Decoupling the influence of vegetation and climate on intra-annual variability in runoff in karst watersheds

Karst landscapes cover 7-12% of Earth's continental area, and approximately 25% of the world's population partially or completely relies on drinking water from karst aquifers. Water shortages are a challenge worldwide in karst mountainous landscapes. Knowledge of intra-annual variability in runoff and the potential drivers of variability is important for optimizing regional water resources use. The objectives of this study were to investigate temporal variations in the distribution of intra-annual runoff during 2003-2017 in six karst watersheds in southwest China and to identify the key drivers of these variations. The Gini coefficient and Lorentz asymmetry coefficient were used to represent intra-annual variability in runoff. Partial least squares-structural equation modeling (PLS-SEM) was used to decouple the effects of climate variables and vegetation dynamics on the distribution of intra-annual runoff. In all six watersheds, the Gini coefficient ranged from 0.15 to 0.59, with a mean value of greater than 1 for the Lorentz asymmetry coefficient. The heterogeneity of intra-annual runoff distribution showed a decreasing trend from 2003 to 2017. Climate variables and vegetation dynamics strongly influenced intra-annual variability in runoff and accounted for 19-63% and 17-67% of the variation in the Gini coefficient and Lorentz asymmetry coefficient, respectively. Vegetation was negatively correlated with the Gini coefficient, and the direct effect of climate on the Gini coefficient was greater than its indirect effect on the Gini coefficient through vegetation. As compared with traditional multivariate statistical methods, PLS-SEM provides additional valuable information, including information on the direct and indirect impacts of climate and vegetation on the distribution of intra-annual runoff. PLS-SEM is recommended as an effective approach for disentangling the coupled relationships between predictors and hydrological characteristics under different circumstances.