# IDF Curve ToolCalculate IDF Curve by several methods

## What is rainfall intensity duration frequency (IDF)?

An IDF (intensity duration frequency) curve is a mathematical function that relates the rainfall intensity with its duration and frequency of occurrence. The rainfall IDF (intensity duration frequency) curves play an important role in water resources management engineering. The rainfall IDF (intensity duration frequency) curves are graphical representations of the probability that a given average rainfall intensity will occur within a given period of time. The IDF curves can take different mathematical expressions, theoretical or empirically fitted to observed rainfall data. The first IDF curve was established as early as 1932, whilst since then many sets of IDF relationships have been constructed for several parts of the world.

Assessment of rainfall is usually done using Intensity-Duration-Frequency curves (IDF curves) for various water resource-related schemes. An IDF curve is a graphical representation of the probability that rainfall with a particular intensity and duration will occur and the probable time interval between storms with similar characteristics.

The rainfall Intensity-Duration-Frequency (IDF) relationship is one of the most commonly used tools in water resources engineering, either for planning, designing and operating water resource projects, or for various engineering projects against floods (Nhat et al., 2006). It is one of the most commonly used tools in water resources engineering particularly to identify design storm events of various magnitude, duration and return periods simultaneously. Historic rainfall event statistics (in terms of intensity, duration, and return period) are used to design flood protection structures, and many other civil engineering structures involving hydrologic flows (McCuen 1998; Prodanovic and Simonovic 2007).

The IDF curves are applied in hydrology to express in a synthetic way, fixed a return period T and a duration d of a rainfall event, and for a given location, the information on the maximum rainfall height h and the maximum rainfall intensity i. Known these parameters, it is possible to build synthetic rain graphs that are useful to the elaboration of flood hydrographs (Dupont and Allen, 2000). It is routine in a hydrological study that the IDF parameters are computed for the return period of 2, 5, 10, 25, 50 and 100 years (Wagesho and Claire, 2016). In this research, I also want to calculate IDF curves in the same return period, during past and future periods.

### What is IDF Curve Tool?

The IDF curve tool can draw IDF charts by using several methods. You have two options to input data. In the first tab, you can input your raw data in 5-min to daily scale then the IDF tool will calculate maximums in the second tab. But also you can input the maximums in the second tab instead of raw data. In the third tab, you can draw IDF graphs for any return periods. A return period is an average time or an estimated average time between events(intensity of rainfall occurrence). You can save the IDF data in an excel file and draw the IDF graphs in Excel.

### Theoretical Extreme Value (EV) Distribution Approach

To illustrate the second approach, let us select the Gumbel (Type I) distribution as our EV distribution. The Gumbel Type I distribution is, where μ is the location parameter and b is the scale parameter.

For other methods, the tool will fit the distribution on the maximum rainfall data then the tool calculates every rainall value that you want, based on the return period. In this tool, we used the Accord libraries to fit the distributions on your data.

Intensity of Rainfall = Inverse Distribution (1 - 1 / T)

You can find the distribution information on free Distribution calculators.

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