The changing rainfall patterns drive the growing flood occurrence in Phnom Penh, Cambodia

Abstract

Study region: Urban flooding is an intensifying issue in the rapidly developing lowland cities of Southeast Asia. Cambodia's Phnom Penh City, located at the confluence of the Mekong and Tonle Sap Rivers in the lower Mekong Basin, recently saw increasingly prevalent flash floods, resulting in casualties and damages. Study focus: While flood planning and impact assessments have been done in the Mekong basin, flood causes in the face of climate change remain insufficiently understood. In this paper, the drivers of the increasingly prevalent floods in Phnom Penh are investigated through analysis of remote sensing and instrumental data. New hydrological insights for the region: With precipitation records from the Climate Hazards Group InfraRed Precipitation and gauge station dataset, a general precipitation increase of 7 mm/year and a notable shift to a wetter regime were observed. Strong periodicities of 1/3-year, 1/2-year, 1-year and ∼5-year cycles were also identified, which generally showed a considerable increase in intensity from 1981 to 2021. Over the past 40 years, precipitation patterns have intensified. Yet, the average water levels of the rivers surrounding Phnom Penh have declined by 1–2 m from 1981 to 2021, indicating the pressure from climate change is a major hydrological driver contributing to Phnom Penh's flash floods. Meanwhile, Phnom Penh's vulnerability to flash flooding is also likely aggravated by drastic land use changes, which have increased the city's impervious surfaces and reduced its wetlands by 30 % since the 1980s.