SEA LEVEL RISE AFFECTING THE VIETNAMESE MEKONG DELTA: WATER ELEVATION IN THE FLOOD SEASON AND IMPLICATIONS FOR RICE PRODUCTION REINER WASSMANN 1, 5 , NGUYEN XUAN HIEN 2 , CHU THAI HOANH 3, 5 and TO PHUC TUONG 4 1 Institute for Meteorology and Climate Research (IMK-IFU), Forschungszentrum Karlsruhe, Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany E-mail: reiner.wassmann@imk.fzk.de 2 Sub-Institute for Water Resources Planning (SIWRP), Ho Chi Minh City, Vietnam 3 International Water Management Institute (IWMI), Colombo, Sri Lanka 4 International Rice Research Institute (IRRI), Los Baños, Philippines 5 Formerly at IRRI. Abstract. In this study, we assessed the impact of sea level rise, one of the most ascertained conse- quences of global climate change, for water levels in the Vietnamese Mekong Delta (VMD). We used a hydraulic model to compute water levels from August to November – when flooding is presently critical – under sea level rise scenarios of 20 cm (= 20) and 45 cm (= 45), respectively. The outputs show that the contour lines of water levels will be shifted up to 25 km (20) and 50 km (45) towards the sea due to higher sea levels. At the onset of the flood season (August), the average increment in water levels in the Delta is 14.1 cm (20) and 32.2 cm (45), respectively. At the peak of the flood season (October), high discharge from upstream attenuates the increment in water level, but average water level rise of 11.9 cm (20) and 27.4 cm (45), respectively, still imply a substantial aggravation of flooding problems in the VMD. GIS techniques were used to delineate areas with different levels of vulnerability, i.e., area with high (2.3 mio ha = 60% of the VMD), medium (0.6 mio ha = 15%) and low (1 mio ha = 25%) vulnerability due to sea level rise. Rice production will be affected through excessive flooding in the tidally inundated areas and longer flooding periods in the central part of the VMD. These adverse impacts could affect all three cropping seasons, Mua (main rainfed crop), Dong Xuan (Winter–Spring) and He Thu (Summer–Autumn) in the VMD unless preventive measures are taken. 1. Introduction Over the last three decades, the Vietnamese Mekong Delta (VMD) has undergone drastic changes in hydrology to improve agricultural production, namely, grow- ing rice (Duong and Cho, 1994; Xuan and Matsui, 1998; Hashimoto, 2001; Minh and Kawaguchi, 2002). New canals and sluices resulted in a complex web of in- terconnected water bodies (see Figure 1) that will further be expanded to improve living condition of approximately 16 million inhabitants. The Vietnamese part of the Mekong Delta covers an area of 3.9 mio ha (of about 6 mio ha of the total Mekong Delta, including the Cambodian part) of which 2.9 mio ha are currently Climatic Change 66: 89–107, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.