The Relationship between extreme rainfall and flood tendencies in Pemali-Comal River Basin, North Central Java

Abstract

Increasing of flood magnitude causes an increase of flood risk which is now recognized as the big problem in most part of the world. It is still debatable whether increasing of rainfall intensity or changes of catchment or both of them are the main cause of flood discharge. Therefore, extreme rainfall and flood discharge need to be investigated. The objective of this study is to analyze the presence of tendency in annual maximum daily rainfall, annual maximum 2-day rainfall, annual maximum 3-day rainfall and annual maximum flood in Pekalongan region and to analyze the relationship between extreme rainfall and flood tendencies. This study based on daily rainfall data that has a period of more than 50 years which covers the last two decades from 57 rainfall stations and annual maximum flood data from 6 gauging stations. The methods used in this study are Grubb and Beck method to detect outliers, whereas Spearman method to analyze the tendencies and linear regression to see the changes of data series for every period of analysis. The result showed that only a few data series of 1-day, 2-day and 3-day annual maximum rainfall have significant increasing or decreasing trend using 95% confidence interval. Design rainfall with 20 years return period changed every decade which followed the direction of the tendency. There was no significant tendency of annual maximum flood with 95% confidence interval, but the data series changed every year when linear regression was applied. The tendency of rainfall and flood discharge do not indicate strong relationship. It means that the tendency of rainfall is not the major reason of the flood tendency, other factor such as land use changes may give the main contribution.