Hydrological connectivity regulates runoff dynamics on grass-biocrust covered slopes: mechanisms and an enhanced SCS-CN model

Slopes covered with both grass cover (GC) and biological soil crusts (BSC) play a critical role in influencing rainfall-runoff processes within semi-arid regions. However, the limited understanding of runoff mechanisms on slopes covered with both GC and BSC hinders the effective utilization of water resources. The SCS-CN model is limited by its neglect of hydrological connectivity, which reduces the accurate assessment of runoff processes over the complex slope surface. This study focused on slopes covered by combinations of GC and BSC. Simulated rainfall experiments were conducted in boxes (2 m×1 m) filled with loess soil, under a rainfall intensity of 90 mm h -1 . Close-range photogrammetry was used to reconstruct microtopography and quantify multidimensional hydrological connectivity indicators. The results showed that GC and BSC affected runoff parameters differently, with GC promoting infiltration and BSC increasing surface roughness and shortening flow length (FL). Consequently, the combination of GC-BSC significantly affected hydrological connectivity, with the FL decreased by 14.38%-40.15% compared to the bare soil, the topographic wetness index stabilized at 1.95, and the runoff coefficient in simplified hydrographs remained stable between 0.55 and 0.6. The original SCS-CN model shows significant deviations in runoff simulation for slopes covered with GC-BSC. In this study, the FL and the runoff retention coefficient ( k ) were incorporated into the SCS-CN model to redefine the potential maximum retention ( S ), and a calculation equation was constructed as S = S 0 + k FL. The revised model improved NSE from -19.92 to 0.75 and reduced RMSE by 87.86% for the validation dataset. This study revealed the regulatory mechanism of runoff generation by the combination of GC-BSC and provided a reliable scientific tool for ecological restoration in semi-arid regions.