The Mechanisms Investigation For Vadose Hydrological Processes Response To Precipitation Events In A Typical Upper Unsaturated Zone,South China

Global carbonate karst area is about 22 million km2,and roughly 20-25%of the global population depends largely or entirely on karst groundwaters for potable water supply.However,the karst distribution areas of China is up 3.44 million km2,which occur over one third of China territory.The krast groundwater resource of China is abandunt,which occupies-25%of total groundwater resource.The karst resources are coming under increasing pressure from social and economic development and ecoenvironmental issues.In recent years,the extreme drought events in Southwest China frequently occur and flooding is serious.Climate change gives great challenges on karst hydrology and water resources.The investigation of karst hydrological processes in the vadose zones have profound implications for better understanding of contaminant transport regimes,karst water resource protection and sustainable utlization.Karst flow systems are associated with a high level of heterogeneity encountered in karstified rocks due to uneven permeability development,which increase the difficulties of the research of hydrological recharge processes.Karst caves are ideal sites to obtain deep insight of the hydrological processes in upper unsaturated zone and understand the relationship between precipitation and drip discharge.In particularly,karst caves provide favorable conditions for investigating a series of hydrological recharge processes and response machanisms,including epikarst infiltration recharge,percolation,transfer and storage capacity.High-resolution monitoring of potential evapotranspiration,soil moisture and vadose cave drip water was conducted in a typical karst upper unsaturated zone,South China to investigate hydrological response processes and estimate the effect of climate change on karst hydrology and water resources.The primary conclusions have been drawn as follows:(1)Changes in potential evapotranspiration ET0 have significant inter-annual,annual and seasonal variation trend.The annual average ET0 in Guilin city is 1137.9 mm from 1951 to 2015.A significant decreasing trends in annual ET0 were observed at the rate of 8.02 mm per decade.Decreasing ET0 trends had been observed mainly in summer,autumn and winter season,while a moderate increasing ET0 trends in spring season.The significant ngative trends of summer ET0 was the primary factor contributing to significant declining rate of annual ET0.Trends in sunshine duration were found to be the main cause of decreasing trends in potential evaporation.Significant trends were identified using the Mann-Kendall test,and the result showed that ET0 mutation occurred approximately on 1967 and 2003 for the period 1951-2015.(2)The variations of soil profile moisture exhibite a distinctive stratification which can be divided into topsoil layer(from surface to the depth of 35 cm),transition layer(depth of from 35 cm to 85 cm)and stable layer(depths ranging from 85 cm to 115 cm).The average values of soil profile volumetric moisture content increase as the depth,characterised by 140 cm>40 cm>90 cm>20 cm.The maximum value appears on the interface between soil and bedrock,and the surface soil moisture content is the lowest.The variations of soil profile moisture are related to climate condition,evaporation and medium permeability differences controlled by communicating with the interface of soil-atmosphere,soil-vegetation,soil-bedrock.(3)The response of soil profile moisture to precipitation was mainly influenced by prior soil moisture conditions.The response processes can be divided into three stages,including moisture absorption process,saturation process,percolation and infiltration process.Absorption process is the primary factor to determine the precipitation threshold and lag time.The 6 mm threshold required to activate soil moisture response in dry season.The response time for surface soil moisture rangs from 0.25?12.5 h.(4)The results of tracer test and drip hydrochemical tracer showed that the XY5 drip site connected with surface by a single conduit.The dripping water of Xiaoyan Cave is mainly recharged by two different flow components,conduit flow characterised by concentrated recharge and diffuse flow.The hydrochemical indexes and isotope compositions of drip water are controlled by distinct runoff types resulted from karst aquifer structure.Conduit flow showed high peaking discharge,higher hydrochemical variations but buffered isotope responses,which was attributed to homogenization vis mixing of meteoric waters in the unsaturated zone above the Xiaoyan Cave.The hydrochemical dynamic variation of conduit flow is primarily related to soil CO2 effect and precipitation effect.The sources of vadose diffuse flow are smaller fracture,soil and epikarst reservoir water with the attenuation of the isotopic fluctuations and hydrological variations are attributed to the magnitude of water rock interaction and residence time,indicating mixed signals of infiltration water in the epikarst storage reservoir.(5)Hydrological clasification of XY5 drip water was identified based on the relationship between maximum drip discharge and coefficient of variation,the result showed that three drip types appeared in XY5 drip site in different periods,including seepage flow,vadose flow and conduit flow.However,the timing of drip type will change as the variations of climate conditions(precipitation amount)and aquifer water condition of karst unsaturated zone.(6)The response relationship of XY5 drip water in Xiaoyan Cave is non-linear.A threshold is critical factor for arising the drpping water response to the precipitation,which is attributed to the pre-event karst aquifer hydrological conditions.74.4 mm rainfall was the threshold for the first drip discharge response from dry to early rainy season,indicating that soil and epikarst reservoir have considerable storage and regulation capacity.The baseflow amount derived from seepage or infiltration flow is 6.52 m3 from dry to early rainy season,which can maintain dripping water for 69.7 days.A threshold of flow generation expressed by precipitation amount must be exceeded as a function of excessing the field capacity of soil and residual capacity of epikarst zone after a long time drought,with the result of a 10-hour rapid response in wet season,while a 9.8 day lagged-time response in dry season.The peak discharge of drip water response to storm event in rainy season is up to 64.74 m3 with the duration time of 41 h,which accounts for?82%of total drip discharge.(7)Vadose water amount is in water balance state for Xiaoyan Cave in annual time scale.The changes of climate conditions mianly reflect in the variations of input and output items.However,the variations of storm flow or conduit flow is larger than baseflow composition for Xiaoyan Cave,arised from intense rainfall and rainstorm events.The baseflow occupies approximately 25?40%of total annual discharge for the upper unsaturated zone above the Xiaoyan Cave.The soil and epikarst reservoir can continuously maintain drip baseflow above 5 months.The correlation analysis between drip discharge and soil profile moisture showed that correlation coefficient for the 140 cm soil moisture and drip discharge is largest,indicating that the drip discharge in dry season mainly originates from the 140 cm depth of soil moisture.(8)Recession curve of XY5 drip discharge is exponential function,which can be divided into three recession segments.The recession rate of ? segment is faster,and the recession coefficient is 0.087,with the duration of 15 h.This segment can be interpreted as fast recharge arised from intense rainfall and rainstorm.The water of ? segment flowed out in 35 h with the recession coefficient of 0.027.The main source is fractures and soil moisture with a lower water flow velocity.The drip discharge keeps stable in ? segment as slow flow,and the recession coefficient is 0.006.The water source mainly derived from soil and epikarst reservior as slower infiltration flow.The average recession coefficient is obtained by master recession analysis with the value of 0.012 for the unsaturated zone in Xiaoyan Cave.However,the recession coefficient of infiltration flow in dry season is 0.0003,which represent the average storage capacity of the epikarst reservoir.(9)The recession curves of drip discharge and soil moisture for the depth of 20 cm,40 cm and 90 cm all are exponential function.The recession coefficient for 20 cm,90 cm soil moisture is 0.019,0.012,respectively.The recession lag time of soil moisture content at 90 cm relative to surface soil is 24.25 hours,while the lag time of 26 hours for drip water.