Effects Of Exposed Rocks On Soil Water Redistribution Around Rocks Under Different Land Use Patterns In Karst Area

Karst areas are ecologically fragile,with shallow soils,and their special dual hydrological spatial structure of surface and subsurface,resulting in the occurrence of"karst drought"in southwest China despite abundant rainfall.There are a large number of outcrops in karst areas,which have an important influence on the redistribution of rainfall,soil moisture infiltration around the rocks and moisture redistribution.In order to investigate the ecohydrological effects of rock outcrops and their influence mechanisms,this study used outcrops≤2.5 m in height at the Huanjiang Karst Ecosystem Observation and Research Station of the Chinese Academy of Sciences in northwest Guizhou to analyze the runoff characteristics of rock outcrops under different vegetation types（tree forest,shrub forest,pure forest,grassland and de-forested land）and different rainfall amounts,and used the bright blue tracer technique and single-loop infiltration method to explore The study was carried out to investigate the reverse water lifting effect of exposed rocks and soil water transport characteristics around the rocks,combined with the monitoring of soil water content at different soil depths and distances（distance from exposed rocks）after rain and under dry conditions（no rain for 5 days）to reveal the influence of exposed rocks on soil hydrological processes around the rocks.The results of the study can provide theoretical basis for soil and water conservation,ecological restoration and economic construction in karst areas,and the main results are as follows:（1）The rainfall in the study area from November 1,2021 to October 31,2022 was1473.4 mm,which was mainly concentrated in April-June,accounting for 60.22%of the rainfall during the whole monitoring period.The total runoff depth of the 37monitored rock faces was 651.29 mm,and the average rainfall-runoff conversion coefficient was 0.46.There were differences in the runoff output characteristics of the exposed rock faces under different rainfall and vegetation conditions:in both the rainy and dry seasons,the runoff depth of the rock faces under different rainfall amounts showed that heavy rain>moderate rain>light rain,and the runoff depth of the rock faces increased significantly with increasing rainfall in the rainy season（P<0.05）;the output of rock surface runoff was greatest in grassland in the rainy season,in scrub in the dry season,and lower in arboreal and pure forest in the dry/rainy season.The rainfall-runoff conversion coefficients were greater in the wet season than in the dry season,0.51 in the wet season and 0.42 in the dry season;the rainfall-runoff conversion coefficients were greater in the wet season than in the dry season;the rainfall-runoff conversion coefficients were greatest in the scrub in both the dry/rainy seasons,lowest in the de-wooded areas in the wet season,and lowest in the arboreal forests in the dry season.（2）The nutrient content of rock surface runoff under different rainfall levels is in the following order:light rain>medium rain>heavy rain,and the nutrient content of rock surface runoff decreases with increasing rainfall.5 types of land use types have the highest TOC,Mg²⁺and K⁺content of rock surface runoff from scrub,the highest Ca²⁺,TN and NO₃^-content from tree forest,the highest NH₄⁺content from deforestation,and the lowest nutrient content from rock surface runoff from grassland.The nutrient contents of penetrating rainfall increased after being transformed into runoff from the exposed rock surface,with Ca²⁺and K⁺contents increasing the most.（3）The staining depth of the soil around the exposed rocks could reach 85 cm,mainly concentrated in the 0?40 cm soil layer.The staining area ratio was at20.37%?59.36%and was highest in the 0?10 cm soil layer,with a mean variation of69.30%?99.60%;the staining area ratio decreased with increasing soil depth.Overall,soil preferential flow development showed an enhanced trend with increasing distance from the outcropping rocks.Soil infiltration rate first decreased with time and then stabilized,with the highest infiltration rate in the first 10 min and reached steady infiltration at 20?90 min（4.24?63.38 mm-min-1）,and the Kostiakov model was the best fit for the infiltration process.The stable infiltration rate of rock foot（RF）was lower than that of non-rock foot（NRF）,and the stable infiltration rate of RF was significantly positively correlated with the roughness of exposed rock surface（P<0.01）,negatively correlated with the soil capacity of 0?10 cm soil layer（S1）and 10?20 cm soil layer（S2）,and negatively correlated with the maximum water holding capacity and capillary water holding capacity of S1 and the maximum water holding capacity of S2.The maximum water holding capacity of S1,maximum water holding capacity of S2,non-capillary porosity and total porosity were significantly positively correlated（P<0.05）.The height of water transported upward by gravity from the exposed rocks was 8.14?15.84 cm,and the ascending process was fast in the first 5 h.After 5 h,the ascending speed gradually decreased until the ascending height was constant.（4）The soil water content around the exposed rocks varied from 30.1%?84.15%after rain to 21.40%?69.13%during drought,and the soil water content after rain was about 1.37 times that of drought.With the increase of soil depth and distance from the exposed rocks,the soil water content of both rain and drought showed a gradual decrease,and its maximum value appeared in the range of 0?30 cm from the exposed rocks,and with the continuous drought,the soil water content of S1 soil layer would gradually decrease,and the soil water content of S2 and 20?30 cm soil layer（S3）would first decrease and then increase.The soil water content of S2 and 20?30 cm soil layer（S3）will first decrease and then increase.Soil water content around exposed rocks is significantly influenced（P<0.05）by soil physicochemical properties and rock surface runoff from exposed rocks,with capillary water holding capacity as the most influential factor after rain and total porosity as the most influential factor during drought;the influence of rock surface runoff on soil water content decreases with increasing soil depth,and recent rock surface runoff output has a greater influence on soil water content.（5）In the area covered by exposed rocks（0?50 cm from the rocks）,the soil water content of S1 soil layer gradually increases as the distance from the exposed rocks becomes farther,and S2 and S3 gradually decrease;in the non-covered area（50?100cm from the rocks）,the soil water content decreases as the depth of the soil layer increases,and the soil water content increases as the distance from the exposed rocks becomes farther.The soil water content increases as the distance from the exposed rock increases.Under the rock cover condition,rainfall was only positively correlated with soil water content of S1 soil layer（P<0.05）,but its effect on the lower soil layers（S2and S3）was smaller.The above results show that the exposed rocks play an important role in the rainfall process in different land use methods,and the rocks can take up 46%of the rainfall on the rock surface to form runoff and carry nutrients to the soil.The results of the study have some significance for solving the problem of"karst drought"and ecological construction in karst areas.