Karst Slope Research Articles

The characteristics and controlling factors of facies-controlled coastal eogenetic karst: insights from the Fourth Member of Neoproterozoic Dengying Formation, Central Sichuan Basin, China

Karst porosity system is widely developed in the Fourth Member of Neoproterozoic Dengying Formation in Central Sichuan Basin of China. The result of the previous exploration in this area based on the theory of telogenetic karst, which mainly focused on the fracture/fault belt especially in the karst slope, leaves much to be desired. Though a systematically geological research in this study, it is found that the dissolution holes are mainly concentrated on the mud mound deposits and granular rocks. Both of them have a greater primitive porosity and permeability than the micrite that originally formed in low-energy environments, which is facies controlled. On the basis of the above and the fact that a tectonic movement has caused a massive subaerial exposure of the formation that only experienced a shallow burial, it is proposed that the paleokarst porosity system is the product of eogenetic karst. Two-stage eogenetic karsts have been interpreted: the syngenetic exposure caused by the short-term relative sea-level fluctuation and the exposure caused by the uplift of the second phase of Tongwan tectonic movement. The meteoric alteration in syngenetic period is commonly marked by the development of intergranular pores but on a small scale. It is believed to be important for providing a good pore-permeability foundation for the later eogenetic karst. The later exposure occurs immediately at the end of the deposition of the Fourth Member of Dengying Formation that only experienced a shallow burial. Since the effect of compaction and cementation in this shallow burial before uplift is relatively weak, only the micrite has been densified and retaining the majority of original pores in the carbonate mounds and grain shoals as paths for karst flow. Moreover, combined with the geological background and seismic profile, it can be ascertained that the karst occurs in a coastal environment. Twelve meteoric lenses have been interpreted after recovering the karst palaeogeomorphology. The scale of the meteoric lens and the reservoir performance have shown a decreasing tendency from the coastal to the inland area, which also confirms this view. The results of this research have put emphasis on the lithologic differentiation in eogenetic karst and the geographical location of karst development thus might provide a reference for further reservoir prediction in this area and other similar places.

Role of underground leakage in soil, water and nutrient loss from a rock-mantled slope in the karst rocky desertification area

Leakage loss of soil and water is the main reason for karst rocky desertification, and the concomitant nutrient loss/leakage carried by soil and sediment is an important factor that affects groundwater quality. Due to the difficulty in measuring the underground leakage of soil water, the amount of nutrient loss/leakage on a karst slope is currently unknown. A karst slope farmland is simulated in a laboratory based on field investigations in slope farmlands in a karst plateau region in Guizhou Province, China. Rainfall experiments were conducted to determine the surface losses and underground leakages of soil, water, nitrogen, phosphorus, and potassium, as well as the effects of rainfall intensity and slope on these factors in karst slope farmlands. Surface runoff occurs only at rainfall intensities greater than 30 mm/h, but underground runoff occurs under all rainfall intensity conditions. Moreover, underground leakage loss is the main mode of water loss. Soil loss on a karst slope farmland is a process that transitions from underground leakage to surface erosion as the rainfall intensity increases. The concentrations of total nitrogen (TN), total phosphorus (TP), and total potassium (TK) in surface runoff increase with increasing rainfall intensity and decrease with increasing slope gradient. The total loss loads of TN and TK in runoff and sediment increase with increasing rainfall intensity or slope gradient. The total nutrient loss loads of TN and TP are dominated by runoff losses, but the TK loss load is dominated by sediment losses.

Sparse Bayesian Learning-Based Seismic High-Resolution Time-Frequency Analysis

Time-frequency (TF) analysis is a useful tool for seismic data processing and interpretation. We introduce sparse Bayesian learning (SBL) to TF analysis and propose a new SBL-based high-resolution TF method. The method decomposes the seismic trace into a series of Ricker wavelets using SBL-based sparse representations and subsequently implements Wigner-Ville distribution (WVD) on the decomposed wavelets to produce TF spectra. By iteratively solving a Bayesian maximum posterior and a type-II maximum likelihood, SBL-based decomposition can sequentially obtain an optimal number of Ricker wavelets with different peak frequencies or phases from a preset wavelet dictionary, and can simultaneously invert for the associated sparse TF pseudoreflectivity with the prediction uncertainty. The WVD of SBL-based decomposed wavelets can assemble TF distribution of the reconstructed signals to approximately characterize WVD of the original data. Therefore, the linear stack of WVD of all decomposed independent wavelets is immune from both the notorious cross-term interferences of the traditional WVD and random noise. Synthetic data example involving thin beds and laboratorial physical modeling data example involving several known multicave combinations are used to demonstrate the effectiveness of the proposed SBL-based TF analysis method and illustrate its advantages over WVD and the orthogonal matching pursuit-based TF analysis method. The 3-D real seismic data example is adopted to test its application potential for interpreting deep channels and the karst slope fracture zone. The results show that the proposed SBL-based TF method is a potentially effective, stable and high-resolution seismic TF analysis tool even in the presence of thin beds.

Rainfall driven transport of carbon and nitrogen along karst slopes and associative interaction characteristic

Transportation of carbon and nitrogen during rainfall events is highly important within karst ecosystems and has particular relevance to understand decoupling relation between carbon and nitrogen cycles. The ability of the karst ecosystem to sequester carbon is currently unknown but has potential to be an important residual terrestrial carbon sink. We hypothesis that the runoff response to rainfall events will vary according to whether the slope belongs to karst area, and carbon and nitrogen have imbalanced outputs response to runoff. Hence, a series of in-situ simulated rainfall experiments at three sites (two karst slopes with different carbonate matrixes and one non-karst slope without a carbonate matrix) were conducted in a typical karst zone in southwest China. We measured runoff discharge, and dissolved carbon and nitrogen during the experiments. The results show that the total runoff discharge increased positively with rainfall intensity, with the carbonate matrix making an important contribution to rainfall reallocation. The dissolved carbon loss from runoff plots is much higher than the dissolved nitrogen, with the dissolved C/N revealing the decoupling relation of carbon and nitrogen cycles in this critical karst zone. This initial carbon migrated by runoff at catchment scale is critical to carbon budgets in rivers. Furthermore, nitrogen leaching from the system is crucial as a nitrogen shortage at karst area in southwest China.

Response of mechanical composition of fractured soil to rainfall in shallow Karst slopes in central Guizhou, China

Field sampling and indoor pipette method was used to analyze the variation of soil mechanical composition of shallow Karst fissures in response to rainfall under different land uses in Karst rocky desertification region in central Guizhou. Results would supply important scientific basis and direction for further research of soil leakage in the Karst areas. The results showed that silt was the dominant component in soil mechanical composition of shallow Karst fissures, with a content of 57%. In response to rainfall, the contents of silt and clay were increased and that of fine sand was reduced in slope cropland, while the content of fine sand was reduced and that of silt was increased in grassland. The changes of mechanical composition of shallow Karst fissures in forest and shrub land were irregular. Moreover, there was a positive correlation between rainfall and the sand content of shallow Karst fissures. The fissures of land use types had effects on the changes of silt and extremely coarse sand contents with the increase of soil depth, with a decreasing trend of silt.

Soil erosion and chemical weathering in a region with typical karst topography

Soil erosion is a global environmental issue that can lead to the loss of nutrient-rich topsoil and even soil desertification, which is more severe in vulnerable and sensitive karst areas. Currently, it is imperative to explore the soil erosion mechanism and the weathering process to help prevent soil degradation in the karst area. In this study, we estimated the soil erosion rate of a representative karst slope in Shilin area of China by 137Cs tracer method. We also analyzed the changes of soil total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) along the soil profiles. The results show that there is mild to moderate intensity erosion in this area, and erosion rates in different locations of slope are in the following order: hilltop > hillside > bottom. The 137Cs content has a certain correlation with TOC, TN and TP content in the soil profile, which indicates they may have similar loss pathways. Furthermore, the chemical index of alteration (CIA), Na/K value and their relation with soil erosion are analyzed and reveal that the studied area is seriously affected by physical erosion and chemical weathering under a hot and humid environment area.

Surface erosion and underground leakage of yellow soil on slopes in karst regions of southwest China

AbstractSurface erosion and underground leakage loss simultaneously exist on slopes with a double‐layered structure (surface and underground) and have caused rocky desertification in karst regions. Because of the great difficulty of directly determining the underground leakage loss of soil and water, soil erosion processes in this region remain unclear, especially underground leakage loss. The aim of this study was to reveal the plot‐scale characteristics of surface soil erosion and underground leakage loss of yellow soils on karst slopes. Simulated rainfall tests and field runoff plot monitoring were conducted to achieve this aim. We found that surface erosion formed on steep slopes (30°) or at greater rainfall intensities (over 50 mm hr−1) and that surface runoff and surface soil loss dominated the total runoff and soil loss on karst slopes at greater rainfall intensities. However, underground leakage loss always occurred at different slopes and rainfall intensities. For small rainfall intensity cases (lower than 80 mm hr−1), underground runoff and underground soil leakage loss dominated the total runoff and soil loss and showed an underground runoff ratio of over 90% and an underground soil leakage ratio of over 44%. Rainfall intensity had significant effects on surface runoff depth and surface soil loss rates but an insignificant influence on underground leakage loss. Slope gradient and slope length influenced the runoff yield, and slope degree had a greater effect on the surface soil loss than had slope length. This study improves our understanding of surface erosion and underground leakage loss in karst rocky desertification regions.

Paleogeomorphy evolution of the Ordovician weathering crust and its implication for reservoir development, eastern Ordos Basin

Gas reservoir development of the Ordovician weathering crust in the Ordos Basin is closely controlled by the pre-Carboniferous paleogeomorphy. Previous studies show that the paleogeomorphy is high in the west and low in the east, and the karst highland, karst slope and karst basin are developed from west to east. With further exploration in recent years, many karst breccia that represent strong karstification, are found in the east area which previously is considered to be the Ordovician karst basin. Thus, it is necessary to revaluate controlling factors of karst paleogeomorphy development from a viewpoint of the dynamic paleogeomorphy evolution, to investigate the paleogeomorphy evolution of the Ordovician weathering crust in geological history and guide further research and prediction of development law of reservoir spaces. In order to reconstruct the paleogeomorphy of the weathering crust in the top of Ordovician in the east Ordos Basin, paleogeography, thickness of residual strata and paleokarst characteristics are well studied. The result shows that a wide range of paleokarst highland occurred in the central to east part of Ordos Basin in the early period, and the karst reservoir spaces were formed; but in the late period, the east part of the basin gradually evolved into the paleokarst basin, the current pre-Carboniferous paleogeomorphy was thus formed, and the dissolution reservoir spaces formed in the early period were mostly filled, accordingly the reservoirs were tight. However fracture networks formed by cave collapse connect intercrystalline pores of dolomite, therefore, the reservoirs can still be well preserved locally and are worthy targets for hydrocarbon exploration.

Characteristics of paleogeomorphology and paleokarstification and the impact on natural gas accumulation: a case study of upper assemblage of Majiagou formation in central Sulige gas field, Ordos Basin, China

The Lower Paleozoic gas reservoirs in central Sulige gas field are mainly accumulated in Ordovician ancient weathering crust karst carbonate reservoirs, and they are especially rich in the upper assemblage of Ordovician Majiagou formation (Ma51–Ma54 sub-members). However, the spatial distribution of gas is unclear yet because of the different denudation degree and uneven development of paleokarstification. This paper mainly focuses on the characteristics of paleogeomorphology and paleokarstification and their impact on natural gas accumulation. Geological stratifying has been done based on the observation of cores and thin sections, and the analysis of drilling, logging and seismic data. The paleogeologic map of pre-carboniferous was depicted according to the ancient weathering crust outcropped strata. Paleogeomorphology reconstruction is based on a comprehensive method combining the moulage method with the residual thickness method. Result indicates that the paleogeomorphology can be divided into western karst highland and eastern karst slope in the area investigated, and the denudation degree of ancient weathering crust in the west is more serious than that in the east. The karst highland can be divided into remnant hill and depression while the karst slope can be divided into gentle hill and shallow depression in detail. Paleokarst section can be vertically divided into epikarstic zone, vertical vadose zone, horizontal phreatic zone and deep slow flow zone based on the analysis of paleokarst identification mark and vertical dissection of single well. By analyzing the relationship between paleokarst characteristics and production data, the study shows that paleogeomorphology and paleokarstification have a significant control on nature gas accumulation and distribution. Gentle hills on karst slope are expected to be the most favorable areas for reservoirs horizontally, horizontal phreatic zone is more advantageous vertically, and second the vertical vadose zone.

模拟降雨下喀斯特坡耕地土壤养分输出机制

模拟降雨下喀斯特坡耕地土壤养分输出机制

Nitrogen loss from karst area in China in recent 50years: Anin-situ simulated rainfall experiment's assessment.

Karst topography covers more than 1/3 of the People's Republic of China in area. The porous, fissured, and soluble nature of the underlying karst bedrock (primarily dolomite and limestone) leads to the formation of underground drainage systems. Karst conduit networks dominate this system, and rainfall takes a crucial role on water cycle at China karst area. Nitrogen loss from the karst system is of particular concern, with regard to nutrient use efficiency as well as water quality, as much of the karst system, including steeply sloping terrain, is used for intensive agriculture. We use simulated rainfall experiments to determine the relationship between rainfall and nitrogen loss at typical karst slope land and then estimate nitrogen loss from the karst soil. The results show that both surface runoff and subsurface runoff have a significant linear correlation with rainfall at all studied sites. Subsurface runoff is larger than surface runoff at two karst sites, while the opposite is true at the non‐karst site. Exponential function satisfactorily described the correlation between rainfall and nitrogen concentrations in runoff. Nitrates accounted for 60%–95% of the dissolved nitrogen loss (DN, an index of N‐loss in this research). The estimated annual N‐loss load varies between 1.05 and 1.67 Tg N/year in the whole karst regions of China from 1961 to 2014. Approximately, 90% of the N‐loss load occurred during the wet season, and 90% of that passed through the subsurface. Understanding the processes and estimating N‐loss is highly valuable in determining long‐term soil security and sustainability in karst regions.

Paleokarst reservoir features and their influence on production in the Tahe Oilfield, Tarim basin, China

This study analyzed the characteristics of cave heights and fillings, influences on cave distribution by unconformity and geomorphology, and production control trend for geological factors. The heights and numbers of caves in different geomorphologies are basically similar. The cave heights are greater near the unconformity surface. On an average, there are higher degrees of cave development and larger cave heights in the karst slopes than in the karst highlands and karst depression. Based on core observations, four types of reservoirs were identified: unfilled caves, filled caves, open fractures and filled fractures. The formation of caves is mainly controlled by faults and underground water surfaces. Faults works obviously in the karst highland, while underground water surfaces obviously in the karst slope. Caves controlled by faults are mostly unfilled, while controlled by underground water surfaces are mostly filled with clastic sediments. According to the results from both first-day and cumulative production, production is not highly correlated with cave heights, but rather storage and connectivity of caves and fractures. Generally, wells with unfilled caves or open fractures have higher first-day and cumulative production than filled caves or fractures. The distribution of high first-day and cumulative production is consistent overall.

Effects of Underground Pore Fissures on Soil Erosion and Sediment Yield on Karst Slopes

AbstractUnderground pore fissure is one of common ways of soil leakage, and the soil loss in underground pore fissure would aggravate the development of rocky desertification in karst areas. A designed steel tank with varied underground pore fissure degrees was used to measure how underground pore fissure affected soil erosion and sediment yield on karst bare slope. We found that sediment yield rates and its distribution ratios obviously differed between the surface and underground. Surface sediment yield rate greatly responded to rainfall intensity conditions, while underground sediment yield rate rarely changed. Both bed rock bareness and underground pore fissure degree had an insignificant effect on both surface and underground sediment yield rate, but the former greatly influenced sediment distribution between the surface and underground. With increasing of bed rock bareness rate, both the average surface and underground sediment yield rates first increased and then decreased. The later posed an effect on underground sediment yield rate, which increased with increase of underground pore fissure degree. With increase of underground pore fissure degree, critical rainfall intensity for producing surface sediment would increase from 0.8–1.3 mm min−1 to 1.3–2.0 mm min−1. The results provide a mechanistic understanding of how underground pore fissure affects soil erosion and sediment production. Copyright © 2017 John Wiley & Sons, Ltd.

The three different types of paleokarstification and reservoir distribution of Leikoupo Formation, Middle Triassic in the northern Sichuan Basin, China

The Middle Triassic Leikoupo Formation in the northern Sichuan Basin is a new discovery sequence for strategy replacement in the gas exploration field. The carbonate paleokarst reservoirs of the Middle Triassic Leikoupo Formation in the northern Sichuan Basin provide a case study of gas exploration in carbonate paleokarst reservoirs. By conducting a comprehensive study of well drilling, drill cores, well loggings, and test data, combined with the analysis of geological background of the Sichuan Basin, it is considered that syndepositional, supergene, and burial paleokarstification can be identified in the Leikoupo Formation, and these types of karstification were vital for reservoirs formation. Syndepositional karstification controlled the formation and distribution of early reservoirs. This type of karstification was limited to high-energy grain shoals, and therefore its horizontal extent was limited by the distribution of this facies. During temporary episodes of relative sea level fall, carbonate shoals were exposed subaerially or within the fresh water lens, and significant selective dissolution occurs, forming moldic pores and intragranular dissolved pores. The reservoirs related to supergene karstification mainly occurred in the area of the buried hill containing carbonate rock at the top of the Leikoupo Formation, and are vertically limited within a depth of 70–90 m under the carbonate corrosion unconformity plane of the Leikoupo Formation. The reservoirs related to supergene karstification formed mainly in limestone breccia and vuggy limestone in surface karst zone, and the high-angle dissolved fracture and medium- and small-scale corrosion holes in vertical vadose zone, and medium and small corrosion holes, dissolved fractures, roof fracture-caves in horizontal karst zone. The karst monadnocks within karst slope are the most favorable microgeomorphic units for horizontal distribution of reservoirs. Burial karstification was related to dissolution of carbonate minerals by organic acids, and was the key factor in optimal transformation of reservoirs. Burial karstification corroded and expanded the pore system left after infilling of the earlier pore system, and mainly was to optimize the transformation the reservoir space redistribution of earlier pore system. The compact carbonate layers related to burial karstification only can be dissolved along the early cracks in the channel and formation cracked reservoirs, but development of this type of reservoir was limited. The reservoirs were influenced by syndepositional, supergene, and burial paleokarstification in the Leikoupo Formation, and carbonate paleokarst reservoirs were formed finally.

Restoration of paleokarst landform and its geological significance: A case from Middle Permian Maokou Formation in Northwestern Sichuan Basin

Based on the analysis of deposition and tectonism, “residual thickness method” is used to restore the paleokarst landform of Middle Permian Maokou Formation in northwestern Sichuan Basin. With the feature of plain with karst hilllock, the paleokarst landform in this area can be classified into three secondary geomorphic units: karst platform, karst slope and karst groove, in which the karst hillocks and monadnocks on karst platform and karst slope are the favorable zones for the development of karst reservoirs, and favorable exploration zones in the next step. Furthermore, in the karst grooves, the Maokou Formation are often denuded into Members Mao 3 or Mao 2, and the seismic profiles show the top of Maokou Formation in karst groove is missing due to erosion. Members Mao 4 and Mao 3 are generally preserved in the karst platform. The seismic profiles across the karst platform and karst groove show that the NE and NW striking erosion grooves were the result of differential uplift and erosion caused by basement faulting at the end of Middle Permian, which then successively developed and formed the NW striking Guangyuan-Wangcang and the NE striking Jiangyou-Guangyuan troughs in Changxing Period. It is suggested to pay more attention to the geological research and exploration of the shallow carbonate platform areas adjacent to the syneclise and trough in fairly deep water.

Characteristics of soil erosion on Karst slopes under artificial rainfall experiment conditions.

Studying soil erosion process and mechanism on Karst slopes has important practical and guiding significance for controlling soil and water loss and preventing rocky desertification in Karst region. The soil erosion process and characteristics on Karst slopes were explored by artificial simulation rainfall experiment. Results showed that the soil and water loss largely came from underground hole (crack) when rainfall intensity was in the range of 30 and 50 mm·h-1, while soil erosion occurred primarily in soil surface when rainfall intensity was 80 mm·h-1. Surface runoff modulus and transport rate both increased with increasing slope, and decreased with increasing underground hole (crack) degree. The underground runoff modulus varied from 0.37 to 0.52, and the underground transport rate changed from 0.81 to 1.93 g·min-1. They both decreased with increasing slope while increased firstly and then decreased with increasing rainfall intensity.

Sedimentary features and exploration targets of Middle Permian reservoirs in the SW Sichuan Basin

The exploration direction and targets for the large-scale Middle Permian gas reservoirs in the Sichuan Basin are hot spots and challenges in current exploration researches. The exploration successes of large gas field of Cambrian Longwangmiao Formation in Gaoshiti-Moxi region, Central Sichuan Basin, indicated that prospective sedimentary facies belt was the basis for the formation of large gas fields. In this paper, based on seismic data, outcrop data and drilling data, the tectonic framework and sedimentary features of the Middle Permian in the SW Sichuan Basin were comprehensively studied. The following conclusions were reached from the perspective of sedimentary facies control: (1) during the Middle Permian, this region was in shallow water gentle slope belts with high energy, where thick reef flat facies were deposited; (2) the basement was uplifted during Middle Permian, resulting in the unconformity weathering crust at the top of Maokou Formation due to erosion; the SW Sichuan Basin was located in the karst slope belt, where epigenic karstification was intense; and (3) reef flat deposits superimposed by karst weathering crust was favorable for the formation of large-scale reef flat karst reservoirs. Based on the combination of the resources conditions and hydrocarbon accumulation conditions in this region, it was pointed out that the Middle Permian has great potential of large-scale reef flat karst gas reservoir due to its advantageous geological conditions; the Middle Permian traps with good hydrocarbon accumulation conditions were developed in the Longmen Mountain front closed structural belt in the SW Sichuan Basin and Western Sichuan Basin depression slope belt, which are favorable targets for large-scale reef flat karst reservoirs.

Recharge processes on typical karst slopes implied by isotopic and hydrochemical indexes in Xiaoyan Cave, Guilin, China

Summary Measurements of the hydrogen and oxygen isotopic composition of precipitation and drip stream water at drip site XY5 in Xiaoyan Cave in Guilin City were conducted to investigate the infiltration and vadose flow processes, and to characterize precipitation–discharge relationship in the overlying karst hillslope area. A marked seasonal variability of oxygen and hydrogen isotopes was observed in precipitation ranging from −18.7‰ to −0.6‰ for δ 18 O and −139‰ to +5‰ for δD, respectively during the study period from November 2013 to October 2014. However, the narrow range observed for cave drip stream water (δD of −43‰ to −22‰, δ 18 O of −7.3‰ to −4.3‰) was attributed to homogenization via mixing of meteoric waters in the unsaturated zone above the cave. Although drip stream waters plot close to or slightly above the local meteoric water line, the weighted average δ 18 O value of precipitation is depleted than in dripwater, which is attributed to evaporation predominantly in the rainy season when cave air circulation is most active. The relationship between surface infiltration or leakage and cave discharge is connected by the distinct models of flow through the epikarst zone. Vadose flow showed high peaking hydraulic responses but buffered rain isotope responses with an attenuate magnitude of 5.0–9.4‰ for δ 18 O. The sudden hydrochemical proxy variability was related closely to increase of discharge and the flushing out of old water with higher dissolved calcite. A threshold of flow generation expressed by precipitation amount must be exceeded to create a peak in discharge, as a function of excessing the field capacity of soil and residual capacity of epikarst zone. The sources of vadose seepage are soil and epikarst reservoir water with the attenuation of the isotopic fluctuations (−4.28‰ to −5.92‰ for δ 18 O) and slight hydrological response, indicating mixed signals of infiltration water in epikarst storage reservoir. The total discharge volume of percolating water is 2084 L derived from piston flow due to the increased hydraulic head in the dry season, which provides a quantitative evidence of the potential adaptive capacity of the system to deal with the extreme droughts.

Role of epikarst in near-surface hydrological processes in a soil mantled subtropical dolomite karst slope: implications of field rainfall simulation experiments

Epikarst exerts a strong control on run-off generation in karst regions, but it is still unclear in karst regions. Our study aimed to demonstrate the effect of epikarst on near-surface hydrological processes in a subtropical cockpit karst region of southwest China, using plot-scale rainfall simulation experiments with different rainfall intensities (low and high) and antecedent moisture conditions (dry and wet). A trench excavated to the epikarst lower boundary allowed identification of flow pathways in the entire soil–epikarst architecture system, thus facilitating the water balance calculations using a conceptual model with the assumption of a two-stage hydrological evolution. More than 70% of the total rainfall water moved vertically through the shallow soil layer and then was redistributed by the epikarst as subsurface flow occurring on the soil–epikarst interface, depression filling on epikarst surface, water held by epikarst and deep percolation. Epikarst water regulation capacity, defined as the sum of depression filling on epikarst surface, water held by epikarst, epikarst seepage flow and deep percolation, was 58 mm (wet antecedent condition) and 223 mm (dry antecedent condition). Total run-off from the soil–epikarst system was dominated by saturated subsurface flow showing a threshold process controlled by epikarst storage capacity (storing as much as 181 mm of rainfall water under dry antecedent condition). Our study proved that despite the epikarst being relatively poorly developed and covered by a soil mantle, it still exerted a strong influence on near-surface hydrological processes and thus should be adequately considered in future modelling of water recharge and depletion dynamics in this integrated soil–epikarst system. Copyright © 2015 John Wiley & Sons, Ltd.

Subsurface flow in a soil-mantled subtropical dolomite karst slope: A field rainfall simulation study

Soil and epikarst co-evolve resulting in complex structures, but their coupled structural effects on hydrological processes are poorly understood in karst regions. This study examined the plot-scale subsurface flow characteristics from an integrated soil–epikarst system perspective in a humid subtropical cockpit karst region of Southwest China. A trench was excavated to the epikarst lower boundary for collecting individual subsurface flows in five sections with different soil thicknesses. Four field rainfall simulation experiments were carried out under different initial moisture conditions (dry and wet) and rainfall intensities (114mmh−1 (high) and 46mmh−1 (low) on average). The soil–epikarst system was characterized by shallow soil overlaying a highly irregular epikarst surface with a near-steady infiltration rate of about 35mmh−1. The subsurface flows occurred mainly along the soil–epikarst interface and were dominated by preferential flow. The subsurface flow hydrographs showed strong spatial variability and had high steady-state coefficients (0.52 and 0.36 for high and low rainfall intensity events). Irregular epikarst surface combining with high vertical drainage capacity resulted in high threshold rainfall depths for subsurface flows: 67mm and 263mm for initial wet and dry conditions, respectively. The above results evidenced that the irregular and permeable soil–epikarst interface was a crucial component of soil–epikarst architecture and consequently should be taken into account in the hydrological modeling for karst regions.