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
Spatial Characteristics Radar
Spatial Characteristics Hyetographs
Temporal Characteristics Intensity –Rate of precip over some period of time Duration Frequency
Review Histograms Normal distributions Extreme value distributions Cumulative Distributions An example with UNC graduate salaries
Review Normal distributions mean Std dev
Annual Rainfall Assumes data follows normal distribution Typically works for annual precipitation data
Annual Precip Trends
Annual Rainfall Histograms and Cumulative Distributions
Different distributions required for shorter time periods
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
Exceedance Probabilities
Exceedance Probability 1.Normalize data to a z value 2.Find cumulative probability for a given value (Table A3.2)
Annual Maxima Takes the highest value for the entire year
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
Partial Duration Series – RDU Airport
Precipitation Frequency Examples from AZ and NC
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
Return Period- Washington, DC Data from Table 2.1 plotted