Contributions of Climate Change and Human Activities to Changes in Base Flow and Direct Runoff in the Huai River Basin, China

Neglecting the changes in runoff components, i.e., base flow ($Q_b$) and direct runoff ($Q_d$), may lead to inaccurate estimations of the relative influence of climate change and human activities on total runoff ($Q$). To evaluate this impact, the base-flow Budyko function was used to quantify the contributions of climate change and human activities to the changes in $Q_b$ and $Q_d$ in the Upper and Middle Reach of the Huai River, China. More than five decades (1950s−2018) of continuous daily streamflow observation data from nine subcatchments were analyzed. Results show that the change points of total runoff generally occurred in the 1980s−1990s. Compared to the prechange period, the climate change-induced $Q_b$ and $Q_d$ changes were ${-11.6-18.4\mathrm{mm}·\mathrm{yr}}^{-1}$ and ${48-78.4\mathrm{mm}·\mathrm{yr}}^{-1}$, respectively, in the postchange period; the human activities-induced $Q_b$ and $Q_d$ changes were ${-133.3-65.7\mathrm{mm}·\mathrm{yr}}^{-1}$ and ${-127.1-43.1\mathrm{mm}·\mathrm{yr}}^{-1}$, respectively. The contributions of human activities to $Q$ changes at Dapoling, Huangchuan, and Huangweihe increased from 26.2%, 24.4%, and 94.3% (when ignoring changes in runoff components) to 50.5%, 85.5%, and 94.4% (when considering changes in runoff components), respectively. Our results demonstrated that ignoring the runoff component change could result in underestimating the relative contribution of human activities and overestimating that of climate change. This implies that the impact of human activities on total runoff change may be more intensive than currently recognized. This study emphasizes the importance of investigating runoff component changes when conducting runoff change attribution analysis.