Variability and trend in the hydrology of the Yangtze River, China: Annual precipitation and runoff

The catchment of the Yangtze River in China has a long history of human occupation and the intensity of human impacts has increased markedly since economic reforms began in the late 1970s. In order to try to assess the impact of these changes on the hydrology of the river, we analyse both the annual flows of the Yangtze River and annual temperature and precipitation for the Yangtze catchment for the period 1955-2011 and for the three sections of the catchment, Upper, Middle and Lower as defined by the location of the gauging stations at Yichang, Hankou and Datong respectively. Mean annual temperature increases downstream from 12.7. °C in the Upper to 16.0. °C in the Lower section. A significant increasing trend in mean annual temperature is detected over the period 1955-2011 in the whole catchment and all subsections. Mean annual precipitation for the whole catchment is 1045. mm ranging from 859. mm in the elevated Upper section to 1528. mm in the Lower section. Precipitation variability is low by world standards with an annual Cv of 0.066. Using the Mann-Kendal and Rank Sums tests we do not find any trend in precipitation in the catchment. Mean annual runoff for the whole catchment is 515. mm ranging from 421. mm in the Upper Catchment to 838. mm in the Lower Catchment. Runoff variability is also low by world standards with an annual runoff Cv of 0.129. For the Middle Catchment we find a small but statistically significant increase in runoff and the runoff ratio over the period 1955-2011, possibly caused by change in the nature of the surface due to accelerated urbanization post 1980 and increased area of water storage. Overall, annual runoff in the Yangtze River shows little response to the major changes occurring in the basin. In a multiple correlation analysis of discharge, precipitation, dam volume, population and GDP, only precipitation is significantly correlated with discharge, explaining 80% of the variance. Widespread reporting of the impact of development on the annual water yield of the Yangtze, especially the impact of dams and notably the Three Gorges Dam, are not supported by this analysis.