Journal of Environmental Science and Engineering B 4 (2015) 161-168 doi:10.17265/2162-5263/2015.03.008 Assesment of Frequency-Magnitude of Extreme Rainfall Events-Case Study of the MeKong River Delta Pham Hai An 1 , Tran Anh Tu 1 , Tran Dinh Lan 1 and Nguyen Ngoc Tien 2 1. Institute of Marine Environment and Resources, VAST, Haiphong 180000, Vietnam 2. Institute of Marine Geology and Geophysics, VAST, Hainoi 1000000, Vietnam Abstract: Extreme rainfall events are primary natural hazards, which cause a severe threat to people and their properties in populated cities, which are normally located in coastal areas in Vietnam. Analysing these events by using a data series observed over years will support us to draw a picture of how the climate change impact on local environments. The purpose of this report is to understand the characteristics of the extreme rainfall events in MEKONG river delta (south VietNam). Daily rainfall data in the period of 30 years for a meteorological station in each area were collected from the Vietnam National Hydro-meteorological Service. The extreme rainfall events were defined as those exceeding the 95th percentile for each station. The analytical results show that the rainfall values (95th percentile) are 37.4 mm/day at Nam Can station, 27 mm/day at My Thanh station, 22.4 mm/day at Hoa Binh station, 23.8 mm/day at Binh Dai station and 22.7 mm/day at Ben Trai station. The highest rainfall data ever recorded are 246.4 mm/day (Nam Can), 174.5 mm/day (My Thanh), 179 mm/day (Hoa Binh), 187.3 mm/day (Binh Dai) and 136.3 mm/day (Ben Trai) during 1983-2012. The result of the Mann-Kendall tests show that there was a significant creasing of the rainfall at Nam Can, My Thanh station in two periods (1983-2012, 1998-2012) while no clear trend of the rainfall was recoreded at Hoa Binh, Binh Dai, Ben Trai station. In order to estimate the return period of the extreme rainfall events, the method General Extreme Value Distribution was used to calculate frequent distribution. The magnitudes of daily maximum rainfall were from 2 to 100 years. The results of return period show that maximum rainfalls are 46.6 mm at Nam Can station (highest) and 31.4 mm at Hoa Binh station (lowest) during 50 years. Similarly, maximum rainfalls are expected to be about 55.1 mm at Nam Can station and 37.2 mm at Hoa Binh station for 100 years. Key words: Generalized extreme value distribution, MEKONG river delta. 1. Introduction In recent years, weather and climate tend complicated. The vagaries of weather and climate have a significant impact on the variability of meteorological factors, especially precipitation. The transformation of rainfall will affect the hydrological cycle and water resources in the climate system, leading to alter the mean values of temperature and precipitation, which increases the volatility of the phenomenon of rain strengthening or weakening causing floods or drought, especially for the MEKONG river delta region. MEKONG river delta region has the tropical climate, hot and humid, influenced by monsoon Corresponding author: Pham Hai An, master, research field: physical oceanography. E-mail: anph@imer.ac.vn. comprehensively. Each year there are two main seasons: rainy and dry season. Every year, there is considerable precipitation, average 1,400-2,200 mm/year. The months with rainy days are at least from December to March, varying from 0 to 8 rainy days/month. The months with the highest rainfall days are from May to October, varying from 15 to 21 rainy days/month. Precipitation is about from 78 to 92% in the rainy season. Compared to other regions in the country, the rainfall in the study area has been changed little. But the remarkable matter is that the region has 2 rainy peaks: the first rainy peak in June, July; the second rainy peak in September, October 10. Between two rainy peak, from late July to early August, there has a short dry period lasting approximately 10 days due to D DAVID PUBLISHING