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For SOI which month had the highest (positive) correlation (copy that plot to your results). : The wetted perimeter P at H.F.L. Recall from lecture slides that a rough estimate of runoff may be estimated using the total rainfall over the catchment during a given period of time, and a runoff coefficient. Volume of water can be easily measured by knowing the catchment area and height of the rainfall. The flood discharge can be evaluated by using various empirical formulae involving area of the catchment and some coefficient depending upon the location of the catchment. Drainage area was the only sig - nificant variable in the revised regression equations. For DMI which month had the highest (negative) correlation (copy that plot to your results). The river's discharge at that location depends on the rainfall on the catchment or drainage area and the inflow or outflow of groundwater to or from the area, stream modifications such as dams and irrigation diversions, as well as evaporation and evapotranspiration from the area's land and plant surfaces. the arithmetic mean (considering only stations within the catchment boundary), the Thiessen polygon weighting system, and, the Isohyetal method. Do the same for both SOI and DMI. Content Filtrations 6. Does this match with your own rainfall station correlations (, Plot a scatter plot of discharge and annual rainfall, add a line to the chart. If you want to look at collecting from different areas of your roof, you can always hit the “Clear Area” button to start over. Rainfall data for each station in the catchment is provide in Table, After doing this exercise I have been able to conclude (, Limitations, assumptions and sources of error (. Disclaimer 9. Where Ai A i is the polygon area (km 2 ), P i P i is the rainfall (mm) corresponding to the i i polygon, and ∑Ai ∑ A i is the catchment area (km 2 ). A major hazard to drinking water safety is presented by ‘precipitation’ events (rain, snowmelt), where large Delineate the boundary of a catchment/contributing area using a Digital Elevation Model and a given point (a specific point for which we want to find the contributing/catchment area). What statistical test can we use to test the significance of the hypothesis? at certain intervals. where, R = Runoff in million m 3. (2 marks), What hypothesis can we make about changes in rainfall amount over time? 4.1.2 Catchment protection Catchment protection is the second step in providing safe drinking water and where, for whatever reasons, source choice is limited it presents a key opportunity to minimise pathogen contamination. Below is a link to a first flush volume and length calculator based on last rule of thumb L = Distance from the critical point to site of the bridge in Km. and the reduced levels of the critical point and the bridge site are 200 m and 50 m respectively. If t = one hour and corresponding i is taken as i„ and the value of I is taken from equation 3.6. The image below shows how differences in roof slope do not change this building’s catchment area. A = Area … 60 min of 10mm/h = 10 mm of rain in one hour We can the assume that we had 5 mm in the first 30 min and 5 mm in the next 30 min. What did you find and what were you expecting? Make a name to save the data (the default should be fine). Privacy Policy 8. To achieve this objective, field measurements, GIS techniq… Describe the results and conclusions of this analysis. The catchment area is the area from which rainfall is collected. The most basic measure of average rainfall depth is the arthimetic average, whereby you sum the observed rainfall at each rain gauge inside the catchment and divide by the number of gauges. Para 4.2.2 The design discharge (Q) normally shall be computed flood with a probable recurrence interval of 50 years. The terms catchment & watershed are found to be used interchangeably. (2 marks), Use the Data Analysis add-in to calculate descriptive statistics (mean annual rainfall, max annual rainfall, min annual rainfall, Standard deviation of annual rainfall, Skewness) for the whole data (you can install by going to Tools->Add-ins->Analysis ToolPack). Here we determine how rainfall correlates with known modes of climate variability (i.e. Measure and record distance shore to shore of each transect. We can do the same exercise except using gridded data (climate data divided into say 0.5 degree spacing using reanalysis information). I want to estimate the discharge at the outlet of an ungauged catchment in order to determine how much water is available for water supply. Input the data range. (5 marks), Break into small groups of about 5 and discuss the results of Table 5. Is the relationship really linear – discuss? 3.2. Rational Formula: The runoff from a catchment can be estimated by using a rational formula for small catchments as follows: R = C.A.P. Mean rainwater supply in m 3 = Mean annual rainfall in mm/year (Need to convert this value in ‘m’) X Surface area of catchment in m 2 X Run-off coefficient. Drag the Year FIELD into the ROWS box. Select Year and Rainfall amount as the FIELDS. Tc = time of concentration for the watershed (hr); time for runoff to travel from the furthest distance (by time) in the watershed to the location where you wish to determine Q p. Our calculation allows the user to divide a watershed into a maximum of five sub-regions represented by different curve numbers. To correlate the index and rainfall select “Correlate with other time series”. Calculate potential supply of rainwater from the catchment area. C = Constant. Note that our calculation allows you to use a variety of units. First you will need to register so you can save data series. This can be done using the Channel network algorithm. Use a PIVOT table again. Figure 5: A guide to the timescales applicable to weather, climate variability and climate change Pacific Climate Futures. Such surface velocity is to be determined for each section and weightage average value is obtained for the purpose of flood discharge estimation. As discussed before, the run-off depends on the following factors: (i) Degree of porosity and degree of saturation of the soil in the catchment area. Empirical Formulae 3. This article throws light upon the top six methods for estimation of flood discharge. 3.4. an arithmetic average of gauged quantities. Engineering, Designing, Highway, Bridges, Considerations, Flood Discharge from Rivers. Determine the relationships between rainfall and runoff to evaluate the nature of the relationship between rainfall and discharge. 3.1 and Fig. area, channel length, channel slope, percentage of impervious area, and percentage of the basin served by detention. (iii) Obstacles to flow such as roots of trees, bushes etc. You are given the measured data for each rainfall gauge in Table 1. Left click on the “Sum of Month” in the VALUES box and remove it. (4 marks), Next, let’s look at the distribution of the rainfall data. Estimate the maximum flood discharge by using the various empirical formulae and compare the flood discharges: This formula is applicable for Madras (Tamil Naidu) State only and as such gives low value which is not considered. Use this tool to locate a gauging station near your rainfall site. What conclusions can you draw from these results? 3.3 since the mean intensity for a small time interval, t, is more than the mean intensity for the whole time period, T. The relation between i and I may be shown as: Where C is a constant and may be taken as unity for all practical purpose. \tag{1} The area is located in Western India within 150 km. The length of the catchment is 30 Km. In such catchment, storms of shorter duration which cause the maximum flood discharge, will not reach the bridge site nearly at the same time and as such run-off in such catchment area will be less than that in a fan-like shape of catchment. of the lowest beds at 500 m upstream and 500 downstream are 107.42 m and 105JO m respectively. The calculation of the catchment: cultivated area ratio is based on the concept that the design must comply with the rule: WATER HARVESTED = EXTRA WATER REQUIRED: The amount of water harvested from the catchment area is a function of the amount of runoff created by the rainfall on the area. Percentage run-off varies from 20 per cent to 70 per cent depending upon the shape and nature of the catchment. 3.2 show two types of catch­ment. Introduction Many water-management programs require streamflow This will require the calculation of the Catchment Area, Runoff Coefficient, Location, Return Period and the Climate Change Factor. ArcGIS can obtain the total area of flowing into this specific point, also called pour point. Discharge is the volume of water moving down a stream or river per unit of time, commonly expressed in cubic feet per second or gallons per day. of India, the values of io for various places of the Indian Union are reproduced in Table 3.2: Time of concentration is defined as the time taken by the run-off to reach the bridge site from the furthest point of the catchment which is termed as the critical point. Once you have the square footage number, enter it into the rainwater collection calculator above. This is the maximum rate at which water will be allowed to discharge from the system. Does rainfall correlate with known modes of climate variability (i.e. IOD and ENSO using the Southern Oscillation Index (SOI)). How “useful” are these statistics for your understanding of the system? Insert PIVOT table (select OK to create a new tab). Other indicators of temporal variability (look for cyclic patterns). Describe the values you got and what they mean. The previous exercise focussed on estimating rainfall and runoff at catchment scales during a storm event. Introduction Many water-management programs require streamflow IOD is calculated by anomalous Sea Surface Temperature gradient between the western equatorial Indian Ocean (50E-70E and 10S-10N) and the south eastern equatorial Indian Ocean (90E-110E and 10S-0N) in units of degrees Kelvin. Km. What areas of Australia have significant correlations and what time of year? To calculate how … The critical intensity of rainfall, Ic, corresponding to the concentration time, Tc, is derived from equation 3.9 considering I = Ic corresponding to T = Tc. But in a stream flowing through alluvium region, the cross-sectional area and the perimeter may change during highest floods due to the scouring of the banks and the bed and as such in estimating the maximum flood discharge, the depth of scour has to be ascertained first and the values of the cross-sectional area and the perimeter may then be calculated by taking levels of the bed at certain intervals. Drainage area was the only sig - nificant variable in the revised regression equations. Rainwater Collection Calculation Formulas and Equations. ... Q = 0.028 PfIcA = 0.028 x 0.10 x 0.60 x 2.98 x 80,000 = 400 cum/sec. type BROOME in the “Select stations” section), and then press “Get stations”. Are they consistent across WA? From the daily record of rainfall, annual rainfall for a zone is determined. The calculation algorithm used may not result in a valid catchment for each selected discharge point. Table 3.4 gives the value of f in equation 3.13 derived from equation 3.12 with the introduction of the coefficient, f, therein. Ensure the station has 10-20 years of available data for this exercise. We then look at how rainfall may be changing over time and how it varies over space across Western Australia. The best way to calculate discharge of a river is to measure it. The time taken by the float to cover a fixed distance is noted by a stop watch and the distance travelled by the float divided by the time taken is the surface velocity of the stream. Cross Sectional Area and Bed Slope 5. In this exercise you are required to calculate the average rainfall (ADR) over the entire catchment using: You are then required to produce an estimate of catchment runoff, which we will assume is equal to the stream discharge out of the catchment ($$Q$$). What variability can you see? The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. The annual rainfall varies from place to place and therefore, the recorded rainfall for a considerable period, say fifty years, is very useful in getting the maximum rainfall recorded during this period. Select the required data across multiple columns (year C to rainfall amount F). (8 marks). Is this long enough for a reliable trend? The methods are: 1. For this we use simulation models. 3.6). The terms “100 years’ value” and “50 years’ value” are defined as momentary peak discharge which occur “on the average” once in 100 years or once in 50 years. iii. where, R = Runoff in million m 3. Content Guidelines 2. A … Based on the triangular approximation, the unit hydrograph peak discharge up (that is, the peak discharge resulting from an amount of excess rainfall Q = 1 unit applied uniformly over the catchment within a specified rainfall duration D) from a catchment of area A is found as. C = Constant. These length may be worked out as below (See Fig. \text{SOI} = 10 \frac{P_{\text{diff}} - P_{\text{diffav}}}{SD(P_{\text{diff}})} What areas of Australia have significant correlations and what time of year? Roof Area (m2) X Precipitation Amount (mm) = Amount Collected (liters) We will examine these during this exercise. Estimation of Design discharge (Q) (Para 4.2 S.S.C) Para 4.2.1 The estimation of design discharge for waterway shall preferably be based, wherever possible, on procedures evolved from actual hydro meteorological observations of the same or similar catchments.”. Calculate the arithmetic mean and write this in your summary table (Table 3). Before publishing your articles on this site, please read the following pages: 1. Three approaches are currently used for estimating the total rainfall: In this exercise you will compare these three different approaches for calculating the ADR and runoff generated by a single rainfall event. The analyses resulted in a relation that can be used for estimating storm peak discharge based on drainage area, basin-average rainfall, and impervious area… The catchment area is computed from the contour map and the flood discharge is estimated from the                              “Run-off ” formula. In order for this command to produce a catchment area, the surface triangle edges must form a channel that slopes toward the discharge point. Another important factor which determines the percentage of run-off is the shape of the catchment. The estimation of maximum flood discharge shall be based on this maximum recorded rainfall. Note any trend and variability in the index. Figure 4: Isohyets on catchment with inter-isohyetal areas. Here you can easily find how much rainwater you can collect from you roof, how much you need to remove from a courtyard, or how much runoff you can expect from an area of land. Find out the peak storm discharge by the Rational Method assuming that the rainfall in 5 hours is 20 cm. What factors may influence it (hint, think about earth orbit)? \], $$$A calculator with the option ‘y to the power x’ (y x) Preferably the IDF-curves (intensity-duration-frequency curves) of the zone studied Analysis of the catchment area First the catchment area with its boundaries will have to be identified on the map. Area based rule-of-thumb: 0.41 liters for every meter squared of roof [verification needed] OR 10 gals for every 1,000 foot squared of roof . The river's discharge at that location depends on the rainfall on the catchment or drainage area and the inflow or outflow of groundwater to or from the area, stream modifications such as dams and irrigation diversions, as well as evaporation and evapotranspiration from the area's land and plant surfaces. ; determine the percentage run-off. As the catchment area gets larger and larger, the possibility of reaching the run-off to the bridge site simultaneously from all parts of the catchment is less and less and as such the value of f is gradually reduced as the catchment area is increased. Throughout the exercise we should remember that rainfall will be influenced by a number of processes that operate on different space and time scales (see Figure 5). It is also called catchment or drainage area. We then want to correlate each grid cell on the earth with our climate index (SOI and DMI) to see how the correlations very spatially. A runoff model is a mathematical model describing the rainfall–runoff relations of a rainfall catchment area, drainage basin or watershed.More precisely, it produces a surface runoff hydrograph in response to a rainfall event, represented by and input as a hyetograph.In other words, the model calculates the conversion of rainfall into runoff. What if any assumptions are there about these statistics? First Approach The general form of the empirical formulae Q = CAn where Q = flood discharge, A = catchment area, C = flood coefficient and n = flood index Both C and n depend upon various factors, like Size, Shape, Location and Topography of catchment Intensity, Duration and Distribution pattern of the storm (continued) 7. Rational Method: If R is the total rainfall in cm for a duration of T hours then the mean intensity of … Choose a rapporteur to report back at the end of the discussion. In general, river discharge is computed by multiplying the area of water in a channel cross section by the average velocity of the water in that cross section: discharge = area … Since discharge is only defined in streams or rivers, water balance calculations are often performed for the catchment area upstream of the discharge measurement site, toward which it is often assumed to drain entirely. Choose a new rapporteur to report back at the end of the discussion. 3.1 and Fig. iii. You will then be emailed a zip file of the station’s data. What relationships do you see between rainfall and climate mode and why? Hence, even if the catchment area, quantity, dura­tion of rainfall etc. Note that our calculation allows you to use a variety of units. Add the site to your cart and submit a data request - check that the data is being delivered as a “package of pre-computed reports”. Do this by plotting a histogram of the rainfall data (you will need to decide on a reasonable bin size) and plotting the cumulative probability distribution (you can ask Excel to generate this automatically when it makes the histogram, or generate it manually – each data point represents a probability of 1/#data points). A watershed is all the land & water which contributes runoff to a common point or outlet point. m per hour. Next work out the ADR based on the other two methods. If you are using the roof of a building for this purpose, calculate the square footage of this area by taking the length of the building, including the overhang(s) and multiply this by the width measurement plus the overhang(s) of the building. is considered then the run-off is given by : Where P = Coefficient depending on the porosity of soil, vegetation cover, initial state of saturation of soil etc. ArcGIS can obtain the total area of flowing into this specific point, also called pour point. For a small time interval, t, the intensity of rainfall, i, may be more as may be evident from Fig. Calculate the appropriate t-value and p-value and enter them also into Table 5. The followings are the methods used to calculate discharge of a catchment area using rainfall data: Runoff Computation Methods. Highlight the station where there is a statistically significant slope. This paper has studied the estimation of peak flood discharge of the Kunur River Basin, a major tributary of the Ajay River in the lower Gangetic plain. (viii) Total quantity of rainfall in the catchment area. Select “Line Fit Plots”. Click on “Monthly Station Data” (right hand menu), select “precipitation” from the “GHCN-M (all)” column, search for your station (e.g. The catchment area is F = 3200 km2, the average annual discharge in the year of 2004 is Q = 85 m3/s, the average annual precipitation P = 1245 mm, try to calculate the following value of this catchment in 2004. The average depth of rainfall (ADR) over the entire catchment can be calculated using: ADR (mm) = ∑Ai ∗P i ∑Ai (1) (1) A D R ( m m) = ∑ A i ∗ P i ∑ A i. Then insert PIVOT table (Create a new tab). If land use and soil cover are homogeneous over the drainage area, a … You’ll need to choose an appropriate runoff coefficient, and calculate the stream discharge at the bottom of the catchment - rember to watch out for unit conversions! Add a trend line and “display the equation” on the chart. In case of catchment area with steep but wooded rock, P from table 3.3 = 0.80. Start by downloading the daily rainfall data for your site from the, Break into small groups of about 5 and discuss the results of your correlations. Some values of the rugosity co-efficient, n, are given in table below for various types of surface conditions. Many factors affect the extent to which the measured rainfall amount at one rain-gauge can be extended to Comparison of flood discharges worked out by various empiral formulae: If R is the total rainfall in cm for a duration of T hours then the mean intensity of rainfall, I in cm per hour taken over the total duration of the storm is given by. Download the answer sheet for this exercise by clicking the download button in the tool bar . Area of cross-section A at H.F.L. How do we frame the hypothesis? When the catchment area is of clayey soil lightly covered, P from table 3.3 = 0.50, values of A, f and Ic remaining as before. \tag{2} the surrounding area: Most hydrological analysis at the catchment scale requires knowledge of the areal distribution of precipitation in the form of an average depth (mm) of water for the entire catchment over a specified time period, using information from a set of unevenly placed rain-gauges spread around the area. Example – Rooftop catchment area made of concrete; Mean annual rainfall in = 700 mm/year (0.7m) Surface area of catchment = 2629 m 2 (See Figure 2) age area to depend on the group. We want to ensure that there are only whole years to calculate the sums or else they will be in error. ... Q = 0.028 PfIcA = 0.028 x 0.80 x 0.60 x 2.98 x 80,000 = 3204 cum/sec. Are the data normally distributed? What may be causing the changes? are the same for both types of catchment, the run-off at the bridge site will be more for fan shaped catchment than for normal single catchment. (, Break into small groups of about 5 and discuss the results of your plots. The expo-nents for the drainage-area ratio were 0.85 for the winter season, 0.91 for the spring season, and 1.02 for the summer season. And tick your previously saved rainfall timeseries and then press “Correlate” and copy the results into Table 6. the age of the record, since the weirs or the barrages are mostly of recent construction. the nature of the rainfall quantity (storm event, daily, monthly). Table 3.1 gives the rainfall record in different parts of the Indian Union for a period of 15 years (1935-1949). This can be done accurately by setting up two ropes stretching across the stream, 5 meters apart. Break into small groups of about 5 and discuss the results of your plots. Fig. The Rational Method is, therefore, very realistic and considers all relevant factors which regulate the peak run-off. Select Month and Rainfall amount as the FIELDS; drag the Month FIELD into the ROWS box. (ii) The shape and slope of the catchment area. So, watershed area is the land area, from where all the surface runoff converges to a single point to give common discharge. Isoheytal map corresponds to rainfall total over a 24hr period. By this method the discharge is calculated from Manning’s formula, Where A = the area of cross section of the stream measured from H.F.L, R = the hydraulic mean depth and equal to the ratio of cross-sectional area, to wetted perimeter, P. S = the bed slope of the stream measured over a reasonably long distance. A watershed is all the land & water which contributes runoff to a common point or outlet point. The catchment area (also known as flow accumulation), can be used to set a threshold for channel initiation. If you are using the roof of a building for this purpose, calculate the square footage of this area by taking the length of the building, including the overhang (s) and multiply this by the width measurement plus the overhang (s) of the building. What will be the peak discharge if the catchment area is of clayey soil lightly covered or of steep but wooded rock? Lastly, we look at the relationships between rainfall and runoff.$$$. Copyright 10. The velocity in this case is determined at site by direct measurement of the velocity in place of theoretical calculation from bed slope etc. (. Highlight any months that have a significant p-value (<0.05). The area is based on the “footprint” of the roof, which can be calculated by finding the area of the building and adding the area of the roof’s overhang. SOI is calculated as follows. (2 marks), Calculate the trend in rainfall for the last 5 years. What are the main features of this annual data set? If losses due to absorption etc. Add the information for your site into Table 5 (and insert the other class data when you can). Next the Attenuation outflow must be specified. (we need that as our unit hydrograph is per 30 min) What do these plots tell you about the rainfall data? One centimeter of rainfall over an area of one hectare gives a run- off of 100 cu. \tag{3} The expo-nents for the drainage-area ratio were 0.85 for the winter season, 0.91 for the spring season, and 1.02 for the summer season. How the rain water reaches the channel or the river from the catchment area is shown in Fig. Do this now and briefly summarise the aims of this exercise, your conclusions after doing this analysis, the limitations of your approach, and any recommendations. Q = Peak discharge, cfs c = Rational method runoff coefficient i = Rainfall intensity, inch/hour A = Drainage area, acre. The average depth of rainfall over the entire catchment can be calculated using: where in this method $$A_{i}$$ is the area between isohyets $$P_{i}$$ and $$P_{i+1}$$ (km2), $$P_{i}$$ is the average rainfall (mm) corresponding to the area $$A_{i}$$ between the isohyets, and $$\sum{A_{i}}$$ is the total catchment area (km2). Plot the annual sums as a timeseries. and is composed of sandy soil with thick vegetation cover. The general procedure for determining peak discharge with the Rational Formula is: Step 1 Determine the drainage area. It is advisable to consider a number of heavy storms spread over a prolonged period and io may be calculated for each case and the maximum value of U shall be taken as the one hour rainfall of the region for the estimation of flood discharge. Rational Formula: The runoff from a catchment can be estimated by using a rational formula for small catchments as follows: R = C.A.P. 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