Surface water isoscapes (δ18O and δ2H) reveal dual effects of damming and drought on the Yangtze River water cycles

Abstract

The water regimes and biogeochemical cycles are remarkably affected by river damming and drought around the world. However, the dual effects on river water cycles have not yet attracted sufficient attention along the Yangtze River because of the relative consistency of annual water regimes. This study first explored the altered water cycle associated with damming and drought using surface water isoscapes (δ18O and δ2H) of river and lake water along the Yangtze River during the flooding seasons of 2016 and 2018, combined with other historical isotopic data from 2003 to 2013. The results show that river water isotopes are markedly varied on both spatial and temporal scales. The relatively more enriched river water representing δ18O values were observed from 2003 to 2007 compared with those in 2018, which were possibly caused by increased evaporative fractionation under abnormal drought conditions in the Yangtze River. Spatially, river water δ18O values exhibited a positive trend from upstream to the lower reaches because of the contrasting recharged sources, complex mixing, and evaporation from upstream to the lower reaches of the Yangtze River. This trend was related to variations in surface water temperature and water evaporation during the flooding seasons evidenced by a significant correlation between river isotopes and river water temperature. Moreover, the large variability in deuterium-excess (d-excess) and δ18O in the downstream could be inherited from the upstream because of short residence time and incomplete mixing in the Three Gorges Reservoir (TGR) during the initial stage of the Three Gorges Dam (TGD) operation (2003–2007). In contrast, the relatively lower variability of d-excess and δ18O was found in the downstream area due to the complete mixing in the TGR, causing the homogenization of reservoir water during the normal operation stage after 2010 (e.g., 2013 and 2018). Drought stimulates evaporative fractionation on river water and result in more negative d-excess values especially in 2006. The dual effects of river damming and drought remarkably changed the longitudinal hydrological connectivity and water cycle dynamics by prolonging the residence time and enhancing the water mixing, which should be thoroughly considered for water resource management, especially for regions with heavily-regulated rivers.