Study On Runoff Generation Mechanism In Xitaizi Experimental Watershed In Sub-humid North China

Semi-humid montainous area is important water conservation area of North China.It has unique charateristics including complex climatic and underlying properties with active interactions between hydrological and energy processes.Research on hydrological mechanisms in this area is of vital importance,which would guide water resources management in North China.Runoff generation is key part of hydrological processes.In this study,an experimental headwater catchment is established in a typical semi-humid mountainous forest of North China and long-term field measurements of hydrological processes have been conducted.Based on experimental observations,long-term and event scale rainfall-runoff response characteristics and corresponding controlling factors have been analyzed,hydrograph separations using stable isotope tracers have been conducted,and a storm runoff conceptual model have been established to characterize storm runoff pathways and mechanisms.Based on numerical modeling and statistics methods,impact of rainfall and hydraulic conductivity heterogeneity on Hortonian overland flow patterns have been analyzed,possibilities of Hortonian overland flow occurrence have been estimated in different climatic and underlying occasions.In sum,runoff generation mechanism in semi-humid mountainous catchment have been revealed based on above multi-scale measurements and numerical modeling.Hydrographs in most storm events showed typical ly thin and high characters,the rising and fallings limbs were mostly meristic,the lag time between rainfall peak and flow peak was short and sometimes zero.Only 1-2 storm events could eventually increase baseflow after storm ceased.The temporal trends of groundwater depth and baseflow were similar,soil moisture c ontent responded more frequently than groundwater,the groundwater system in study area was rarely-discharged.Temporal variability of rainfall isotopes was larger than stream water and groundwater,the stable isotopic compositions of groundwater and stream water were very similar.Two-component hydrograph separation results showed that peak discharge in storm runoff was dominated by new water while baseflow consisted of stably released groundwater.Peak runoff generation mechanism is variable source area flow in most rainfall events.Saturated zone generatin surface flow distributed in riparian zone with an area less than 1% of total catchment area.Because of high permeability and low rainfall intensity,in more than 95% of total events,Hortonian overland flow couldn't occur in the study area.Runoff generation mechanisms in semi-humid mountainous catchment can be summarized as two aspects: when rainfall intensity and duration are lower than certain threshold,surface runoff generation is consisted of grou ndwater runoff,direct rainfall on channel and saturation overland flow,run-on infiltration happens frequently in the area,storm peak discharge is mainly consisted of direct rainfall and variable source area flow.In extreme storm events,when rainfall intensity meets the threshold of Hortonian overland flow occurrence,runoff generation mechanism is mainly groundwater baseflow and Hortonian overland flow,peak discharge is dominated by rainfall processes.