The Estimation of Probable Maximum Precipitation (PMP) for 24 Hours Duration in the Upper Brantas River Basin, East Java

The probable maximum precipitation (PMP) is defined as the highest precipitation (measured by a rain gauge) for a given period that is meteorologically reliably recorded in the rainfall station. Estimation of the PMP is very important, especially for the safety design of dams and reservoirs, because precipitation data are available for a much longer period of time than those for river floods. Analysis of the daily rainfall, which was used to estimate the PMP values of the upstream Brantas River showed high consistency, so that the collected rainfall data were reliable. These facts were shown by a straight line produced by each rainfall station based on the double mass curve test. The PMP estimation for upstream Brantas River varied between 419 and 556 mm/day. The estimated PMP values were highest at the Birowo station and lowest at the Tangkil station. They were then not correlated to the maximum daily rainfall observed at a rainfall station. This means that there are other factors as described by the Hershfield equation that resulted in higher estimated PMP values. Therefore, the safety design of dam and reservoir construction in this area has to consider the estimated maximum PMP reaching 556 mm/day.