How to determine the 90% of the average annual rainfall

1) Intensity-Duration-Frequency (IDF) Charts. These charts show the relationship between the rainfall intensity (rate) and the corrisponding rainfall duration (minutes). This data is required for 1 and 2 year statistical return periods. 2) Depth-Duration-Frequency (DDF) Charts. These charts show the relationship between the rainfall depth (volume) and the corrisponding rainfall duration (minutes). This data is required for 1 and 2 year statistical return periods. 3) Volume of the 90% rainfall event, i.e. only 10% of rainfall events have a cummilative depth of more than this amount. 4) 90% Shortest Time between rainfall events, i.e. only 10% of rainfall events have between them a dry time less than this time. 5) Volume of yearly rainfall for "water shed" classification as per LEED. in plain english this means that it is the volume of rain coming down for an entire rainfall 24 hour equivalent event AND the event is the one that statistically happens most of the time. I think the reason the userguide wording is comfusing is because the statistical concepts are confusing and these calculations are usually done by an expert civil engineer. I also am not 100% certain of this area. Links: http://www.hydrocad.net/pdf/TR-55%20Chapter%203.pdf http://www.bossintl.com/download/SoilCharacteristics.html http://www.bossintl.com/download/RunoffCurveNumbers.html also google RUN-OFF VOLUME CALCULATION - TR-55 Method Stromwater Retention Volume - Schueler's Simple Method RUN-OFF RATE CALCULATION - Rational Method