 Hello, I am Dr. Siddish Kumar Kashi, Professor, Department of Civil Engineering, Valshan Institute of Technology, Solapur, presenting a topic deriving unit hydrograph. Learning outcomes of this session, at the end of this session, the students will be able to explain significance of unit hydrograph for computing ordinates of stream flow hydrograph. The students will be able to calculate coordinates of unit hydrograph using given stream flow and base flow data. Now, let us see the steps for creating unit hydrograph. We create it from stream flow data. Synthetically, we also can have the SP-10 minute unit hydrograph, CINDAS hydrograph, SC-SUL hydrograph, time area unit hydrograph, Biccler, Gamma function with unit hydrograph, Fittite distributions, Geometric. But today, we will be concentrating the hydrograph based on the stream flow data. Steps in determining unit hydrograph. Unit hydrograph can be derived from a total stream flow hydrograph at a given stream gauge location along with the following information. The information necessary is the basin area, the basin average rainfall depth and the duration over which the excess precipitation has occurred. Now, see carefully the basic input for deriving unit hydrograph. The first input is the stream flow hydrograph, time and the varying flow over that. Second input is the river basin area, which is contributing to the stream flow. So, just say this is a river basin and this is a river system. We also need the basin average rainfall. That is the average rainfall in a particular storm event over basin. And we also need the information regarding the duration of excess rainfall. It means excess rainfall, which converts the rainfall into stream flow at the surface flow. It is excess over the deep infiltration and storages that one can understand. Step one is select appropriate precipitation event. Appropriate in the sense when deriving unit hydrograph is important to start with an archived hydrograph in which the quick response runoff portion is from one single storm event. Secondly, the storm should have produced its excess precipitation with nearly uniform coverage over space, that is speciality. And another, it should be also uniform over time. So, such storm we need. Second step is remove base flow contribution. Means what? This is a time, this is a flow and this is a total stream flow at a gaging station. But we know that the total stream flow always includes the base flow, that is groundwater contribution, as well as the quick response. So, we should be able to separate this quick response runoff from the base flow. And when we separate, when we are able to separate this, then this particular part will be now seen in this way. After removing the base flow from hydrograph, this will be quick response runoff only with respect to time, the flow in this way. Because this is ultimately going to matter your unit hydrograph. Step three is calculate quick response volume. The total volume of water from quick response runoff needs to be calculated. This is done by summing the areas under the QRR hydrograph for each time step. In this case, hourly time step. If you see this, so this is a surface flow. The surface flow has come over a specific period of time. So, we need to convert it into a certain depth to calculate the volume of quick response runoff. This will give you better idea. Determine the excess precipitation depth from basin. So, this is the basin area. This is the total volume excess. When this total volume excess which we have calculated, when we divide by this particular basin area equivalent to this, we will be able to get the depth. We will be able to get the equivalent uniform runoff depth for the whole basin, average over the basin. So, step four, determine excess precipitation depth for basin. So, simple formula volume by area that is average depth. So, that is why we need volume and that is why we need the basin area to find average depth. For example, assume we have a basin area of 100 square kilometer, which is equal to so many square meters and calculated volume of quick response is 2 into 10 is to 6 cubic meter. Then the depth will be volume divided by area that is 0.02 meter that is 2 centimeter. Now, adjust the quick response hydrograph. The excess precipitation depth probably won't be exactly one unit as unit hydrograph requires. Then we have to adjust it. How? So, we have to adjust the QRR hydrograph to show that the response from one unit would be. So, hydrograph unit divided by excess precipitation that will give the adjustment factor. Suppose, hydrograph unit is 1 centimeter and excess precipitation is 2 centimeter. So, the adjustment factor will be half. Now, how to use it? Adjust the quick response hydrograph. Once we multiply each point on the hydrograph by our adjustment factor here it is 0.5. Our resulting unit hydrograph is for exactly 1 centimeter of excess precipitation. It means previously we had response of 2 centimeter. By adjustment factor 0.5 we multiplied by 0.5 to get the response of 1 centimeter in excess that is the simple principle adopted here. So, determine duration of unit hydrograph. A particular rainfall event has a particular duration. So, when you to determine that the duration of unit hydrograph refers to the continuous time period during which one unit of excess rainfall or precipitation has occurred. If it took 6 hours for one unit of excess to occur we have a 6 hours unit hydrograph. In the same way if 1 centimeter rainfall excess has taken place in 3 hours we say it is 3 hours unit hydrograph. If it has taken place in 8 hours it is 8 hours unit hydrograph in that way. Remember the unit hydrograph duration does not refer to the duration of stream flow response but rather it refers to the duration of rainfall duration of precipitation. The difficult part of determining the duration of unit hydrograph is estimating which portion of entire precipitation event actually contributes to excess precipitation. Why so? Because when rainfall starts it does not give full response in form of a surface flow. But in initial stages infiltration is there. In initial stages we will also find that water is strapped on the leaves of trees it needs some time to fill in the depressions and thereafter it gives a stream flow response. That is why we need to specifically select the duration of rainfall excess. Recall that the water that infiltrates and percolates into deeper storage and base flow is not a part of excess precipitation. That is why the time of excess precipitation will be always lesser than the time of storm. We can estimate this portion of the precipitation by applying a constant loss function to the rainfall. Recall that we have already calculated the depth of excess precipitation to be 2 centimeters. Now we need to know how long it took for the excess time to occur. So we move this loss function line such that the amount of precipitation above the line is equal to the depth of excess precipitation that we already calculated for the basin. Below that line the precipitation goes to long term storage above the line it is excess precipitation. How so we will be able to see here. Now precipitation started from say 0 1 2 3 4 5 6 7 8 9. So the time of precipitation was 9 hours but actually what we find that rainfall excess started from say 2 o'clock and it exceeded up to say 8 o'clock. Why so? Because in this particular portion the rate of rainfall is lesser than the rate of infiltration that is why there is no contribution towards the surface of in this case. So here we separate the deep infiltration from the excess rainfall in terms of depth and here we separate it for a time. So here determining the duration of excess precipitation calculated excess precipitation depth is 2 centimeter. So this is the calculated depth and this part of rainfall has loss in infiltration. So excess precipitation depth is 2 centimeter which is accumulated over this particular period and these depths are accumulated and this is a constant loss function which takes into account the loss of precipitation depth due to infiltration and other things. Now when the excess precipitation bar graph of 6 hours it means the bar graph above this that we have taken here. Notice that the amounts from hour to hour of this graph are not too uniform this is typical. For purpose of calculating unit hydrograph duration however we assume that all excess precipitation occurred uniformly with a period of time. We need to assume this that's why we select a storm suitably which will give all these values reasonably same. So ultimately we arrive at the final unit hydrograph at the end of these steps we have a 6 hour unit hydrograph it means these are the 6 hour where over which we have excess precipitation it shows the steam flow response to 6 hour of excess PPT that produced one unit of depth. So reflection spot define the following the unit hydrograph the rainfall excess and tell us how to derive unit hydrograph from a cache field and a rainfall data which is given to you. These are the references used for this presentation. Thank you.