Chapter 3 - Computations of probable maximum precipitation estimates

Abstract

Extreme precipitation events are projected to be more frequent and intense, exceeding known historical records. Most of critical water infrastructures are often designed considering the probable maximum precipitation (PMP) values. Information of the PMP is necessary for designing high-hazard structures such as large dams and spillways, flood protection works, levees, and nuclear power plants. The World Meteorological Organization (WMO) recognizes several methodologies to estimate PMP. The two main methods are: (1) a statistical approach known as Hershfield statistical method; and (2) the physical approach known as the hydrometeorological method. Depending on the availability and limitations of data, both methods are accepted and are widely used. This chapter introduces the application of the statistical method in estimating the PMP values particularly the use and development of Hershfield statistical method application using datasets of Malaysia and Bangladesh. Introducing the concept of homogeneous region into the envelopment process of the frequency factor, the Malaysia datasets has shown an increase of the PMP values compared to conventional method based on state boundary, while for the Bangladesh datasets, a method to reduce over estimation of the PMP values were presented. A universally accepted calculation technique is still absent. Methods is constrained by the length of the record, and the lack of uncertainty estimates. It is obvious each method has its advantages and disadvantages. So, it can be concluded that there is lack of a unique method for determining the upper bound of rainfall. Therefore, attention should be given to selecting a particular method for a particular region.