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How to Calculate a Hydrograph

Hydrographs are graphs that illustrate the change in hydrologic variables over a time period. Hydrographs are used for different types of water bodies, such as lakes, streams, oceans and water wells. The purpose of a hydrograph is to illustrate the change is discharge of a body of water in order to predict the amount of runoff for future precipitation events. There are a sequence of calculations that one can perform to obtain the peak discharge from a hydrograph.

Things You'll Need

Graphing software
Curve Number Tables for Hydrograph
Tables for Rain Type

Instructions

Calculate Values to Create a Hydrogaph
- 1
  Collect all relevent information. Subdivide the water body to be used in the hydrograph into subareas that are homogeneous or the same. Determine the drainage area of each subarea in square miles. Calculate the travel time (Tt) in hours, the time of precipitation concentration (Tc) in hours and summation travel time to outlet (Summation multiplied by Tt) in hours for each subarea. Tc is the total of the travel times along the hydraulic path in the subarea. Tt is the time that it takes water to travel through a subarea. Travel time summation to outlet is the time it takes for water to flow from the bottom of the subarea to the bottom of the entire body of water.
- 2
  Calculate the weighted runoff curve number for each subarea using the equation CN= summation(CN of subarea x sub area)/total area. The curve numbers should be given for your soil type when working with watershed areas.
- 3
  Determine the rainfall amount (P) in inches. Determine the total runoff, in inches and the area multiplied by the runoff in miles squared inches for each subarea. The runoff can by found using the drainage area.
- 4
  Compute the initial abstraction (IaP) values in inches for each subarea. The initial abstraction values can be found using the curve numbers and the P value is the rainfall amount.
- 5
  Develop hydrograph discharge summary to determine the peak discharge and time of peak. Determine the rainfall type for the summary, locate tables for your rainfall type. Use the correct time of concentration, IaP value, and summation travel time to outlet. Choose the hydrograph time to start on the graph and look for the unit disharge that corresponds to the hydrograph time. Mulitply the unit disharge from the subarea's area times runoff value. This will give you the specified time. Repeat for each subarea and record the disharge summary in a table.
- 6
  Graph the total discharge values at each time to develop the hydrograph for your water body. You can use any graph software, such as Excel or Minitab.

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