The Potential Effect And Mechanism Of Land Use Patterns On Groundwater In The Loess Plateau

In tablelands with deep loess layers on the Loess Plateau,groundwater is an important source of water for production and living,and it accounts for a large proportion of the water use structure.However,the land use patterns of the Loess Plateau are quite different.There are a lot of studies on the relationship between land use patterns and the degree of soil desiccation,but the impact of land use patterns on the groundwater replenishment mechanism and groundwater quality is still unclear.In this study,we analyzed the soil moisture characteristics(vertical moisture curve,soil water storage,water deficit,desiccation level)under different land use patterns at a typical plateau area of the Loess Plateau.Furthermore,we used hydrogen and oxygen water stable isotopes to analyze the conversion relationship between precipitation,soil water and groundwater.We also analyzed the regional groundwater recharge processes based on the chloride ion mass balance method(CMB)and evaluated groundwater quality through the nitrate nitrogen content in the groundwater.The main findings are as follows:(1)Land use has a significant impact on soil moisture characteristics.The mean value of soil water content and soil water storage at 0-18 m ranged from agricultural land>low-age forest land(10-year-old)>high-age forest land(20,26-year-old).The water content changes of the three land types tended to be consistent when the soil depth was greater than 11 m,which indicates that the soil moisture content has little correlation with land types at this depth.The three land types showed different degrees of desiccation at different depths(0-3 m,3-11 m,11-18 m).The whole agricultural land was lightly dry except for soil layers at 3-11 m.Degree of desiccation of the young forest land showed a trend of first decreasing(light desiccation)and then increasing(medium desiccation),maintaining slight desiccation overall.The desiccation degree of the elderly forest land was regarded as serious,and the soil layers was deeply dry.The chemical intensity is greater than that of farmland and young forest land.Analysis of factors affecting soil moisture shows that vegetation type and soil texture can effectively affect moisture content.(2)Land use affects the transformation relationship between precipitation,soil water and groundwater on the Loess Plateau.Using hydrogen and oxygen stable isotope tracing method to analyze the isotopic characteristics of precipitation and groundwater in the study area,it is found that the water vapor sources of precipitation in the study area varied with seasons.The precipitation from July to September is affected by the Pacific Ocean air mass,and the winter is mainly affected by the Arctic Ocean transported by the westerly belt.The influence of water vapor and local evaporation.The time when groundwater is recharged by precipitation is mainly concentrated in the late summer and autumn,and precipitation in spring and winter does not contribute much to the restoration of groundwater.The isotopic composition between deep soil water and groundwater under different land types shows that the supply of groundwater from agricultural land is dominated by piston flow,while the water infiltration of apple forest land is coexisted by piston flow and preferential flow.(3)There are obvious differences in groundwater replenishment mechanisms and water quality for different land types.The average groundwater recharge of the farmland in the study area is 43.1 mm/a,accounting for 7.5% of the average annual precipitation.While,the average recharge of the young orchard and the old orchard are 29.6 mm/a、21.5 mm/a,respectively,accounting for 5.2% and 3.7%of the average annual precipitation.The main channel of groundwater replenishment for farmland is piston flow,which accounts for 84.6%of the total recharge.The recharge channel for old orchard is both plug flow(56.3%)and priority flow(43.7%).Among them,as the forest age increases,preferential flow proportion increases.It takes about 13-95 years for precipitation to reach the groundwater layer from the surface using the tritium tracer and the average soil moisture infiltration rate,suggesting that the land use changes in the past ten years have not affected the groundwater layer.But if the hysteresis persists,it will definitely be affected.The sustainability of groundwater has a serious impact.The average nitrate-nitrogen content and the annual nitrate-nitrogen input of groundwater under different land use patterns are represented as agricultural land<low-age forest land<old-age forest land,suggesting that the existence of a large number of elderly apple forests on the Loess Plateau will lead to a decrease in groundwater recharge and a change in groundwater quality.It will create a deep impact on the regional water cycle.The area of aging forests must be controlled to maintain the hydraulic budget between precipitation and groundwater and promote the sustainable development of regional ecology.