Contrasting runoff response times regulated by vegetation and climate changes in typical dry and wet basins

Study Region: The Hailar River Basin in northeastern China, the Xijiang River Basin in southern China, and the Dongjiang River Basin in southeastern China.
Study Focus: Changes in runoff timing under concurrent climate change and large-scale afforestation are still poorly understood in typical catchments. This study analyses four decades of daily streamflow together with reanalysis climate data and satellite-derived forest cover in the three basins, all affected by major afforestation projects. Center timing, flood timing and spring flood timing indices are derived using circular statistics, and generalized additive models are applied to quantify nonlinear relationships between timing indices and hydroclimatic and vegetation variables and to separate the relative contributions of climate variability and forest expansion.
New Hydrological Insights for the Region: Runoff time has shifted significantly under the combined influence of climate change and forest expansion, with changes in flood timing being more pronounced than shifts in mean runoff timing. Afforestation delays flood timing much more strongly in the  semi-arid basin than in the humid, whereas in humid regions earlier and more concentrated precipitation can still advance floods. Changes in precipitation regime and antecedent soil moisture emerge as primary controls on center timing and spring flood timing. These findings highlight that afforestation policies must be tailored to local hydroclimatic context, while implementing it more cautiously in water-scarce basins to balance flood mitigation against water availability.