Abstract: The effect of input date aggregation on flow simulation was studied at different time scale in the catchment upstream Guchengzi hydrological station of Nuomin river, upper reaches of Nenjiang river, using the Soil and Water Assessment Tool(SWAT) with six watershed delineations, each with a different number of subwatersheds. The results showed that overall accuracy of flow output over a monthly or annual time scale was not affected from watershed delineation, but the daily output that captures hydrologic events was affected by watershed delineation. Average flow at different time scale were not affected by changes in sub watershed size, but the runoff yield increased as the increase of subwatershed amount, and the relative error reached up to 35.4%. The model performance was sensitive to precipitation, and the better simulation accuracy for wet years (above average rainfall) compared with dry years (below average rainfall) for the study area. The watershed delineation had a more profound effect on flow simulation for dry years and summer. There was a reasonable watershed delineation level for distributed hydrological model, and the critical source area accounted for 2% of drainage area. Increasing the number of sub watersheds over the level of watershed delineation couldn't significantly affect the runoff yield. Besides, reducing the number of subwatersheds would create instability of runoff prediction, and it might not fulfill the demand of spatial pattern analysis.

Key words: Distributed hydrological model, SWAT, Watershed delineation, Runoff, Precipitation