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.
