Alteration Of Hydrologic Regime And Its Driving Factors Of The Yellow River Basin

The Yellow River Basin is the birthplace of the Chinese nation,an important ecological security barrier in China,and a major energy and grain production base and economic development region.In recent decades,the hydrological regimes of the Yellow River basin has been greatly altered by climate change,human activities and regional natural endowments.Therefore,the study of hydrological regimes in the Yellow River basin is expected to further enrich and develop the hydrological characteristics of the Yellow River basin,understand the mechanism of the hydrological cycle,and provide scientific basis for the rational use of water resources and ecological protection in the basin.Several statistical approaches were employed to investigate the temporal trends and regime shifts in the time series(1950-2019)of runoff and sediment load at hydrological stations in the Yellow River basin.The Mann-Kendall test was applied to identify their temporal trends,and the Pettitt test was used to detect the abrupt points on the runoff and sediment load.The IHA-RVA method was used to analyse the hydrological regimes at the hydrological stations in the Yellow River basin before and after anthropogenic disturbances,and to assess the degree of the hydrological indicators at each station.In order to detect the major cause for the decrease in runoff and sediment load,the elasticity of runoff from the Choudhury-Yang equation and the double mass curves were applied to quantitatively assess the effects of climate change and human activities.Changes in precipitation,potential evapotranspiration and vegetation(NDVI)were analysed to reveal their effects on the changes in the runoff and sediment load.The main findings are as follows:(1)The Yellow River basin was a heterogeneous source of runoff and sediment load,with uneven regional distribution,including the source area as well as the upper reaches.The inter-annual variation of runoff and sediment load was significant in the Yellow River basin,showing a decreasing trend of fluctuation from decade to decade.And the distribution of runoff and sediment load within the year was extremely uneven,mainly concentrated in the flood season,and the concentration of tributaries is highter than that of the main stream.Both the annual runoff and sediment discharge tended to decrease in the Yellow River basin,with a significant trends(p<0.05),and majority of hydrological stations experienced two abrupt changes.The change points in the mainstream were consistent with the timing of the construction of gigantic reservoirs.The abrupt changes of runoff and sediment load in the tributaries generally occurred in late 1970s and 1980s,with some tributaries experiencing a second abrupt change in the late 1990s.(2)The runoff depth in the sub-basins of the Yellow River basin decreased from south to north,with high values mainly distributed in the source area of the Yellow River,the upper reaches of the Tao River and Wei River,and the Datong River;the high values of sediment modulus were concentrated in the middle reaches of the Yellow River between Toudaoguai and Longmen stations.Both runoff depth and sediment modulus in the Yellow River basin showed a significant decreasing trend(p<0.05),with the average rate of reduction of sediment modulus in the middle reaches of the Yellow River exceeding 120 t/km~2/yr.Prior to the 1980s,soil erosion in the middle reaches of the Yellow River was extremely severe,with the sediment modulus exceeding 15000t/km~2/yr,whereas after 2010,the sediment modulus decreased by more than 80%,to5000 t/km~2/yr in all sub-basins.(3)Compared to the period 1960-1979,the hydrological indicators of flow in the Yellow River basin have generally reached a moderate or high degree of RVA since2000,but the degree of RVA in hydrological conditions varied from basin to basin.Except for the Yellow River source area,the average monthly flow of the main stream decreased in the the flood season and increased in the non-flood season;the annual extreme flow tended to mean,but the time of occurrence varies greatly between different zones.The hydrological regimes of the tributaries changed more dramatically than that of the mainstream.In addition to some months of individual stations,the average monthly flow and the annual extreme flow generally decreased,while the minimum flows varied considerably from tributary to tributary;the frequency and duration of high flows decreased;and the rate of change of flows generally decreased.(4)Climate change in the Yellow River Basin is characterised by a decrease in precipitation and an increase and decrease in potential evapotranspiration;vegetation cover has improved significantly.In most sub-basins,changes in substratum are the main driver of changes in runoff,followed by precipitation,with potential evapotranspiration contributing the least;the contribution of human activities to the reduction of sand transport is higher than 80%compared to runoff.In the Yellow River basin,especially on the Loess Plateau,soil and water conservation engineering measures such as terraces and check dams can regulate flow and alter the process of sediment production and transport in the basin,and have a significant impact on runoff and sediment.