Spatiotemporal Characteristics of Rainfall Concentration and Seasonality in Johor River Basin, Malaysia: Application of the CHIRPS Precipitation Product

An important factor affecting the climate is the amount of rain that falls in a given year. An imbalanced rainfall distribution results in periods of excessive rainfall over certain location that can increase the risk of flood, and contrarily a periods of drought, which hinder plant and crop growth. Therefore, a spatiotemporal assessment of rainfall concentration is important in case of Johor river basin (JRB) in identifying area that is prone to flood, and to ensure the utilisation of water resource can be optimise and conserve throughout the year. In this study, gridded-based CHIRPS rainfall dataset (period 1983–2018) was used to calculate rainfall concentration based on the concentration index (CI). In addition, the performance of Seasonality Index (SI) and Precipitation Concentration Index (PCI) was investigated to define the rainfall regime in JRB. The mapping of the spatiotemporal distribution of CI was done to understand the pattern and changes in the days of heavy rainfall across JRB. High CI is associated with strong physical processes like convection, while low CI can be construed as the result of regular patterns like the fronts' constant passage. The trend was then analysed with non-parametric Mann–Kendall (MK) at 95% significance level. The results showed that SI can give a better classification of rainfall regime in JRB than PCI, due to broader classification. Under SI classification, the rainfall regime in JRB was mainly characterised by ‘equable but with a definite wetter season’ type. Generally, the CI values is high (≥ 0.60) across the basin, varying from 0.61 to 0.72, with the highest in the north-western and western area. The shift in rainfall concentration was observed, as CI significantly increased at 27 grid points along the eastern border at the upstream and middle part of the basin. 25% rainiest days during the NE monsoon made up more than 67% and up to 83% of the annual rainfall for all grid points. Therefore, it is inferred high-intensity rainfall events contribute by the total rain in the basin, which give the highest overall percentage. The results enhance the knowledge on the local climate in Peninsular Malaysia (PM), and also give considerable important information to water resource management, flood mitigation plan and also valuable for agricultural planning.