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To investigate the hydrologic impacts of land use changes on streamflow for an urbanizing watershed with paddy fields, a grid-based daily hydrologic model was adopted. The model was calibrated with two years (2000-2001) of observed streamflow data and validated using five months (2001) of measured soil moisture data and one year (2002) observed streamflow data. After model test, the model was run to estimate impacts of urbanization on each hydrologic component with the land use data sets for 1986, 1994, and 2002 generated from Landsat TM satellite imageries. Total runoff increased from 41.4 percent (1986) to 44.9 percent (2002) for 5.4 percent increase in urban areas, implying direct runoff increase exceeding baseflow decrease. Urbanization affected the proportions of direct runoff for each land use category more than the change in total runoff and the ratio direct runoff to total runoff. The change ratios in direct runoff for urban, paddy, and forest were 14.3 percent, -9.8 percent and -6.7 percent for 5.4 percent increase, 4.6 percent and 3.4 percent decrease in each land use area, respectively. The results indicate that paddy field plays an important role for runoff regulations, and the evaluation method can assist regional policy makers in developing land management strategies that minimize hydrologic impacts on streamflow.