# discharge calculation from catchment area

Q = Peak discharge, cfs c = Rational method runoff coefficient i = Rainfall intensity, inch/hour A = Drainage area, acre. Volume of water can be easily measured by knowing the catchment area and height of the rainfall. Plot the annual sums as a timeseries. Determine the relationships between rainfall and runoff to evaluate the nature of the relationship between rainfall and discharge. Are there statistically significant changes in rainfall? Figure 2: Catchment map, station locations, and isohyetal map for a storm event. Q = Peak discharge, cfs c = Rational method runoff coefficient i = Rainfall intensity, inch/hour A = Drainage area, acre. (2 marks), Calculate the trend in rainfall for the last 5 years. VVR111 – Hydrology and Aquatic Ecology. 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. Lastly, we look at the relationships between rainfall and runoff. 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. You should always go back to your original objectives (or aims) and see what you have learnt from your analysis. 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. The area measurement will appear right above the map. Then insert PIVOT table (Create a new tab). These length may be worked out as below (See Fig. The velocity is least in the vicinity of the bed and banks and mean at the centre line of the stream at a point 0.3 d below the surface where, d, is the depth of water (see Fig. Once you have the square footage number, enter it into the rainwater collection calculator above. The catchment area is the command area of a river wherefrom the river gets the supply of water. Add the Pearson correlation coefficient (r2) value as well. In case of catchment area with steep but wooded rock, P from table 3.3 = 0.80. Are they linear or non- linear and why? 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 . This means that the hydrological boundaries of the catchment are assumed to correspond to the topographic watershed boundaries. 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. Calculate potential supply of rainwater from the catchment area. For this we use simulation models. \mathrm{ADR\ (mm)\ =\ }\frac{\sum{A_i\ast P_i}}{\sum A_i} 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. After that we explore how different modes of climate variability affect annual rainfall. Note that our calculation allows you to use a variety of units. From the daily record of rainfall, annual rainfall for a zone is determined. You will then be emailed a zip file of the station’s data. (2 marks), What hypothesis can we make about changes in rainfall amount over time? Engineering, Designing, Highway, Bridges, Considerations, Flood Discharge from Rivers. Clean up the plot as required and submit on your answer sheet (1 mark). Highlight any months that have a significant p-value (<0.05). Click on “Monthly Station Data” (right hand menu), select “precipitation” from the “GHCN-M (all)” column, search for your station (e.g. Drainage area was the only sig - nificant variable in the revised regression equations. 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 … C = Constant. C = Constant. What if any assumptions are there about these statistics? \tag{2} 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”. This will require the calculation of the Catchment Area, Runoff Coefficient, Location, Return Period and the Climate Change Factor. Catchment-Run-Off Method 2. Is this long enough for a reliable trend? 3.6). Plagiarism Prevention 4. A watershed is all the land & water which contributes runoff to a common point or outlet point. 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. 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. Hence, even if the catchment area, quantity, dura­tion of rainfall etc. Km. Describe the values you got and what they mean. \tag{3} 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. 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. The followings are the methods used to calculate discharge of a catchment area using rainfall data: Runoff Computation Methods. 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. Prohibited Content 3. Clean up the plot as required and submit on your answer sheet (1 mark). may be found out by dividing the area into strips such as BPC, PCDO, ODEN etc. 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. 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. Select “Line Fit Plots”. Cross Sectional Area and Bed Slope 5. 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. the arithmetic mean (considering only stations within the catchment boundary), the Thiessen polygon weighting system, and, the Isohyetal method. Calculate average by … You will then see graphs and options for further integrating the data. This value may be compared with the theoretical worked out value and a final value may be selected. SOI is calculated as follows. The best way to calculate discharge of a river is to measure it. The methods are: 1. Therefore, it may be noted from the illustrative example that the peak run-off is very much dependent on the nature of the catchment, other factors remaining the same and varies from 400 cum/sec to 3204 cum/sec when the degree of porosity and absorption of the catchment area is very high or very low. A = Area … In the present case for catchment area composed of sandy soil with thick vegetation, A = 800 sq.km = 80,000 hectares ; P from table 3.3 = 0.10 ; f from table 3.4 = 0.60 ; Ic = 2.98 cm/hour. The estimation of maximum flood discharge shall be based on this maximum recorded rainfall. (viii) Total quantity of rainfall in the catchment area. where, R = Runoff in million m 3. Below is a link to a first flush volume and length calculator based on last rule of thumb Ensure the station has 10-20 years of available data for this exercise. 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). Introduction Many water-management programs require streamflow The catchment area (also known as flow accumulation), can be used to set a threshold for channel initiation. 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. To correlate the index and rainfall select “Correlate with other time series”. 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.”. Go the Department of Water web site. The catchment area of the stream or river upstream of the bridge site is obtained by marking the ridge line of the contour map and measuring the area enclosed by this ridge line with the help of a plan meter or tracing paper graphs. Examine the relationship between annual rainfall and annual Runoff. ArcGIS can obtain the total area of flowing into this specific point, also called pour point. The length of the catchment is 30 Km. \end{equation}\]. What areas of Australia have significant correlations and what time of year? The calculations are not exact but this volume of rainfall calculator can be used to easily detect the total amount of rainfall you can collect from the roof top, from the courtyard as well you can find the total run off over a particular land area. The R.L. The phrase “on the average” means all the peak discharges as observed over a period of 100 years or 50 years as the case may be and average of the peaks is taken. A major hazard to drinking water safety is presented by ‘precipitation’ events (rain, snowmelt), where large Para 4.2.2 The design discharge (Q) normally shall be computed flood with a probable recurrence interval of 50 years. 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. Area of Cross-Section and Velocity As Observed At Bridge Site 6. Access the rainfall from the previous site that you used in the previous section. Download the answer sheet for this exercise by clicking the download button in the tool bar . The values of P for various conditions of the catchment area arc given in Table 3.3: In addition to the coefficient, P, another coefficient, f, is introduced in the formula for calculating the run-off. In order to calculate the flow discharge produced by a large storm (convective type) in the catchment in, it is necessary to calculate the average depth of rainfall (ADR) over the catchment. If t = one hour and corresponding i is taken as i„ and the value of I is taken from equation 3.6. 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 ). Go to “Monthly climate indices” and then select DMI. Individually (make sure you have at least one person per station but you can have more than one if needed). Select the required data across multiple columns (year C to rainfall amount F). the nature of the rainfall quantity (storm event, daily, monthly). The catchment area is the area from which rainfall is collected. ... Q = 0.028 PfIcA = 0.028 x 0.10 x 0.60 x 2.98 x 80,000 = 400 cum/sec. This formula (originally devised for Northern India but can now be used in most of the states of India with the modification of the value of the coefficient C) is given by: The area of a catchment is 800 sq.km. Other indicators of temporal variability (look for cyclic patterns). The formula is used to calculate the runoff for each month and then the values are added to arrive at the annual runoff. 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. Step 2 Determine the runoff coefficient, C, for the type of soil/cover in the drainage area. where, R = Runoff in million m 3. What variability can you see? You are given the measured data for each rainfall gauge in Table 1. The previous exercise focussed on estimating rainfall and runoff at catchment scales during a storm event. To annotate this page, make sure you do so in the ‘Hydrology Class 2020’ group - you can join it here. 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. Choose a new rapporteur to report back at the end of the discussion. 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. is the bed line BCDEFGHI which is the summation of the length of line BC, CD, DE etc. 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.”. (5 marks). Clean up the plot as required and submit on your answer sheet (2 marks). area = 254900000 m2area =54.9 km Time (min) Discharge (m3/s) 0 0 15 4.5 30 10 45 12.5 60 11 75 9 90 6.5 105 4 120 2.5 135 1 150 0 a) What is the size of the catchment area? the distance from the rain-gauge to the centre of the representative area. A watershed is all the land & water which contributes runoff to a common point or outlet point. Copyright 10. Break into small groups of about 5 and discuss the results of your plots. The specified location results in a catchment with no area . 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. Once we have computed ADR, we can now estimate the likely amount of runoff. One centimeter of rainfall over an area of one hectare gives a run- off of 100 cu. Some values of the rugosity co-efficient, n, are given in table below for various types of surface conditions. 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. the characteristics of the local storm patterns (convective versus low intensity events). After the determination of the mean velocity of the stream, the flood discharge is obtained by; In some cases it may be possible to have the maximum flood discharge measured at weir or barrage sites. Many factors affect the extent to which the measured rainfall amount at one rain-gauge can be extended to To achieve this, we use a large number of weather stations across WA to quantify the variability of rainfall in space and time. For DMI which month had the highest (negative) correlation (copy that plot to your results). Click on “Correlate with a time series” and choose either SOI or DMI. Know how different modes of climate variability such as IOD and ENSO affect annual rainfall. If V, is the velocity at surface, Vb is the velocity at bottom and Vm is the mean velocity then their relationship may be established in the following equation. (, Break into small groups of about 5 and discuss the results of your plots. The area is located in Western India within 150 km. 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