Long-term homogeneity and trend analysis of seasonality and extreme rainfall under the influence of climate change in Johor River basin, Malaysia

Abstract

Abstract There is a growing concern over the unprecedented shifts in seasonality and extreme rainfall over the last century across the globe. This study investigated 24 rainfall stations in Johor River basin (JRB) for homogeneity over the period 1970–2015 and followed by analysis of rainfall seasonality and extreme rainfall trend. Homogeneity test was conducted for annual and seasonal rainfall using a hybrid of Standard Normal Homogeneity test (SNHT), Pettit test, Buishand Range test (BRT), and Von Neumann ratio (VNR) test. The JRB climate was then divided into several classes using the Seasonality Index (SI). For trend analysis, RClimDex was utilised to compute eleven extreme rainfall indices as recommended by ETCCDI, consisting of frequency indices, namely, number of heavy rainfall days (R10), very heavy rainfall days (R20), extremely heavy rainfall days (R25), maximum number of consecutive dry days (CDD) and maximum number of consecutive wet days (CWD), and intensity indices, namely, daily maximum rainfall (Rx1day), maximum five-day rainfall (Rx5day), annual total wet-day rainfall (PRCPTOT), simple daily intensity index (SDII), very wet day rainfall (R95P) and extremely wet day rainfall (R99P). Mann-Kendall (MK) trend test was applied to show the area within JRB that is subjected to change at 95% significance level. Then, the flood vulnerability area due to significant change in extreme rainfall was assessed based on the past flood record. The results showed that the rainfall series during North East (NE) and South West (SW) monsoon are more homogeneous than annual rainfall series with the presence of inhomogeneity in some stations being associated with the influence of climate variability. Although non-significant, the SI results show increasing trends with rainfall being concentrated in the downstream area. During the NE monsoon, frequency index of R10 showed a significant increasing trend at thirteen stations. Meanwhile, R20 and R25 showed increasing trend at five stations, mainly in the downstream and at the west of the basin. During the SW monsoon, R10 showed a significant increasing trend at six stations, but R20 and R25 showed a significant decreasing trend at two and one station, respectively. Generally, the results showed that increasing extreme rainfall in the form of frequency indices are more prominent throughout JRB particularly at the end of the NE monsoon. Increasing severity of previously recorded flood prone area is expected, which required improved flood mitigation strategy.