Eco-hydrological Functions Of Weathered Bedrock In The Loess Hilly Region

Weathered bedrock,an imperative transition zone connecting the overlying soils and the underlying fresh bedrock in the Earth’s critical zone,extends the water cycle from vegetation canopy to bedrock.However,their hydrological regulation functions have not received sufficient attention in loess hilly regions.In this study,the weathered bedrock in loess hilly regions was set as the research object.Using in-suit observation and simulation experiments,the spatial distribution and water movement characteristics of weathered bedrock in the hillslope were revealed,the bedrock groundwater evolution and soil moisture changes in response to rainfall in check dams were clarified,the hydrological regulation effects of impermeable layers on the profile hydrological processes were resolved,the conservation mechanisms of weathered bedrock on ecohydrological processes were analyzed,and a weathered bedrock-soil-plants ecohydrological model was finally constructed.This study would provide new indicators for the eco-environment evaluation of the“Returning Farmland to Forest Program”,which could also broaden the research category of the water cycle in the loess hilly critical zone.The main conclusions were:（1）Weathered bedrock was widely distributed under soils in hills,which had significant impacts on the profile hydrological processes.Drillhole investigation showed that the weathered thicknesses exceeded 5.0 m,which was closely related to slope position,geological topography,and weathering degrees.The infiltration and evaporation simulation experiments showed that with the increase of weathered bedrock thicknesses,the stable infiltration rate was increased linearly by 0.5-5.7 times,and the maximum evaporation intensity reached 15.4±2.2mm d^-1.Profile spatiotemporal moisture changes monitoring in woodland and shrubland hillslopes showed that weathered bedrock optimized the profile hydrological configuration.Rock moisture was about twice that of soil moisture,and the profile water storage increased linearly with weathered bedrock thicknesses.（2）Groundwater recharge and discharge processes above the weathered bedrock in check dams showed significant seasonal fluctuations,whose recharge time from cumulative rainfall was 69-250 days,while the discharge time caused by soil evaporation,vegetation transpiration,base flow and bedrock fissure flow was 66-774 days.A single rainfall event(>10 mm d^-1)could visually affect topsoil（<50 cm）moisture.Moreover,rainwater mainly infiltrated into soils and shallow groundwater through substrate inflow with an infiltration rate of 0.8-3.5 cm h^-1,whose changes were closely related to soil depth,land use types and rainfall intensity.Furthermore,under continuously decreasing annual rainfall scenarios（2018-2021）,soil water storage in check dams showed a fluctuating downward trend.At the end of the observation period,soil water storage and infiltration depth showed the trend of farmland<scrubland<grassland<shrub-grass mixture abandoned land.These results indicated that the shallow-rooted herbaceous might be ideal for sustainable water resources.（3）Weathered bedrock acted as impermeable layers,intercepting the precipitation in limited soil bodies,regulating vegetation transpiration and water consumption processes,and indirectly improving the seasonal rainfall utilization efficiency.The profile water storage per unit volume was directly related to the depth of impermeable layers,which were affected by the differences in overlying vegetation and precipitation events.Compared with the control group,soil water storage in alfalfa and caragana maximum increased by 20.4%and 6.6%under0.6 m restrictions.Moreover,the impermeable layers changed the allocation of above-and belowground biomass.Due to the differences in canopy structure and root configuration between alfalfa and caragana,there was the greatest effect on their growth at 0.6 m restrictions.The aboveground biomass decreased by 22.6%in alfalfa and 33.5%in caragana,while the belowground biomass changed by 138.7%in alfalfa and-66.7%in caragana,respectively.（4）Weathered bedrock had positive ecohydrological conservation functions,which improved the vegetation water supply ranges and variabilities under drought stress while storing excess water under abundant rainfall.Rock moisture was always higher than soil moisture,showing a significant stratification characteristic.The profile water storage had a positive linear relationship with weathered bedrock thicknesses.Compared to bulk soils,adding 20 cm and 50 cm weathered bedrock particles increased the profile water storage（0-170 cm）by 13.9%and 20.1%in alfalfa,while 5.3%and 12.2%in caragana,respectively.Moreover,with weathered bedrock thicknesses increasing,the leaf length,width,area,and water potential between alfalfa and caragana showed a significant increase trend,which further improved the water consumption and transpiring rates.（5）The weathered bedrock-soil-plants ecohydrological model described the circulation,transformation and balance processes of rock moisture,soil moisture,groundwater,plant water,surface water,and atmospheric water in loess hilly regions.By storing excess rainwater,hindering water infiltration loss and broadening the root activity space,weathered bedrock regulated the ecohydrological processes,which played the dual roles of alleviating soil desiccation and conserving water,providing new perspectives for coping with climate change.