Detection of changes in flood data in Victoria, Australia from 1975 to 2011

Changes in observed streamflow records for 131 catchments from the state of Victoria in southeast Australia are investigated for the 1975-2011 study period. Seven flood indices are considered which are derived from annual maximum, peak-over-threshold (POT) and monthly maximum flood series. Trend analyses are undertaken using Mann-Kendall (MK) and seasonal MK tests along with the bootstrap resampling approach to account for serial correlation. A common trend direction in all months in streamflow data for the majority of the stations has been identified by Van Belle and Hughes' homogeneity test. Trend analyses results show the percentages of stations exhibiting significant downward trends ranges from 4 to 35% at 5% significance level, which is generally higher than the percentage of stations to occur by chance. Good agreement is found between trends in the AM and POT flood magnitude time series, and separately in the POT flood frequency time series, but not between flood magnitude and their associated frequency series. More stations with significant negative trends have transpired in the AM than in the POT flood data series. Resemblance in the results between both the MK and seasonal MK tests for most of the catchments is also noticed.