In VO, it is possible to add IDF groups (IDF Group) . However, to calculate IDF manually, you can do the following steps:
To calculate IDF, we need recorded precipitation values.
For each year, select the maximum precipitation with different duration. For example, precipitation with duration 15, 30, 45, 60, 90, 120, 180, and 360 min. Do frequency analysis by fitting a suitable statical distribution on each duration. So, we can calculate precipitation (P) with the desired return period (T) for each duration ( ). For example, where t is the duration, and T is the return period.
Select the maximum daily precipitation for each year. Do frequency analysis by fitting a suitable statical distribution on maximum daily precipitation. That way, we can calculate daily precipitation ( ) with desired return period (T). For example, .
Suppose that the vertical axis of the IDF graph is precipitation value (P). At the end, we can calculate I = P/time . The reason for this assumption is that we have recorded the amount of precipitation, and we don’t have its intensity.
The equation for IDF curves is R = at^b which
We apply the log on both sides of the equation.
Now, we have a linear equation like A = B + C log t. In this equation, we have already calculated A, but B and C are unknown.
To calculate B and C, we need to do a regression for each return period. For example, for the return period of 10 we have:
A = B + C log t is equal to y = 0.4089x – 1.2485
B = log a = – 1.2485
C = b = 0.4089
So, for T=10 we have: R = 5.3432*t^0.4089
If we do R/duration, we will have the intensity.
Bahmani, R. Radmanesh, F. Eslamian, S. Khorsandi, M. Zamani, R. Proper rainfall for peak flow estimation by integration of L-Moment method and a hydrologic model. International Research Journal of Applied and Basic Sciences 2013 Vol.4 No.10 pp.2959-2967 ref.11
Bahmani, R. Eslamian, S. Khorsandi, M. Hosseinipour, Z. Combination of L-Moments Method and Hydrological Model for Design Flood Hydrograph Determination. World Environmental and Water Resources Congress 2013 : Showcasing the Future . 2013. doi:10.1061/9780784412947.320