How to Find the Areal Distribution by Isohyetal Method


In this graphical technique How to Find the Areal Distribution by Isohyetal Method, lines of equal rainfall, called isohyets, are drawn over an area of precipitation point measurements. The magnitude and extent of the resultant rainfall areas of coverage are used to estimate the areal precipitation value

  • The isohyet method is superior method especially when the stations are large in number.
  • An isohyet is a line joining points of equal rainfall magnitude.
  • It is Potentially the most accurate approach
  • Plot gauge locations on a map
  • Subjectively interpolate between rain amounts between gauges at a selected interval
  • Connect points of equal rain depth to produce lines of equal rainfall amounts (isohyets)
  • Compute areal rain using:


The annual precipitation for for the following four gauging station is given

  1. Parachinar guage station= 781.7mm(2013)
  2. Forest Institute Peshawar = 396.2mm(2013)
  3. Abbotabad guage station=1269mm(2013)
  4. Dir gauge station=1058.43mm(2013)


  1. Find the location of these gauging station on google earth
  2. Draw the Isohyetal line in the catchment area
  3. Calculate the area enclosed by consecutive Isohyetal lines
  4. Find the areal distribution by Isohyetal method

Areal Distribution Of Annual Precipitation By Isohyetal Method in GIS;

  • Locate the stations using Google earth
  • Georeferencing using QGIS
  • Add shapefile for ppt, data
  • Interpolation
  • Contouring
  • Finding out areas enclosed by isohyetal lines
  • Areal ppt distribution

Step No:01 Insert CSV File of PPT Data and its Co Ordinates:

Step No:02 load raster image

Step No:03 do labeling

Step No:04 draw connecting lines

Step No:05 draw isohytal lines by contouring

How to Find the Areal Distribution by Isohyetal Method Using isohyetal Method for Average Precipitation

  • P=(1/53025.5((396+500/2*020.797+(500+600/2)(065.722 )+(600+700/2)(107.22)+(700+800/2)(156.109)+(800+900)/2(195.882)+(900+1000/2)(195.882)


  • P=915.25mm
  • Thus the average precipitation by this method is nearly the same as the Thiessen method

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