Study On The Hydrological Process Of The Earth Critical Zone In The Loess Plateau

After decades of comprehensive treatment of large ecological projects on the Loess Plateau,such as Grain for Green and Gully Land Consolidation projects,while the ecological barrier functions have been greatly improved,the natural evolution process of soil hydrology has been changed obviously.The dried soil layer has even been formed on the profile,hindering and even cutting off the groundwater recharge path,affecting the ecological functions of vegetation and the sustainable utilization of water resources.In the loess hilly and gully region with limited water resources,groundwater is very important for the growth of vegetation,residents’life,and economic development.However,thick loess,broken terrain,and strong human disturbance aggravate the complexity of the groundwater recharge process by rainfall,leading to its internal mechanism still being unclear.Therefore,after systematic scientific evaluation of the Gully Land Consolidation project,the soil deep profile→line→area→slope-gully system→watershed multi-scale research system were established at the treated and untreated watersheds（Yan’an Gutun treated gully and Luochuan Shuangchagou）.Long-term（2017-2021）,high-frequency,and high spatial resolution field in-situ observations were conducted to study soil water distribution pattern,rainfall infiltration recharge depth,dried soil layer restoration,vertical and lateral flow paths,and rainfall recharge process at different spatial-temporal scales.The results can not only provide data support and scientific basis for the sustainable utilization of water resources in the Loess Plateau,but also have important theoretical and practical significance for revealing the mechanism of water cycling at different scales.The main conclusions were as follows:（1）The scientific evaluation of the Gully Land Consolidation project is evaluated systematically.The Gully Land Consolidation project expanded the agricultural farmland area（the newly created land accounted for 7%of the sub-project area）,increased crop yield（the yield of 30%sub-project increased by 1.5 times）,and improved the surrounding ecological environment（the average NDVI increased by0.0376,the slope stability improved,and the farmland no/weak swamping）.There were no significant differences in clay,silt,sand,bulk density,and saturated water conductivity between treated and untreated gullies,but the organic carbon content in treated gullies was significantly lower than that in untreated gullies.The spatial dependence of soil physical and chemical properties in the gully was as follows:saturated water conductivity>bulk density>soil particle composition>organic carbon.（2）The spatial and temporal characteristics of soil water was clarified.The vertical distribution patterns of soil water in farmland,grassland,shrub land,and forest land were significantly different.The water consumption depth was the most serious in the 22-year-old Robinia pseudoacacia forest.Soil water infiltration of the Robinia pseudoacacia forest lasted from August（60 cm）to March（420 cm）,which could effectively alleviate the annual soil water deficit.The lateral water flow of rainy season along slope length exceeds the vertical infiltration.The existence of temporal stability horizon and point of soil water on slope.On the three-dimensional scale,the most stable horizon and point were 180 cm of H₁ and 220 cm of H₂₁,respectively.（3）The relationship between soil water infiltration depth and rainfall events was established,and the restoration processes of the dried soil layer was explored.The degree of soil water infiltration increased linearly with the increase in rainfall,and the response threshold was 16.0 mm.When the rainfall was 20～30 mm,all the observation sites were charged and the efficiency of converting rainfall into soil water was 39%.Following the variation degree of soil particle composition,the infiltration degree of soil water was midslope>upslope>downslope.Based on the stable field capacity,the average dried soil layer thickness and water content were 396 cm and 9.0%,respectively.The uneven distribution of rainfall events and the existence of microtopography explain the weak/moderate spatial variation and the strong temporal variation of the dried soil layer indices.（4）The response process of groundwater table fluctuation to rainfall events was illustrated.The annual fluctuation degree of groundwater table is positively correlated with the annual rainfall at Yan’an Gutun treated gully watershed,and only the rainstorm event（about 150 mm）could cause the short-term fluctuation of groundwater table.In normal years,the recharge of groundwater was dominated by the rainy season.In a wet year,the average annual rise of groundwater table was 133 cm,and the spatial heterogeneity was weakened.The annual variation of groundwater table is moderate at the Luochuan Shuangchagou watershed.The threshold of rainfall events to trigger the groundwater table response was 9.6 mm.The recharge range of an individual rainfall event to the groundwater system was 4～112 cm.The lag time of groundwater table response to rainfall events has spatial heterogeneity,and the heterogeneity decreases with the increase of annual rainfall.（5）Revealed the internal mechanism of the groundwater recharge process by rainfall.The local rainfall recharged the reservoir water,soil water,and groundwater.Vertically,the maximum recharge depth of rainfall on the sunny slope,shady slope,and gully was 260 cm,300 cm,and the diving surface,respectively.In a normal year,the lateral flow of 0～100 cm along the slope was formed in the rainy season.In a wet year,after the rainy season,a connecting recharge belt nearly 300 cm deep was formed along the slope length.There was hydraulic relation between deep soil water near and gully groundwater.On the slope,the dried soil layer caused by the project of Grain for Green project intermittently or continuously blocks the water migration to the deep layer,which forms the connection of rainfall,vegetation,and soil water.The Gully land consolidation project is implemented to increase soil water storage and raise groundwater tables,led to the rainfall rapidly recharged to groundwater by the combination of piston flow and preferential flow,forming the connection of rainfall,vegetation,soil water,and groundwater,and through the vertical region of the Earth’s critical zone on the Loess Plateau...