A thesis s ubmitted in partial fulfillment of the requirement for the degree of Maste r of Science, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 1991

Ho Chi Minh City is located in the downstream area of the Dongnai-Saigon River System. The river system is affected by the tide from the sea, and the lower parts are affected by salinity. Following the construction of Trian Hydropower Plant and Dautieng Irrigation Dam on Dongnai and Saigon Rivers, salinity conditions in Ho Chi Minh City have changed. It has been observed that there is a close relationship between the discharge from the reservoirs and the salinity in the downstream area. The construction of the dams has caused positive effects on the salinity in the fresh water regime area, and negative effects in the estuarine portion. Rice production in Thuduc has increased due to higher water discharged in the dry season, which resulted in a decrease of salinity. On the other hand, a decrease in rice production has occurred in Duyenhai District, because of higher salinity resulting from a reduction in water discharge from the reservoir in comparison with natural discharge in the rainy season. A software package, MIKE- 11, was applied to the area to study the salinity intrusion. The Hydrodynamic and Transport-Dispersion Models were calibrated and verified to get the best fit between computed and observed water levels and salinity. Then the models were applied to determine the relationship between discharge from the reservoirs and the salinity in the river system. This provides a basis for an economic analysis for optimization of discharge operations.