ENV* K245 Water Resources Engineering home

  • Rainfall Characteristics
  • Rainfall distribution
  • Relative Humidity
  • where T is in °C

  • Water Vapor
  • mass of water = absolutehumidity*height*area

  • Precipitation
  • Rainfall Measurements
  • Note: xg > 2Ho; where xg is the minimum distance of a gage to a nearby tall object with height Ho

  • Point Precipitation
  • P = S (Pm*W)/S W

  • Areal Precipitation (see Sec 4.5, p. 205 et seq.)
  • P = S AiPi/S Ai

  • Frequency Analysis
  • rank the n items of data from highest to lowest (if we want a rare high event) or lowest to highest), assign a number m corresponding to the rank
  • calculate a probability P that the item at rank m will be exceeded:
  • Hazen’s formula gives:

    Fa = (2n-1)/2y; the third highest item of ten would have P = (2*3-1)/(2*10) = 0.25

    [or sometimes we might use P = n/(y+1) so for example the 3rd highest item of 10 would be P = 3/(10+1) = 0.27]

      1. plot each item above its probability
      2. sketch the line that fits the points and find the rainfall, etc, that corresponds to P=1/T
      1. find the mean and standard deviation of the items:
      2. mean = X = å x/n

        Standard Deviation
      3. look up in a table of z values (ie, the normal distribution) the z value corresponding to P=1/T, call this value K (the z value for the
      4. our T-year event is then given by:

    xT = X + Ks

  • Probable Maximum Precipitation (PMP)
  • Gross and Net Precipitation
  • Distribution of the Precipitation Input

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    Anthony G Benoit
    (860) 885-2386