Temporal variation in summer monsoon intensity since 1873 and its influence on runoff in the drainage area between Hekouzhen and Longmen, Yellow River basin, China

Controlled by continental monsoon climate, runoff of the Yellow River shows large temporal variability. How runoff responds to the changing summer monsoon intensity is important both in theory and in water resources management. The earliest hydrological observations on the Yellow River started in 1919, and thus, runoff data are available only for the past 90 years. Using data of the summer monsoon intensity (SMI), the temporal variation of SMI since 1873 and the resultant variation in runoff are dealt with for the drainage area between Hekouzhen and Longmen, Yellow River basin. At the 128-year scale, the variation of SMI can be generalized as a trend of decreasing first, then increasing and then decreasing again. At the time scale of 50 years for which concurrent data are available, the temporal variations in both SMI and precipitation show some decreasing trend. At the time scale of 80 years for which concurrent data were available, the temporal variations in SMI and natural runoff also show a decreasing trend. For 5-year moving averages of these variables, the synchronous trend of variation is more obvious. This fact indicates that the variation in SMI results in the variations in precipitation and runoff in the study area. Based on data of SMI and natural runoff for the period from 1920 to 2000, a regression equation between the 5-year moving averages of SMI (SMI 5m ) and natural runoff ( Q wn,5m ) was established, which shows that 72% of the variation in Q wn,5m can be explained by the variation in SMI 5m . Using this regression equation, Q wn,5m for the period from 1873 to 1919 was reconstructed. At the 128-year scale, the natural runoff from the study area shows a trend of decreasing first, then increasing and then decreasing again, in response to the similar variation in the summer monsoon intensity.