| The source region of the Yellow River(SRYR)is located in the east of the Tibetan Plateau with semi-humid and semi-arid climate,and characterized by high elevation,extremely low temperature,wide-spread permafrost and seasonally frozen ground.Under the climate change effects,the discharge of the SRYR has undergone magnificant change in the past 6 decades.However,the main causes of discharge change in the SRYR and frozen ground roles in discharge change in the SRYR is a still unknow and debatable question.This article aims discuss these unsolved questions.Based on the Budyko framework,we analysed the causes of discharge inter-annual change.The catchment characterised parameter change of one-parameter Budyko type equations represents the catchment water balance change.We modified the traditional separation method for catchment characterised parameter change analyses,the variations of the catchment characterised parameter in the past 5 decades of the SRYR were obtained by 11-year moving average method.The step-wise regression method was used in the attribution analysis to select the explanation variables and determine the relationship between the explanation variables and discharge change of the SRYR.It found that climate change is the main controlling factor of discharge change in the SRYR,and the human activities impacts are negligible.In the two sub-basins,JM and TNH,where permafrost widely underlain,and the catchment characterized parameters were significantly correlated with maximum frozen depth(MFD).The analyses conclude that permafrost degradation will increase catchment discharge in somhow.For the characters change of runoff in different season,we choose JM catchment to investigate the response of rainy season runoff,snowmelt runoff and winter baseflow to climate change.Moving T-test was used to detect the time series change point,the results indicate change points of snowmelt runoff and winter baseflow are close to temperature change points.Winter baseflow is mainly controlled by rainy season runoff but frozen ground degradation alters the relationship between winter baseflow and rainy season runoff.Frozen ground degradation increased winter baseflow discharge but dedreased direct snowmelt runoff generation.The direct snowmelt runoff remained a same level between 1961 to 1997,but decreased 34.75%between 1997 to 2013.Since1980,the direct snowmelt runoff coefficient decreased with the decreasing freezing index.With the warming climate,decreasing accumulated snowfall and degradating frozen ground,we deduce that the spring floods could disappear in the future.A 2D groundwater infiltration and discharge model,with conductive-advective energy transport and with freezing and thawing processes,considering saturated and unsaturated zone was established by Sutra Ice model to simulate and analyse long-term frozen ground degradation impacts on groundwater discharge.The simulated results indicate that the effect of permafrost impermeability decreased with temperature increase and permafrost degradation,grounderwater discharge to slope decreased and discharge to the river bed increased.The slope discharge is positively correlated with evaporation,permafrost degradation decreased grounder discharge to slope which will increase runoff.Meanwhile,the results indicate the rate of river bed discharge to total groudwater dishcharge exponentally increase with time when the temperature increase rate remains the same.Embedding the obtained exponential equation into the modified ABCD model,we get a further modified model-A~*BC~*D~*in considering the hydrological effects of frozen ground degradation.A~*BC~*D~*model simulated changes of infiltration water distribution into the soil moisture and groundwater during frozen ground degradation period agreely,indicating the practicability of the A~*BC~*D~*model in cold regions.This article provided new opinions for scientifical explanation of discharge changes and permafrost degradation hydrological impacts in the SRYR. |
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