Study on water and sand simulation in Loess Plateau considering slope difference of land surface

Most sediments in the Loess Plateau of Yellow River basin originate from the gullied-hilly loess terrain, with approximately 50% deriving from gully systems which is the dominant geomorphological features. Accurately simulating the water and sediment processes in this area remains challenging due to the intricate sediment generation mechanisms within the slope-gully-river cascading systems. This study presents an enhanced version of the physically-based distributed hydrological model WEP-SED to reflect the influence of topographic slope variations on sediment production and transport processes.

The WEP-SED employs a three-tiered hierarchical structure (slope-gully-river continuum) to simulate coupled water-sediment dynamics (Fig. 1), which includes splash erosion, runoff & overland flow erosion, conflux & erosion in slope-gully, gravity erosion, conflux & sediment transport, and conflux & sediment transport.In the new model, the contour band in the sub-watershed is changed to upper-middle-down slope band, which is designed to better resolve slope-dependent erosion dynamics. This spatial discretization methodology accounts for both hydrological flow paths and local slope gradients, enabling more precise representation of erosion processes across varying topographic conditions, especially the mechanism of seriously soil erosion in the steep slope terrain and sedimentation in the valley floor of the gully. The refined sediment transport mechanisms within each slope band are schematically depicted in Figure 2. The breakpoint for the three slope band is 10°, one is the first one from top to bottom, the other is the first one from bottom to top, where the slope is just change over 10°. In the upper gentle slope band, the splash erosion and runoff & overland flow erosion is considered; in the middle steep slope band, splash erosion, runoff & overland flow erosion, conflux & erosion in slope-gully, gravity erosion is considered; in the down gentle slope band, splash erosion, runoff & overland flow erosion, gravity erosion, conflux & sediment transport in gully and river is considered.

The enhanced model was implemented in the Nanxiaohe sub-watersheds to investigate erosion-sediment dynamics during seven flood events in August 2009. It indicates that the model performs a relatively good fitness in simulating the water and sediment processes, and reflects the erosion difference in seven flood events. According to the model simulation results, the middle steep slope band constituted the dominant sediment source (70%), followed sequentially by down gentle slope band (27%) and the Upper gentle slope band has the smallest contribution. Thus, the enhance model could reflect the slope impact on sediment erosion and transport in Loess Plateau, which could be used for the benefit evaluation of soil and water conservation engineering projects.

Fig 1. A schematic illustration of the model structs and principle of the WEP-SED model.

Fig.2 Schematic diagram of geomorphic unit division