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As it is known, MSMA 2 doesn’t contain IDF coefficient for Sarawak; the IDF Coefficient for Sarawak is contained inside Sarawak Hydrology yearbook.

For the benefits of Sarawak C&S consultants, we have scanned the IDF data from this book, the softcopy of the scan can be found here.

Sarikei IDF coefficients

Not only that, we have also extracted the data from the scan by using Plot Digitizer, and put inside online for everyone to use.

Example: IDF Coefficient for Sarikei


Blue is the computed rainfall Intensity, red is the actual rainfall intensity obtained from site. 

Note that from the above graph, our fitting coefficients are very, very good and can reproduce the observed Intensity correctly with very little error, no more than the measurement error.

How is the \(\lambda\), \(K\), \(\theta\), \(\eta\) being calculated?

The Intensity rainfall formula is given as below:


Given all the measured data between \(I\), \(T\), \(d\), how do we come up with \(\lambda \), \(K\), \(\theta \), \(\eta \)?


  1. We can first assume an initial sets of  \(\lambda\), \(K\), \(\theta\), \(\eta\), computed an initial \(I\), and then compute the square difference between the measure \(I\) and computed  \(I\), ie: \(e=(I_{computed}-I_{measured})^2\).
  2. And then we pick another sets of \(\lambda\), \(K\), \(\theta\), \(\eta\), and then compute the new \(I_{computed}\), and compute the \(e\) again.
  3. Do this a few rounds to find the fitting constants that correspond to the minimum \(e\), and then we can get our actual fitting constants!


Software tools and plugins:

Google Sheets allows one to do all the above nicely via a plugin called OpenSolver. OpenSolver is a linear, integer and non-linear optimizer plugin that can be used in conjunction with Microsoft Excel and Google Spreadsheet.

You can duplicate the Google Sheets in the above folder, install the plugin, specifying correct range of data, the equation for fitting purpose, and then you will be able to come up with your own fitting constants. The guide for using the OpenSolver for Google Sheets can be found here.

You can even do this for the next upcoming rainguage station, without relying on any other software vendors!