The objective of the study is to assess the water quality improvement resulted from the rearrangement ofthe irrigation water supply systems at Mankyeong River Basin in Korea. There is a mixed type of watershed composed of urban and rural areas in the region. When the current intake of irrigation water installed at the Eo-u weir is moved downward to the confluence of Jeon-ju stream, it was expected that the water quality of main stream could be significantly improved