Sensitivity of Salinity Intrusion to Sea Level Rise and River Flow Change in Vietnamese Mekong Delta-Impacts on Availability of Irrigation Water for Rice Cropping

The combined impact of sea level rise and reduction of the Mekong River flow in the dry season on salinity intrusion and rice cropping in the Vietnamese Mekong Delta was assessed in this study. The MIKE11 model was used to simulate flow and salinity intrusion from December to June for the medium-term (mid-2030s) and long-term (mid-2090s) scenarios using data derived from the SRES B2 climate change projection. The sea level rise values for two scenarios were +20 cm and +45 cm, while the rates of change for the Mekong River flow were -15% and -29%, respectively. The results obtained for the mid-2030s and mid-2090s scenarios show that the 2.5 g/L saline front is likely to shift upstream by 10 km and 20 km in the main river channels, and up to 20 km and 35 km in the paddy field, respectively. The simulated salinity intrusion results were then used to compute durations of available water for irrigating rice cropping. The results indicate that area where triple rice crops are possible, will be reduced by approximately 71,000 and 72,000 ha, while single crop areas will be increased by approximately 38,000 and 179,000 ha for the mid-2030s and mid-2090s scenarios, respectively. Using GIS techniques the delta was divided into three areas reflecting different rice crop vulnerability levels; areas of high and medium vulnerability measured approximately 200,000 ha and 400,000 ha, respectively.