Effect of soil-rock structures on the characteristics of rainfall-runoff responses in epikarst zones
This study was conducted at the Puding Karst Ecosystem Research Station,Chinese Academy of Sciences.Three square columns were set up in the experiment to simulate different soil-rock structures.Each column has a cross-sectional area of 1 square meter and a height of 2 meters.The station is located in a warm,humid mid-subtropical monsoon climate zone,with an average annual rainfall of 1,315 mm and an average annual temperature of 15.1℃.The soil-rock structures in this area are diverse,mainly comprising limestone,dolomite,and associated soils.This study aims to reveal the impact of complex soil-rock structures on hydrological processes in the karst areas of Southwest China.In this study,physical models of three typical soil-rock structures(Column a:thin limestone soil over limestone blocks;Column b:thin limestone soil over dolomite gravel;Column c:thick soil over limestone gravel)were constructed,the underground runoff processes formed by natural rainfall were observed,and the water balance and runoff response characteristics under different soil-rock structures were compared.The research findings provide scientific insights into better understanding of the transformation and effective utilization of water resources in karst areas.Three square columns were used as experimental devices in this study to simulate different types of soil-rock structures(combinations of limestone,dolomite,overlying lime soil,and yellow soil).During natural rainfall events,evaporation and runoff of the soil columns with different soil-rock structures were compared and analyzed.Various methods of statistical analysis were employed to quantitatively assess the impact of these structures,revealing the hydrological processes of the complex soil-rock structures in epikarst zones.The main results are as follows.(1)Water Balance:As soil thickness increased(20 cm,23 cm,85 cm),evaporation increased and runoff decreased.Dolomite gravel(Column b)retained more water than limestone blocks(Column a),leading to higher evaporation and lower runoff.Seasonal runoff variability was significant in areas covered with thinner soil,in which flood and drought are prone to occur.(2)Rainfall-Runoff Relationship:Linear regression revealed a threshold-linear two-stage pattern.Runoff started after reaching a threshold,becoming linear beyond that.The increase of soil thickness raised the rainfall threshold for runoff formation.Column a had the highest runoff efficiency,while Column c had the lowest.The specific surface area of the gravel in Column b is larger,so its evaporation is twice as much as that of Column a,and the regression slope is lower.(3)Impact of Rainfall Patterns:Small-event rainfall(Patterns B and D)caused higher peaks in Column a,while large-event rainfall(Patterns A and C)caused higher peaks in Column b.Thicker soil in Column c delayed initial runoff and peak flow,especially in small events.Additionally,the initial runoff response time of soil-rock structures with thin soil layers depends on the rainfall process,while the initial runoff response time of soil-rock structures with thick soil layers is negatively correlated with soil moisture content.This study reveals the significant impact of different soil-rock structures on hydrological processes in the karst areas of Southwest China.The physical properties of soil-rock structures(e.g.,soil thickness and characteristics of underlying rocks)not only affect the water balance but also determine the rainfall-runoff relationship and runoff characteristics under different rainfall patterns.Firstly,the soil layer thickness is a key factor influencing water balance and underground runoff response.Thicker soil can significantly delay the response time of underground runoff,reduce peak flow,and increase the rainfall threshold required for runoff generation.Additionally,the physical properties of rock fractures affect the water balance,with dolomite gravel under the soil having a higher water retention capacity than limestone blocks,leading to higher evaporation losses and lower runoff.Lastly,the response of water balance and underground runoff to different rainfall patterns varies significantly with soil-rock structures.Low-fracture limestone blocks beneath the soil(fracture porosity of 8%)have high water conductivity,allowing small rainfall to easily infiltrate and form substantial peak flows.In contrast,dolomite gravel beneath the soil,with a higher water storage capacity(fracture porosity of 40%),facilitates the accumulation of infiltrated water during heavy rainfall,leading to larger peak flows.
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