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How to Calculate the Drainage Area

By Yvonne Bernard
Drainage basins are at work all around us.

Drainage area is an alternate term for drainage basin. 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. In the field of hydrology, calculating drainage area is a critical element in many practical applications. In determining the peak runoff of watersheds, the drainage area is needed. Calculating drainage density or calculating storm drainage for secondary roads begins with the drainage area. This concept reaches into many fields of engineering.

Approach I

Step 1

Acquire United States Geological Survey (USGS) topographic maps at 1:24,000 scale that cover the drainage network of interest.

Step 2

Trace the drainage area of interest on the topographic map.

Step 3

Outline the drainage area with a polar planimeter by moving the tracer point along the previously traced area.

Step 4

Record the enclosed area in Vernier units.

Step 5

Convert the Vernier units to the desired area measurement based on scale of map and conversions provided with the planimeter.

Approach II

Step 1

Acquire United States Geological Survey (USGS) topographic maps at 1:24,000 scale that cover the drainage network of interest.

Step 2

Trace the drainage area of interest on the topographic map.

Step 3

Digitize the drainage area into a Geographic Information System (GIS) file using the GIS unit and the add-on digitizer.

Step 4

Compute the drainage area using the accompanying software from USGS.

Approach III

Step 1

Acquire United States Geological Survey (USGS) topographic maps at 1:24,000 scale that cover the drainage network of interest, and trace the drainage area of interest on the topographic map.

Step 2

Place a sheet of 100 dot-per-square-inch transparent overlay on top of the drainage area on the USGS map.

Step 3

Count the total number of dots falling within the drainage area and half of the dots falling on the lines.

Step 4

Find the correct conversion factor, noting map scale. For a 1:24,000 scale, the correct per dot area conversion factor is 0.003889 kilometers (km) squared.

Step 5

Calculate drainage area by multiplying the total number of dots by the conversion factor.

 

Things You Will Need

  • USGS topographic maps
  • GIS unit with digitizer and software
  • 100 dot-per-square-inch transparent overlay
  • Polar planimeter
  • Ruler

About the Author

 

Yvonne Bernard began her writing career in 1994. She has written articles for “Design News” and published an engineering text book, “Environmental Engineering,” for McGraw-Hill. Bernard is also a member of several engineering societies. She has a Bachelor of Science in mechanical engineering from the University of Illinois.