A multi-factor integrated method of calculation unit delineation for hydrological modeling in large mountainous basins

Hillslope-based distributed hydrological model has become an essential tool to simulate hydrological processes in mountainous areas, while how to properly delineate hillslope with key factors still remains to be answered. In this study, we propose a conceptually simple and computationally efficient method, the hillslope-asymmetry-elevation-band-aspect-based (HEA) delineation method, for large mountainous basins. Among these three factors, elevation band and hillslope aspect could represent the spatial heterogeneity of each hillslope in vertical and horizontal directions, respectively. More actual flow routing in each hillslope could be characterized due to the consideration of hillslope asymmetry and elevation band. The performance of HEA method is examined by conducting hydrological simulations with HEA-based basic calculation units (BCUs) in the Nu River basin in Southwest China. Simulated hydrographs agree well with the observations at different sites with Nash-Sutcliffe efficiency coefficient (NSE) greater than 0.75, indicating the HEA delineation method works well for the large mountainous basins. Further numerical experiments are carried out to quantitatively investigate the role of HEA delineation factors in influencing streamflow process and the contribution of homogeneity of underlying surface and meteorological forcing in influencing streamflow process in different aspects. The results show that: the total streamflow is overestimated (underestimated) without consideration of hillslope asymmetry (aspect); while it is overestimated (underestimated) in wet (dry) season without consideration of elevation band. In addition, reduced heterogeneity in underlying surfaces and meteorological forcing leads to underestimated streamflow in different aspects, of which about 80% and 20% can be attributed to underlying surfaces and meteorological forcing, respectively.