Revealing the influence of topography and vegetation on hydrological processes using a stepwise modelling approach in cold alpine basins of the Mongolian Plateau

Abstract. Topography and vegetation are critical factors influencing catchment hydrology; however, their individual contributions are often underestimated in hydrological models. This limitation is particularly evident in cold, mountainous regions such as the Mongolian Plateau, where observational data are sparse. To address this, we employed a stepwise, top-down modelling strategy based on a flexible modelling framework to systematically assess the influence of topography and vegetation on hydrological processes in the Bogd Uliastai and Zavkhan Guulin river basins. Beginning with a lumped model (FLEXL), we successively integrated snow processes (FLEXL-S), topographic distribution (FLEXD), and finally, a landscape-based parameterization accounting for vegetation heterogeneity (FLEXT). Both FLEXD and FLEXT outperformed the lumped models in simulating runoff and snow water equivalent (SWE). Interestingly, FLEXT showed similar performance to FLEXD – likely due to limited vegetation heterogeneity – it offers more physically realistic parameterization by explicitly representing landscape units, suggesting its potential in more complex basins. The ratio of snowmelt runoff to streamflow was quantified as 23.6 % ± 0.7 % and 15.9 % ± 1.3 % in the Bogd Uliastai and Zavkhan Guulin river basins, respectively, with peaks in spring and a clear increase with elevation. At high elevations, runoff is primarily snowmelt-driven, resulting in delayed and gradual runoff, whereas lower elevations dominated by rainfall generate rapid runoff. Controlled by distinct dominant hydrological mechanisms, different landscape units contribute unequally to streamflow. This study underscores the pivotal roles of topography and vegetation in runoff generation and demonstrates the effectiveness of a stepwise modelling framework for improving hydrological understanding in cryospheric and data-scarce regions.