thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Engineering and Technology

Thesis (M.Eng.) - Asian Institute of Technology, 2005

Under the rapid development of agricultural cultivation, water quality is a rising problem in many watersheds of Vietnam. Intensive agriculture has raised a public concern when huge amount of fertilizer and pesticides was used without proper management obviously making a severe impact to human health. In this study, Soil and Water analysis Tool (SWAT), a model is well designed for rural ungaged basin is used to simulate water quality problems in Cong watershed (Vietnam). Digital Elevation Model was provided by United States Geologic Survey agency with resolution of 90 m. Land cover was provided by Ministry of Agricultural and Rural Development (MARD) for 3 years 1996, 1999, 2003 classified from LANDSAT images. Digitized stream network and soil map (map scale 1:50000) were also obtained from MARD. Meteorological data was obtained from Ministry of Natural Resources and Environment (MONRE). Land operation and fertilizer information was provided by MARD and field surveys. Rain fall data (six stations with recording duration from 1961-2003) were processed by using Integrated Quantity and Quality Model (IQQM) while a back-calculation process was implemented to estimate the natural inflow to Nui Coc reservoir (at the outlet of upper Cong watershed). Baseflow filter was also used to determine the proportion of baseflow by using Baseflow program. Results from calibration and validation processes show fitted results between observed and simulated results on monthly basis, quite acceptable on daily basis for both natural period (1961 1970) and intensive agriculture period (1994 2001) as well as validation period (2002). Sediment and nitrate yield maps were extracted from the longterm simulation results (1994 2001). The highest sediment and nitrogen yield sub-basins were respectively indicated. SWAT simulation results suggested that sediment is not a severe problem with Nui Coc reservoir but nitrogen loads are the real threats. Moreover, the simulation points that high concentration of ammonia, which endangered Thai Nguyen downtown residents in relatively long period of time, occurred in Nui Coc reservoir. In brief, SWAT proved its ability in simulating the water quality problems in watershed level. It promised as a useful tool to assist water quality management process practically in Cong watershed.