1. GEOG 441 Watershed Systems Precipitation Monday 1/26/2009

2. Precipitation Characteristics –Spatial –Temporal Frequency Analysis

3. Precipitation Units- Volume (L 3 ) 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

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 1.Normalize data to a z value 2.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 T return = (n+1)/m n= # of years of observation m= rank

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