Understanding Hydrological Procedure No. 4 and Its Application

I was privileged to give a training on "Understanding Hydrological Procedure No. 4 (2018) and Its Application" organised by Malaysia Water Partnership (MyWP) and UNESCO-IHP Malaysia. Hydrological Procedure No. 4 (HP 4) is a procedure to calculate peak floods in ungauged rural catchment of Malaysia. It was originally developed in 1974 and updated in 1987 covering only Peninsular Malaysia. In this update, the HP4 has considered all the hydrological data up to 2017 with covering areas include Peninsular Malaysia, Sarawak and Sabah. HP4 was developed using Regional Flood Frequency Analysis (RFFA) based on Index Flood Method. The assumption of this method is that the flood characteristics in a homogeneous region are similar in terms of its Mean Annual Flood (MAF) and Flood Frequency (FF). MAF formula was developed using multiple-linear regression with acceptance criteria R2 > 0.7 and PBIAS < 25%. For FF formula, it was developed using the goodness-of-fit test (Ztest) where a distribution is acceptable when Ztest < 3. The detailed formulas and its regions can be found on the HP4 manual in this Link. The most important things that I was highlighted during the training are: - All data used in this analysis has limitation (i.e. spatially, temporary and accuracy). - Every method (regression and statistics) has its own considerations and limitations. Finally, all models are wrong but some of them are useful. Hopefully, this hydrological procedure will be useful especially for young engineers and students.