The impact of watershed delineation on hydrology and water quality simulation.

Flood routing is a significant calculation for predicting watershed responses, involving discharge and pollutant exports. The computation of flow routing is highly relative to the relationship between downstream and upstream subbasins. A watershed could always be divided into several subbasins based on its topography and stream distribution. How detailed of the delineation of the stream distribution in a watershed would influence the modeling accuracy of flow routing and the prediction of watershed responses. The objective of this work was to discuss the effect of watershed delineation on runoff and pollutant transport predictions. When the number of divided subbasins increases, the stream distribution could be delineated more clearly. The case area was usually regarded as two subbasins only. In the present study, the case area was divided into 43, 25, 15 and 9 subbasins respectively. If the modeling result under 43 subdivisions is assumed to be the actual situation, the relative error of runoff simulation due to the simplified delineation of stream distribution is only around 25% in two subdivisions. However, the relative error of suspended solids (SS), total nitrogen (TN) and orthophosphate (Orth-P) simulation can reach 85%, 71 and 70% in two subdivisions respectively. The uncertainties or errors induced by too much simplification of watershed delineation could be carried over and amplified to the pollutant transport process and the modeling results of pollutant exports.