A multi-scale assessment of the effects of runoff/sediment discharge in karst catchments as shown by vegetation-related remote sensing indicators

Remote sensing technology development has facilitated the emergence of numerous vegetation-related indicators, which may reflect the hydrological and sediment transport processes operating at different scales. However, few studies have investigated which vegetation-related indicator can best capture the variability in runoff and sediment discharge at multiple time scales, especially in ecologically fragile karst regions where the relationship between vegetation and hydrological process is highly complex and variable. To address this issue, five satellite-derived vegetation-related indicators (Normalized Difference Vegetation Index (NDVI), Kernel Normalized Difference Vegetation Index (kNDVI), Enhanced Vegetation Index (EVI), Leaf Area Index (LAI), and Solar-Induced chlorophyll Fluorescence (SIF)) were selected to assess their associations with runoff and sediment discharge across six karst catchments in Southwest China during 2003–2020, and these associations were conducted in nine karst catchments of France and Central United States to validate the findings of this study. The Mann–Kendall trend test results indicated a significant increasing trend in vegetation, whereas sediment discharge generally exhibited a decreasing trend. Continuous Wavelet Transform analysis detected significant cyclic oscillations in runoff (9-13 months) and vegetation-related indicators (8-16 months), but no persistent periodicity was observed for sediment discharge. The Cross Wavelet Transform and Wavelet Coherence analyses were then applied across all 15 karst catchments. The proportion of areas showing significant coherence between vegetation and runoff ranged from 4% to 33%, whereas this proportion between vegetation and sediment ranged from 6% to 29%. Subsequently, the technique for order preference by similarity to ideal solution was employed to evaluate the relative explanatory capacity of hydrological variables for vegetation-related indicators. The results identified SIF as the most effective explanatory variable for both river runoff and sediment in global karst areas. These findings provide new insights into the complex relationships between vegetation and hydrological processes in karst catchments and offer valuable guidance for improving water and soil resource management strategies.