Key Processes Of Water Cycle In Check Dams In The Wind-Water Erosion Crisscross Region

Check dam is an important part of water cycle in small watershed and a significant area for food production in Loess Plateau,it is of great significance to accurately understand grasp the space-time characteristics of soil moisture shallow groundwater under different land uses in check dam,to clarify the soil water supply capacity of check dam scientifically guide the agricultural production regional ecological restoration of check dam.In this study,we choosed two check dams（A and B dams）in Liudaogou watershed,the spatial distribution of soil resistivity in check dam was explored by electrical resistivity tomography,the spatial distribution of shallow groundwater in check dam was analyzed after the relationship with soil moisture content was established,we also studied the dynamic change process of shallow groundwater in check dam ground and estimated shallow groundwater storage.By monitoring the change of soil moisture content in different land use types（cropland,grassland shrubland）in the check dam,explored the spatial temporal distribution characteristics of soil moisture in three land use types,calculated the soil water storage,analyzed the soil moisture consumption supplementary depth under three different land use types,determined the profit loss degree of soil moisture.At the same time,combined with meteorological data soil moisture data,the main water consumption depth of Salix matsudana was analyzed,the sap flow characteristics of Salix matsudana and Populus simonii Carr were explored by thermal diffusion method in check dam and the responses to environmental factors were analyzed.The conclusions of the study are as follows:（1）The soil resistivity of the check dam has increasesd,then decreased,then increased with depths.The upper soil resistivity is higher than the lower one,the upper soil resistivity has greater variability;the soil resistivity and soil moisture content showed a significant power function relationship.In the vertical direction,the soil moisture content of the shallow surface layer of the check dam showed that the upper layer is lower than the lower layer,the overall trend is to first decrease then increase with stratification.In the horizontal direction,the shallow surface groundwater content is gully tail>gully head>the middle.The groundwater storage in A and B check dams were seasonal changes,the end of winter to the following summer for soil moisture consumption period,late summer early winter for the accumulation period,summer water storage is the smallest,the value is 85,949,94,196m³,the winter water storage value reached the largest,its value is 93,486,99,309 m³.The groundwater storage in check dam is changing seasonally,the water storage is decreasing gradually from late winter to summer,the water storage is increasing sharply in late summer early autumn,the water storage in summer is the smallest,the water storage is the largest in winter.（2）The soil moisture content of farmland,grassland and shrubland in the check dam has obvious seasonal variation characteristics,which is synchronous with the change of precipitation has a certain lag.The order of soil water storage of the three land use types was farmland>grassland>shrubland.In normal water years,the water consumption of 0-200 cm soil was shrubland>grassland>farmland.In dry years,the consumption of soil water storage was grassland>farmland>shrubland.However,in the past two years,the soil water storage of all land use was negative.The range of soil water storage of 0-200 cm farmland at the end of season was-37.2-18.0 mm,that of grassland was-51.4-29.7 mm,that of shrubland was-122.9-8.0 mm.From the supplementary depth,farmland and grassland can be infiltrated to about 100 cm soil depth,while the soil water storage of shrubland can not be supplemented for a long time,the soil water storage in check dam is in a negative compensation state.In terms of vegetation configuration,grassland can be appropriately increased,the proportion of high water consumption shrubs can be reduced,agricultural production activities can be carried out on the premise of avoiding soil erosion.（3）There was a lag between sap flow initiation and solar radiation change.Soil moisture difference had no significant effect on sap flow initiation.The diurnal variation curves of sap flow density of Salix matsudana and Populus simonii Carr showed a double peak pattern.In order to reduce water loss,sap flow density of Salix matsudana at noon decreased significantly or even nearly stopped transpiration at high soil moisture content.From 2019 to 2020,the daily transpiration rate of Salix matsudana ranged from 2.0 kg kg·d^-1 to 2.2 kg·d^-1,that of Populus simonii Carr was about 1.6 times that of Salix matsudana,ranging from 3.2 kg·kg·d^-1 to 3.5 kg·d^-1.The daily sap flow rate of Salix matsudana and Populus simonii Carr was significantly positively correlated with solar radiation,saturated water vapor pressure difference air temperature（P<0.05）,the correlation degree from high to low was solar radiation>air temperature>vapour pressure deficit.In the growing season of 2019,the water storage consumption of Salix matsudana at 0-600 cm was-90.2 mm,the water storage at 100 cm and 200 cm was significantly positively correlated with the daily liquid flow rate（P<0.05）,which was an important source of transpiration water consumption.When transpiration is strong or shallow soil water storage is insufficient,Salix matsudana could use deeper soil water,even shallow groundwater.Considering the sustainable water resources management and regional water cycle changes,it is not suitable for planting deep-rooted high water consuming plants in check dam and its surrounding areas.