e. The 5 year-storm is a storm that has a 20% probability of occurring in a given year, for a given area, and the 100-year storm is a storm that has a 1% probability of occurring in a given year, for a given area. However, the drainage network appears in raster format. Drainage Area - The watershed runoff area or surface runoff area in the case of a parking lot. Septic drainfield pipe diameters: The usual minimum septic drainfield pipe inside diameter is 4". And yes, there used to be tables correlating rainfall rates and roof sizes to gutter sizes and pipe sizes. 36; or 12 drains required. The 100-year design storm covers the entire basin with uniform constant rainfall intensity until the design dis-charge at the crossing is achieved. The modified equation is: Q = (C x I x A) / 96. 40. For this reservoir, the drainage area is (πx10002) = 3 141 593 ft 2. Our online tools have been designed with simplicity in mind removing the need for any prior specialist industry knowledge. 3. q w = Well Flow Rate. Generally, drainage systems will be designed to handle the 5-year storm at a minimum, and the 100-year storm at the maximum. gov/Watershed calculation using ArcGis In these cases, the flow per unit area of the statistics for the streamgage would be multiplied by the drainage area for the selected site to estimate the statistics for the selected site. The two methods give different answers for drainage area. or . Example - 5: Vertical Offset. Mar 7, 2018 · Here's my stats for anyone who wants to confirm the calculations: pond will be . Use Clip Feature. Using a 10,000 sq. 623. A t is the floor area, and the answer you're trying to reach. Jan 30, 2024 · Enter the path's length and width as 5 m and 1 m, respectively. Find the dimensionless radius r De from Equation 9-5: Step 4. Example: Rational Method used to calculate 100-year design storm. of roof area is directed onto 3,000 sq. Line Density window appears, select Input Polyline Features ( Drainage ). The flow rate values in the example above are about 1 to 2 gpm higher than what I calculate with Drainage Density (Dd) = Stream Length / Basin Area . 5:1 bevel the factor is 1 in. • Rainfall intensity is the same over the entire drainage area. 78 inches per hour. The tool will then give you the volume of excavation you need for your French drain system. At zoom level 8 or greater State/Region selection will be enabled. Step 2: You have zoomed in sufficiently to select a state or regional study area. 78) (14. The measurement of drainage density provides a hydrologist or geomorphologist with a useful numerical measure of landscape dissection and runoff potential. Then scroll all the way down and hit Continue to calculate selected basin characteristics 12. Hydrology Tools download DEM data from https://earthexplorer. This method is less accurate or predictable as the percent impervious surface area Aug 16, 2015 · Do you want to learn how to measure the area and perimeter of a watershed using ArcGIS software? Watch this video tutorial and follow the steps to create your own map and analyze the spatial Generally there are two methods by which the quantity of storm-water is calculated: 1. 849 for metric or 1. This report should contain the various drainage areas that were used A = Drainage area, hectares, ha (acres) K u = Units conversion factor equal to 360 (1. Remember when making your selections that Roof Sep 12, 2023 · A drainage area defines the geographic extent of the land surface that drains to a specific point (the outlet). Where: D b = Drainage Density L = Total Length of all Stream Channel A = Area of Basin. 23, where, Q is the storm-water run-off in gallons per minute (GPM) C is the runoff coefficient, I is the rainfall intensity in inches per hour. The result is the number of drains needed. 318 if using imperial units, C is the roughness coefficient of the pipe material, R is the hydraulic radius (cross-sectional area divided by perimeter), and S is the slope of the pipe. I need to calculate for every basin/sub-basin, the drainage area at every elevation contour along the stream. (S:\Tutorials\Tufts\Tutorial Data\Watershed Delineation by Pour Point) Note: if your area is fairly flat, the watershed area delineation will not be accurate. In rainwater catchment, our job is to catch that rain and use it with purpose. The 5 steps you need to find out the number of roof drains needed for your commercial roofing project. 1- Area of an example stream crossing watershed (A) = 100 acres. A T = Total Area of Field. Buffer trade areas create a buffer area around the locations that are of interest to you. 75 m³ of gravel, which weighs 1. the drainage area of sampling sites. The calculated area is only an estimation. It is a measure of the rate at which water Similarly for a 2,000 ft long well, a = (L/2) +745, 1,745 ft. Mar 2, 2020 · 4) In an irregularly-shaped drainage area, a part of the area that has a short time of concentration (T c) may cause a greater runoff rate (Q) at the intake or other design point) than the runoff rate calculated for the entire area. Inlet Calculations. Distance is the drain length. The openings to drainage inlets are typically covered by a grate or any other perforated surface to protect pedestrians. Mar 17, 2015 · For example - attached image shows the point that was digitized as a destination for upstream drainage area calculation. 6, or 33 drains required. where. " The drain outlet should be sized large enough so that it will safely pass the maximum flow through the grate, without creating water buildup. 0, Manning Equation: d = [ 3. Stormwater Studio has the ability to design and analyze a system of HEC-22 style inlets along with your underground pipe system. Performing the calculation necessary for the design and installation of a drainage field just got a lot easier with this simple but powerful excel size calculator. Calculate the drainage radius re: Step 2. In this calculator, you will learn how Manning's equation works and how to use it to determine water flow. The catchment where the selected point of interest falls in is split in two and the sub area upstream is isolated from the remaining area; Box 4. roof area, the 3 inch rainfall will produce 18,700 gallons in one hour at the rate of 312 gallons per minute. ft. The drainage basins are created by locating the pour points at the edges of the analysis window (where water would pour out of the raster), as well as sinks, then identifying the contributing area above each pour point. Input the daily flow of wastewater (in gallons per day) and the absorption rate (in gallons per day per square foot) into the respective fields. = Definition. Ridges and hills that separate two watersheds are called the drainage divide. The tools guide the user step by step allowing fast and simple roof drainage design whilst ensuring full compliance with BS EN12056-3 rainwater drainage design standard. S w = Water Saturation which is the volume fraction of the porosity that is filled with interstitial water. 2. Than the field calcultor expression was: A watershed is the upslope area that contributes flow—generally water—to a common outlet as concentrated drainage. To run through the vectorization process, follow the steps 4. See the references at the bottom of the page for more complete tables including impact of slope. 2- Runoff coefficient (C) = 0. Dec 12, 2023 · Help control runoff and promote the natural movement of water. Example: The location of Kansas City, MO shows 6" of rainfall per hour. q r = Field Flow Rate. The tool that implements this method of estimation was not available at the time of the Apr 12, 2020 · Calculate the catchment area of river using Google Earth Pro Roofing Material Calculator. This is also a nice time to enter the filter fabric overlap length if you plan to use a filter fabric for your French drain. Reply: Excellent Jump to the final step, calculate Drainage Density in ArcGIS. 0104 × A × i (ASCE 7-16 Equation C8. t pss is time to reach pseudo-steady state in hours Feb 3, 2024 · How to Use. Population Field selects Grid_code, then choose Area Jan 19, 2022 · where A is the well's drainage area in square feet and the radius is in feet. The catchment area is the area which intercepts the rain. Line Level: https://amzn. Consult a Water Table Management Professional for design criteria information. Figure 1. 1) Find the drainage area in a stormwater management report. Jul 10, 2024 · This open channel flow calculator will help you find water flow velocity and volumetric flow rate through an open channel, given the channel's roughness coefficient, slope, and cross-sectional area. Drainage area is an alternate term for drainage basin. May 9, 2017 · Watershed calculation using ArcGis. This results in a raster of drainage basins. Instantly, the calculator tells you that you require 0. It has built-in capability to analyze. A = drainage area (in acres) Assumptions: 1. The calculator shows your excavation area is 5 m². The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. These are most commonly based on a distance from the POI, but they can be adjusted in a number of ways. An alternative method using the watershed tool: https://support. Calculate the gas cone ratio δ g from Equation 9-7: The Roof Drain Calculator below is based on the maximum amount of rainfall in inches per hour of the hypothetical "worst storm in the past 100 years" and which has a two percent chance of occurring in any given year. ) x (1 ft / 12 in) x (1 hr / 60 min) = 0. The impervious surfaces equaled 25 ft in length and 40 ft in width. ft × 1 inch/hour × 0. 0 in English Units) Assumptions inherent in the Rational formula are as follows:(6) • Peak flow occurs when the entire watershed is contributing to the flow. The watershed consists of surface water --lakes, streams, reservoirs, and wetlands --and all the underlying groundwater. (2. So even one WC DFU is 4, pipe will be 3 in and not 2 in. esri. A stormwater management report is an engineering report that is usually required for a land development project to be approved by the local municipality. Flatter slopes results in slower runoff rates and provide more time for rainfall to infiltrate into the soil. Now, if you have a drain that can handle 500 GPM, you would need: Number of Drains = Total Capacity / Drain Capacity = 4,500 GPM / 500 GPM = 9 drains. You will use the same the stream gage data point and DEM that you used in Tutorial 1 - ArcGIS Basics for Water Resources Mapping. 0103896. So the peak runoff is: Q = (0. A particular pipe size on a given grade will only carry a certain amount of water. To create the input D8 flow direction raster, the Flow Direction tool must be run Calculate primary and secondary drain size and the resulting rain load, R. Step Length = Distance / Step Number. On a highly permeable landscape, with small potential for runoff, drainage densities are sometimes less than 1 kilometer per square Sep 23, 2019 · Fractional drainage area, E 1 , of well 1 vs. The rational method equation is: Q = C x I x A/96. INTRODUCTION Drainage area is an important basin characteristic in studies of surface water. (Table 2. rate ratios, q 1 ,q 2 Drainage area of Well 1 as a function of flow rate-ratios is obtained by; Figures - uploaded by Samir Prasun Author content A = Area of Drainage Zone (in acres) The equation above can be modified to give you runoff in gallons per minute. In cases where a roof has a complex shape, such as in the image to the right, measuring the dimensions and areas of each part of the roof to calculate the total area will result in a more accurate measurement of area. Drainage area calculations are used in several water quality models and indices. ) Estimate the roof drain outlet size you will likely use. Your selection will dictate the data used to perform basin delineation and flow statistics calculation. Thus. Below, we cover the top three methods for calculating catchment area: Method 1. This method is less accurate or predictable as the percent impervious surface area Dec 25, 2023 · 🌐 Welcome to our YouTube channel! In this comprehensive tutorial, we'll guide you through the process of creating a detailed Drainage Density Map using ArcG These calculations are based on the Manning’s Roughness ASAE EP 260. The 5-year Sep 13, 2023 · The drainage network is generated for the study area according to the level of detail specified during the calculation in the raster calculator. 54. Rain falls. Calculate the Total Rock Needed: Total Landscaping Rock Needed (in cubic yards) = (Total Area x Depth) / 27 Total Landscaping Rock Needed = (300 square feet x 2 inches) / 27. Sep 29, 2020 · To calculate drainage density, two essential parameters are needed and these parameters are Total length of all stream channel (L) and Area of basin (A). Calculate drainage area by multiplying the total number of dots by the conversion factor. 40) (1. ) Determine your building’s location on the above map to determine the approximate rainfall as measured in inches per hour. Drainage Intensity represents the steady state drainage rate when the water table midway between parallel drains is at the soil surface. For example: 50,000 / 4,400 = 11. For example, if 5,000 sq. Drainage Density is performed using Digital Elevation Model (DEM). Example: For a 4" hourly rainfall and 4" leader, each drain can handle 4,600 sq. Jan 9, 2011 · Step 5. to/2PwovJ4When installing drainage pipe in your yard for a downspout or sump pump discharge proper planning can save you time and m In order to determine the carrying capacity of the proposed Drain, the Manning’s Formula has been used. Runoff rates are less sensitive to moderate variations in slope. 9 = 4,500 GPM. Example: 150,000 sq. Enter the Diameter of the pipe (inches): Enter the Grade (%): %. 3 Plastic Tubing Drainage Chart and should be used for estimating purposes only. 3), and solving for t, we get. 5 times the The Prinsco Drainage Calculator estimates the capacity of tile drainage systems. 8) and Tv (E. . Then, to determine the amount of runoff in gallons, multiply the total area in square feet by . Expansion of the ideal well model . The term "drainage area" is defined as the land area where precipitation falls off into creeks, streams, rivers, lakes, and reservoirs. Up-stream navigation is performed on the centerline network and the program calls drainage catchments that have a 1:1 relation to the NHD reaches; Box 3. Note: Remember to convert the depth from inches to feet (divide by 12) before using the formula. 87 gallons per hour/sq. divided by the 4,600 sq. For a vertical well located at the center of a drainage circle, the time to reach pseudo-steady state is t DA = 0. Step 1: Use the map or the search tool to identify an area of interest. Let’s solve an example; Find the well drainage area when the total area of field is 11, well flow rate is 9 and the field flow rate is 6. Jan 4, 2018 · A = Drainage Area, Units: acres (1 acre = 43,560 sq. Using this value in Equation 12 the obtained area is 3 074 103 ft 2 which is a reasonable value for this case. Then we multiple the area by . Step 4: Divide total roof area by area found in Step 3 to obtain the number of drains required. 3 days ago · First use the Hazen-Williams equation to find the velocity of the fluid: v = k × C × R 0. Depth of water (in 1 hr): 3. Dec 2, 2015 · There are several method that can be used to calculate drainage, but the rational method is probably the easiest and most widely used method. 30 (loam woodland soil, from Table A-1) May 31, 2022 · Calculate Drainage Density and Prepare Drainage Density map using ArcGIS. 1. 0104 is calculated from a string of conversion factors: (7. Take the roof’s total square footage and divide by the total square footage handled by one drain. The side‐bevel dimensions are determined by multiplying the width of the culvert by a factor. Now, on the lower left select Continue button 10. 6/24/2014 David Howard, AIA said: What is the general area of a flat (1/4"/1'-0" slope) single-ply roof for each roof drain (ratio for the number of roof drains / roof area). 013 m, Hydraulic gradient of the Drain i, mean slope of the Drain, 3. 26 tonnes. Primary Drain. 8. Conducting a percolation test is the first step, this calculator guides you through the steps from the start, leaving you with a design to follow. 55 % Carrying capacity of the Drain, How to calculate the quantity of drains required: ) Calculate the roof area to be drained. In simplest terms, it is an area of land where water from natural sources, such as melting snow or rain or melting ice, pool and join another existing body of water, such as a river or lake. I used raster resolution of about 30. Therefore, the calculator uses only four categories of slope – flat (2%), moderately flat (5%), moderately steep (10%) and steep (above 15%). (See Figure 7). For a 1. This method is documented on page 9 of a report by Ries (2006). A pour point is the point at which water flows out of an area. The word "watershed" is sometimes used interchangeably with drainage basin or catchment. If the drainage area is given in acres, it has to be converted to square feet using the relationship 1 acre = 43,560 ft 2. Let’s solve an example; Find the drainage StreamStats provides access to spatial analytical tools that are useful for water-resources planning and management, and for engineering and design purposes. 36 inches of soil covering at the very top in order to finish the bedding, the overall final depth of a septic drain field is going to be approximately 3 feet to 4 feet. 23 where: Q = Storm Water Runoff (in gallons per minute, gpm) C = Coefficient of Runoff I = Rainfall Intensity (in inches per hour) A = Area of Drainage Zone (in square feet) A = Drainage area, acre. /ft. Knowing the catchment area is the first step in calculating the volume of water that can be caught from rainfall. A drainage basin is an area that drains water and other substances to a common outlet. Upon completion, a full Oct 16, 2009 · The constant 0. Formula 1. 4) Q = 10. In this equation, k is either 0. The drainage area was calculated as 14. The design watershed charac-teristics are homogenous. This area is normally defined as the total area flowing to a given outlet, or pour point. drainage network (lines) within the watershed. 208 · ( n / K n) · ( Q p / ( S o 0. The “Raster to polyline” tool is used to “vectorize” the drainage network. The watershed calculation uses the eight-direction pour point algorithm (D-8) to calculate the flow direction at each location, along with a = Drainage area in acres Since our watershed is a relatively steep pasture with heavy soil, the C value is 0. The peak discharge occurs when all segments of the drainage area are contributing to the runoff from the site. The coefficient of runoff or the maximum rate of runoff Roof Drainage Calculators. A quick video demonstrating the process for delineating drainage basins in ArcGIS Pro. 63 × S 0. equals 32. Overland flow. ii. 0312 gallons per minute/sq. The forms to be used in documenting the drainage-area determinations and the filing system for storage of these forms are outlined. The formula for calculating drainage density: D b = L / A. preadsheet user’s guide. The theoretically derived IPR equations can be expanded to include the production of gas and saturated oil. You will also learn about the most below to obtain roof area that can be handled by each leader. This is because parts of the area with long concentration times are far less susceptible to high-intensity rainfall. 75-1 acre, 18'=20' at dam so 9'-10' average depth, 6 acres drainage area using contour edges from topo map (10 acres if use midpoint of top countour area - see question 1), 36" annual precipitation. Q = A (m)2/3 (i)1/2 n Where A, is the cross-sectional area of the Drain n, surface roughness of the Drain, taken as 0. Other common terms for a drainage basin are watershed, basin, catchment, or contributing area. Example Calculation. The command defines the surface region with a depression low point (catchment point), delineates the region with a boundary, and calculates the area of the catchment region. A yellow delineated drainage basin will appear on the screen 9. Larger watersheds contain many smaller watersheds. Sizing and Location–For most indoor locations, the grate free area should be 1. of roof area. For a 1:1 bevel, the factor is 1/2 in. There are many methods available to estimate the time of concentration including the Kirpich formula, Kerby formula Jun 14, 2019 · The formula for calculating the well drainage area: A w = A r [ qw / qr] Where; A w = Well Drainage Area. 0 . Compute the distance from the WOC to the bottom of the perforations D b: Step 3. 1) Substituting this in Eg. Click the “Calculate” button, and the tool will generate the optimal drain field size in square feet. 30 inches (NOAA) Tributary area (primary drain): 100-ft by 50-ft = 5,000 ft 2. b = 745 ft, and drainage area = pab/43,560 = 94 acres. Calculate the dimensionless perforated length ɛ by applying Equation 9-6: Step 5. Find reservoir drainage area. The calculator cannot account for complex shapes based Using the formula: Total Drainage Capacity = 5,000 sq. If average areas are Jul 7, 2024 · Enter your French drain depth, width, and length in our French drain calculator. In the figure, outlet is at the bottom elevation. usgs. Mar 2, 2020 · Time of concentration is a critical component in some analysis methods for calculating peak discharge from an area. Again, these figures might vary depending on the size of the establishment, local Select the point of interest; Box 2. EPA’s National Stormwater Calculator (SWC) is a software application tool that estimates the annual amount of rainwater and frequency of runoff from a specific site using green infrastructure as low impact development controls. To convert that cross-sectional area to the diameter of the pipe, remember that the formula for the area of a With 1 foot of gravel layering below each trench of the septic drain field and 3 feet, i. So, in this example, you would need nine drains to efficiently handle the roof’s drainage requirements. Flow rate (volume) to maintain roof drainage: Q = 0. Next, input the depth of the excavation as 15 cm. In both the above methods, the quantity of storm water is a function of the area (in hectares): the intensity of rainfall and the coefficient of runoff. Nov 30, 2022 · For cascading drain calculation; you may follow formula given below; Step number = (Start IL – endMaxIL – Distance*Gradient) / Cascading drop. Step 3: Drainage Area Hydrology, Peak Discharge, and Treatment Volume (Tv) Repeat the procedures of Step 2 as needed to determine the post-development Land Cover and corresponding Site Rv (Equation 11. Creating a buffer trade area map showing your Grate free area is defined as "the total area of the drainage openings in the grate. Using the Drain Field Size Calculator is a straightforward process. (see below) Use the Catchment Area command to analyze water runoff and display the surface drainage area. The map-based user interface can be used to delineate drainage areas, get basin characteristics and estimates of flow statistics, and more. The purpose of this analysis is to Drainage Fields. 3-1*) where: Q – flow rate to maintain drainage rate Jan 1, 1991 · and dimensionless time based on drainage area, t DA. co Minimum drain size is also usually specified in local drainage ordinances; 12-15 inches is a common minimum for street collection culverts, Local ordinances often require the size of storm drains to not decrease in the downstream direction. How can I find the area that will drain to Pond #1 (or pond #2 or 3) after each storm? I would like to find a way so I can find the drainage area to look like FIGURE 2 with a closed polygon Your drainage field should be set out in a continuous loop fed for an inspection chamber. Solution. Click on the Drainage Area Delineation icon Mar 29, 2020 · How to Size Your Yard DrainFDM's Online Yard Drain Supplies: https://www. The followings are some ways for determining a drainage area. ft. Once you have your percolation test results to work out the floor area which your drainage filed should cover you can use the below calculation; A t = P x V p x 0. The number of roof drains shown should be verified with local code requirements. The SWC is designed for use by anyone interested in This tool calculates the three standard coefficients (drainage rates) recommended by Skaggs (2017) for characterizing the hydraulics of subsurface drainage systems. 3. Inlets are located at the upstream end of a Line and can be in a sag or on a longitudinal slope and can be of any size. The Rational Method 2. The Procedures for the delineation of drainage-basin divides and the types of maps to be used also are described. It can be part of a larger watershed and can also contain smaller watersheds, called subbasins. 2: Catchment Area. Please ref figure for better understanding my view. Checks the capacity of drain tile on existing drainage systems; Sizes the piping needed on the acreage to The top‐bevel dimension is determined by multiplying the height of the culvert by a factor. Open the Arc Toolbox, Arc toolbox window appears to expand Spatial Analyst Tools, then expend the Density subgroup, Select Line Density, and double click it. Drainage Inlets - The receptors for surface water collected in ditches and gutters to enter the storm drainage system. 8 Meters to compute the area of a cell; and used extract values to points tool to get the flow accumulation value at my selected point. 5) ] 3/8 A = drainage area (in acres) Assumptions: 1. The minimum pipe size needed for a catchment area of 23 m2 m 2 is calculated to be 23 cm2 c m 2 using the rule of thumb in Equation 6 as shown below: The needed cross-sectional area of gutter is 23 cm2 c m 2. Final IL = Start IL – [step number * [step length* (gradient + cascading drop)] Note: i. Jun 24, 2014 · How to Calculate the Drainage Requirements for Flat Roofs Reader Question: is there a rule of thumb for number of roof drains needed on flat roofs. Buffer trade areas. com/store=====French Drain Man – Michigan’s Yard Water Drainage Ex Apr 20, 2022 · Can anyone please help me figure out how to find a drainage area of a surface? Please look at the surface in FIGURE 1 with 3 locations for proposed ponds. Example -1 : Drainage Pipe (Soil) Pipe Sizing. So that would be 25 ft x 40 ft = 1,000 sq. 48052 gal / cu. frenchdrainman. A watershed is a special kind of drainage area that extends from the starting point (the outlet) upstream all the way to the headwaters. A simplified table is shown below. Empirical Formulae Method. Example - 2: Drainage Pipe (Waste) Pipe Sizing. Drainage area of a stream at a specified location is the area measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream. 1 4. If you choose to make a French drain with pipe Step 5: Calculate drains needed. I the figure there are six elevation contours (2 black top-bottom, 4 red in between) in the basin/sub-basin. 25 Explained. 4 acres, and the intensity has just been shown to be 1. Septic drainfield gravel (aggregate) specifications: Typically, washed gravel, 3/4" to 2 1/2" (pretty big) gravel is placed for a depth of 12" under the drain line piping ("septic drain tile" in some references). Note: Use the Create Catchment From Surface or Create Catchment From Object For example, a 3 inch per hour rainfall will build up to a 3 inch depth on a flat roof in one hour, at the rate of 1. A flat roof is a great example of a catchment area. It is a land feature that can be identified by tracing a line along the highest elevation between two areas on a map, often a ridge. The boundaries between watersheds are termed drainage divides. Under Basin Characteristics drop down arrow select Drainage Area (DRNAREA) 11. ft) h = Net Pay thickness, Units: Feet; ∅ = Porosity which is the fraction of a rock volume that is void space that is available to store fluids. From Figure 6, the intercept, t ssri, of the minus-one slope straight with the radial flow regime line takes place at 43,6 hr. The steeper the grade of the installed pipe, the more water it will carry. 25 The peak runoff during a 2-year storm in our watershed would The Capture Ratio is the ratio of the pervious area receiving the runoff (such as a lawn area) to the impervious area that generates the runoff. Example - 3: Sizing of building drainage sewer (3D Isometric) Example - 4: Sizing Vertical Stacks. Examples for Sizing Drainage Piping. The outlet, or pour point, is the point on the surface at which Area = length x width. 1. The Create Watershed command allows the user to perform a watershed analysis on loaded terrain data to find stream paths as well as delineate the watershed areas that drain into a given stream section. Note that our Rational equation calculator allows you to use a variety of units. Example 4. You can use the calculator in reverse, too! Contact your state’s water resources department (or its equivalent) to obtain rainfall depth duration frequency data for your area. of lawn area then the Capture Ratio would be 3,000 / 5,000 or 60%. vk qa dj gy vh sz tb rf cu hb