A tree-ring based streamflow reconstruction for the Taohe River (Gannan Plateau, China) associated with the Asian-Pacific Oscillation since 1325 CE

•We present the first tree-ring-based reconstruction of the Taohe River streamflow.•The instrumental record underestimates drought risk in the Taohe River.•Positive Asian-Pacific Oscillation increases the Taohe River streamflow. The Taohe River on the Gannan Plateau of the northeastern Qinghai-Tibetan Plateau (QTP) is the second largest tributary of the upper Yellow River. It drains a watershed of 25,527 km2, supporting 2.29 million people. Therefore, Taohe River streamflow is crucial for regional water supply, economic development, and ecosystem stability. However, the observed streamflow is too short to capture the full range of natural variation, which could hamper the assessment of hydrological risk under ongoing climate change. In this study, we presented the first tree-ring based Taohe River streamflow reconstruction, which explains 44% of the variance in instrumental data (1961–2016 CE) and provides a long-term perspective for assessing recent streamflow variations in the Taohe River. The reconstruction shows that the Taohe River had the highest streamflow during the 20th century, and severe long-term droughts have occurred frequently before. Thus, using instrumental streamflow records for water resources assessment may underestimate drought risk and misguide water resources management policies. Composite diagnostic analysis shows that Taohe River streamflow variation is influenced by the Asian-Pacific Oscillation (APO). A positive APO may strengthen southerlies in the lower troposphere at mid-latitudes in East Asia and increase upward air convection over the QTP and surrounding areas, which then increases precipitation and results in higher streamflow in the Taohe River. Furthermore, dramatic warming in QTP during the 20th century led to the positive phase of APO, which is likely a significant contributor to the recent unusually high streamflow in the Taohe River. This study presents a long-term perspective on Taohe River streamflow variation and provides a reference for the assessment of regional water security.