a minimum overlap of 6 meters. Now that overlap of 6 meters will come only through the overlap of the wetted diameters and the overlap will be decided also by the spacing which you have utilized. So this spacing is also related with the type of nozzle which has been selected as a result you can now appreciate that nodal selection is a very iterative it has to be a very iterative process is very very uhh can be subjective also because there are many sizes of nozzles which are available which when you decide it can be uhh they can be quite close if I give the nozzle selection to 3 different people they might come out with some nozzles which are which are keeping track of all their requirements or within their uhh their uhh their setting which they have decided on because all these things can change drastically the way you are going to operate the system. So you cannot standardize these things they are highly subjective but from that angle only all these constraints have been put they have been made elaborate elaborated through various uhh different uhh criteria so that you force the designer to be within those constraints and with that thing in mind there are many tables which have been developed to help the designer in looking at or in picking up these proper uhh design parameters and the tables I would not uhh give you the tables here I will just tell you that what type of tables are available what are the various parameters which they are uhh utilizing. For example there is a table which gives the wetted diameter n meters for different under different pressure the nozzles of different diameters so for different nozzles running under different pressures you have their wetted diameter or the wetted diameter varies because if you want to uhh uhh select a wetted diameter which is which is a desirable wetted diameter you can use this table. There can be another table which discharge in liters per second the other parameters are the pressure and the nozzles of diameter as you have in the case above the table above. So for different example I can give you the values of uhh some of the nozzles of diameters the range can vary this is from 240 kPa to around 380 kPa so we have the discharges which are uhh under these conditions what will be the discharge available. Similarly the many other tables there is another table which give you the average gross application all these tables will be available in the source material the average gross application dg in centimetres per hour in this table discharge per nozzle the discharge per nozzle is known for different nozzles in liters per second and uhh these discharges vary from 0.126 liters per second to 0.631 liters per second now depending on this theta plus the sprinkler spacing which is the mean lines spacing and the literal spacing for different combinations of spacing for example this table gives this is in meters for different combinations of spacing and for different discharge per nozzle what will be the average gross application which you will get obtained and gives the information on that so you can make the selection directly whether how much gross application uhh average gross application you will get in under those circumstances when you select a particular nozzle so that way there are many such aids which have been provided for making the designers effort quite uhh minimal and uhh the designs can be can be taken up quite quickly but the fact remains that this is a very very uhh specialized job in the sense that you must understand what you are doing once you understood the thing properly then there is no problem you can uhh make the designs without any uhh any big problem. Let us take a one small example to just go through how we make use of what we have gone through so far how we actually make use of those uhh those details or those elements which we have studied so far in an actual case we will take a small very small case where we have uhh some available data and how we can make use of that available data along with the various conditions which we have imposed to come out with a relevant and proper design. The data given is quite elaborate we are assuming that we are uhh using a side roll system the climate of the region is crop period requirement 7 millimeters per day then you will need some meteorological data and the meteorological data uhh this is this pertains to the peak period and the meteorological data is given in the form of the maximum and minimum temperature the minimum temperature is 10.5 degree centigrade maximum temperature absorbed is 27.8 degree centigrade. Now this you can obtain the average temperatures over the days 19.2 degree centigrade similarly the other meteorological law variables which are available are the maximum relative humidity is 93 percent and the minimum relative humidity is 39 percent which again gives mean relative humidity of 66 percent. Then you have the wind speed at 2 meters height is given to be 6.4 kilometers per hour. Or if you want to convert that you might need it in meters per second 1.78 meters per second. Besides this basic data there are some other data which are relevant and which is which are required for the design purpose they have been taken accordingly. The total allowable depletion is 82 millimeters maximum application rate we call it as 0.76 centimeters per hour or 76 millimeters per hour and along with this you have the design constraints the design uniformity which is desirable is 85 percent and the adequacy level is 90 percent. Now these are the various uhh data available there is one more uhh segment of data which is available that the spacings like to have is dependent on what type of equipment you already have available with you. So from that angle the main line spacing should be either 15 meters or 18 meters and the lateral spacing should be 12 meters. These are very realistic uhh constraints which will be normally you will feel that they are there because of the way things are either the farmer is already having some equipment so he will not uhh with each design he would not like to buy new equipment. There might be some flexibility in terms of uhh he has two different types of uhh pipelines or one of the pieces available how he can join them. All those things are equally important. Now besides this basic data there are some constraints on the operations also. So on the operations side the constraints which are there they are the system is to the operating time 20 hours out of 24 hours. This is quite likely that out of the 24 hours the 20 hours are kept uhh for operation the remaining 4 hours are for removing the pipelines and uhh those activities you need to have some time which will be the time when the system would be running. It is also advisable to run the system, it is desirable that the system should run on operating pressures which are this will reduce the operating operation cost. So the operator wants to run the system on the minimum possible pressures operating pressures. Another constraint use single nozzle sprinkler head. Based on this data and based on all the desirable uhh constraints you have to design the system. When you say you have to design the system what you will what you will design it for the design should include the selection of nozzle diameter, what size of the nozzle should be used, what should be the operating pressure, what should be the wetted diameter, nozzle discharge is once you have found out the selected the nozzle diameter and the operating pressure of the nozzle discharge can be known but you are interested in knowing the nozzle discharge also. You are interested in finding out the gross application, the actual time of operation. We had said that we will try to confine our time within the irrigation interval but the actual time of operation can be much lower or it can be within the irrigation interval. How much will be the actual time of operation that is what we have to find out and that will be a function of what are the other parameters which have been selected. I think we will stop here because you would not be able to finish the thing within the remaining time. I will give the answers to any questions if you have, okay. So we will take up this exercise in the next class.