Due to its ecological diversity, the island of Borneo is particularly vulnerable to the impacts of climate change. Positioned at the centre of the Maritime Continent, Borneo was the focus of this study, which thoroughly investigated the spatiotemporal trends of extreme rainfall across the region. By utilising the Expert Team on Climate Change Detection and Indices (ETCCDI) extreme rainfall indices, the study introduced a novel threshold-based likelihood classification to enhance the assessment of Mann-Kendall trend tests, enabling clearer identification of spatiotemporal patterns and providing practical guidance for risk assessment and decision-making. The Climate Hazards Group InfraRed Precipitation with Station (CHIRPS) daily rainfall dataset (1981–2022), covering 943 grid points, was used for this purpose.

The results showed that Borneo is expected to face a high-likelihood increase in extreme rainfall frequency ranging from 34.6 % to 52.6 %, with the central mountainous region experiencing the highest rise of 52.6 %. The affected areas are expected to expand southward and eastward, notably impacting southern Borneo (up to 36 grids) and East Kalimantan's eastern coast (up to 7 grids), with increases also observed in coastal-northern Sarawak, Brunei, and northwestern Sabah. These findings demonstrate the added value of combining multiple Mann-Kendall test variants with a likelihood framework to strengthen confidence in trend detection and effectively identify hotspots like southern Borneo and East Kalimantan's eastern coast. These results provide local governments, disaster management agencies, environmental planners, and water resource managers with a stronger scientific basis to develop targeted adaptation strategies and mitigate climate change impacts in Borneo.

Highlights:

• Comparative analysis of modified Mann-Kendall test variations for Borneo's extreme rainfall trends.
• High likelihood of significant rainfall trends observed across central mountainous regions.
• Threshold-based categorization identifies low, medium, and high likelihoods of rainfall changes.
• Geographic range of extreme rainfall projected to extend southward and eastward in Borneo.

Figure 1: Spatial distribution of PRCPTOT, R10, R20, R25, R95pTOT, and R99pTOT in Borneo

Source:

Zulfaqar Sa’adi, Lelavathy Samikan Mazilamani, Nurzalikha Sa’adi, Mohd Hadi Akbar Basri, Nor Eliza Alias, Zulkifli Yusop, Zainura Zainon Noor, Ricky Anak Kemarau, Mohammed Sanusi Shiru, Andleeb Masood. The likelihood of a significant trend based on a family of Mann-Kendall tests for extreme rainfall in Borneo. Physics and Chemistry of the Earth. Volume 139: 2025, 103959. (https://doi.org/10.1016/j.pce.2025.103959)

Date of Input: 31/07/2025 | Updated: 26/09/2025 | ainzubaidah