Response of gully morphology and density to the spatial and rainy-season monthly variation of rainfall at the regional scale of the Chinese Loess Plateau

Gully erosion normally causes land degradation, especially on the Chinese Loess Plateau (CLP). However, research on gully spatial variations at larger regional scales, particularly considering the influence of rainfall, remains limited. The effects of monthly rainfall variations during the rainy season (June-September) on gully morphology and density have also been less explored. To narrow these gaps, we conducted a study in five representative catchments along a rainfall gradient in the CLP.Using high-resolution satellite imagery, we visually interpreted 2511 gullies and computed multiple gully morphological and density parameters, including length, area, volume, number density, length density and area density, etc. Long-term (1961-2014) monthly rainfall data and other environmental factors were analyzed to understand their impact on gully morphology and density.Results revealed that gully length, width and depth were crucial parameters representing gully morphology, exhibiting obvious variation across catchments. Generally, gully dimensions increased with higher rainfall, while densities decreased. Gully number density (GND) had a more significant impact on regional gully erosion degree than gully scales (length and area).Gully length and depth were primarily influenced by upslope runoff, while width mainly by runoff and potentially freezing and thawing. Rainfall in July and June-September, respectively, and especially the upslope catchment maximum slope were critical factors influencing gully head retreating and bottom incising, while June rainfall played a key role in sidewall collapsing. These result from the interplay of gully development sub-processes, monthly rainfall characteristics, and concurrent vegetation conditions. NDVI and July-September rainfall primarily determined GND at this scale, with the protective effect of rainfall-contributed vegetation outweighing rainfall's promotion on gully occurrence.Our study highlights the importance of tailored gully erosion control measures that consider monthly rainfall variations and the significance of vegetation restoration in controlling new gully formation in the CLP and similar regions with comparable climatic and underlying-surface features.