Water storage in reservoirs built from 1997 to 2014 significantly altered the calculated evapotranspiration trends over China

Global or regional mean evapotranspiration (ET), calculated as the difference between observed precipitation (P) and runoff (R), has been widely used as the standard method to investigate the long-term variability of ET and evaluate hydrological and climate models. The increasing number of reservoirs built in China has led to a considerable amount of water storage, which could significantly alter regional water budgets. This change in terrestrial water storage (S) because of the newly built reservoirs reduces the observed R but does not directly translate into ET. The present study is the first to quantify this reservoir effect on the estimated ET at the national and basin scale from 1997 to 2014 in China. The results show that the reservoir total storage capacity in China increased by 0.38x10(12)m(3) from 1997 to 2014. If this change in S is not considered, a significant increase in the calculated ET of 4.2% per decade is derived. However, after this change in S is taken into account, the calculated trend of ET decreases to almost zero for the period 1997 to 2014, which is consistent with the negligible changes in the determining factors for ET, including precipitation, surface incident solar radiation, and air temperature. The impact of reservoirs and the determining factors of ET are further explored in six major river basins of China, which confirmed our conclusions at the national scale. This study indicates that the impact of S due to reservoir construction on calculated ET trends must be considered.