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GEOG 441 Watershed Systems Precipitation

GEOG 441 Watershed Systems Precipitation. Monday 1/26/2009. Precipitation. Characteristics Spatial Temporal Frequency Analysis. Precipitation. Units- Volume (L 3 ) or Depth/Time (L/T) Input in the mass balance Continuous random variable. Spatial Characteristics. Point measurements.

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GEOG 441 Watershed Systems Precipitation

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  1. GEOG 441Watershed SystemsPrecipitation Monday 1/26/2009

  2. Precipitation • Characteristics • Spatial • Temporal • Frequency Analysis

  3. Precipitation • Units- Volume (L3) or Depth/Time (L/T) • Input in the mass balance • Continuous random variable

  4. Spatial Characteristics • Point measurements

  5. Spatial Characteristics • Radar

  6. Spatial Characteristics • Hyetographs

  7. Temporal Characteristics • Intensity • Rate of precip over some period of time • Duration • Frequency

  8. Review • Histograms • Normal distributions • Extreme value distributions • Cumulative Distributions • An example with UNC graduate salaries

  9. Review Normal distributions mean Std dev

  10. Annual Rainfall • Assumes data follows normal distribution • Typically works for annual precipitation data

  11. Annual Precip Trends

  12. Annual Rainfall Histograms and Cumulative Distributions

  13. Annual Rainfall Histograms and Cumulative Distributions

  14. Annual Rainfall Histograms and Cumulative Distributions

  15. Different distributions required for shorter time periods

  16. Exceedance Probabilities • [dimensionless], the relative frequency associated with a random variable attaining a value greater than some specified value. • For example- calculate probability that annual precipitation in any given year will exceed 1.0 m

  17. Exceedance Probabilities

  18. Exceedance Probability • Normalize data to a z value • Find cumulative probability for a given value (Table A3.2)

  19. Annual Maxima • Takes the highest value for the entire year

  20. Partial Duration • All values above a certain base • Closely related to annual values for long return periods • Could be more (frequent) or less (infrequent) values than annual maxima for a given threshold

  21. Partial Duration Series –RDU Airport

  22. Precipitation Frequency Examples from AZ and NC http://hdsc.nws.noaa.gov/hdsc/pfds/index.html

  23. Return Period • What is the probability of a storm of X magnitude this year? • Inverse of exceedance probability Treturn= (n+1)/m n= # of years of observation m= rank

  24. Return Period- Washington, DC Data from Table 2.1 plotted

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