I've created an idf curve based the Santa Clara County data. They use a specific equation to calculate the intensity (Santa Clara Valley Water District's Return Period-Duration-Specific (TDS) Regional Equation). I have calculated the intensities, see attached "Rainfall Intensity Spreadsheet". When I input these intensities for the 5, 15, 30, and 60 minute durations in order to create an idf curve, the intensity values on the curve Hydraflow produces are less than what I input (see "IDF curve"). I've checked my input into the table and it is correct. The intensity values are a little lower than those required by the County.
For example, the 2 yr - 5 min required is 1.7, but Hydraflow gives 1.58. It's not off by much but it's different when it should be the same.
What am I doing wrong?
The Hydraflow tools provide a quick method to calculate the standard BDE coefficients by entering three durations.
You may want to review the regression values of the Santa Clara rainfall to see if it fits better with a polynomial set of equations.