Statistical analysis of rainfall variations in the Bangkok urban area, Thailand

Abstract

An effect of urbanization as well as regional climate change on the urban rainfall level has been found in many cities. Bangkok is a megacity that has experienced a rapid growth and expansion in the past few decades, and this has led to parts of the population moving from the inner city to the urban fringes and suburbs. This research applied statistical analysis to study the variations in the annual and maximum 1-day rainfall levels from 1982 to 2010 in a Bangkok urban area of 100 km2 that covers the inner city and western urban fringe. The trend in the temperature over this period was also investigated as a preliminary indicator of the effect of urbanization and revealed that the temperature in Bangkok was still increasing and at a far higher rate than in the rural area in the nearby province in the west (Nakhon Pathom). The moving average and Mann–Kendall analyses revealed that the annual rainfall level significantly increased in the urban fringe in the western part and slightly decreased in the inner city in the eastern part of the study area, while the maximum 1-day rainfall decreased all over the study area except for in the commercial area in the eastern part where the trend was unclear. However, frequency analysis between 1982–1996 and 1997–2010 in the commercial area revealed a long return period rainfall (5–10 years or more) increase compared with that in the other areas. The causes in the variation in both the annual and maximum 1-day rainfall are also preliminary investigated in this study.