Karst Catchment Research Articles

Illuminating the complexity of hydrogeological regimes in fault-influenced, anticlinal karst aquifers: Insights into the recharge patterns in tectonically complex karst catchments

For many countries, karst aquifers are a significant source of potable water; thus, understanding their hydrogeological behavior is of great importance. However, in tectonically complex karst areas, knowledge of their hydrogeological behaviors has remained largely limited due to the high heterogeneity, often necessitating the use of costly and time-consuming techniques. In this study, spring discharge hydrographs, water balance estimates, hydrochemical and stable isotopic analyses, geomorphological surveys, and a consideration of the geological and structural setting of the study area were used to investigate the hydrogeological function of karst aquifers drained by two large permanent springs: the Bibitalkhone (Bt) and Sabzab (Sa) springs, located in a highly faulted catchment in the Zagros Mountains (southwest Iran). The results obtained from these different approaches suggested that fault-controlled, external waters from neighboring karst aquifers are responsible for more than two-thirds of the total discharge recorded at both springs. In this study, this predominantly conduit-type recharge is referred to as “regional (or external) autogenic recharge”. A smaller portion of the springs’ discharge (∼25% and ∼5% of the Bt and Sa springs’ discharge, respectively) originates from internal waters associated with the springs’ host aquifer, which is supplied by rainfall and infiltrates into the subsurface via joints and fractures. This type of recharge is largely diffuse and is referred to here as “local (or internal) autogenic recharge”. A relatively large portion of the Sa Spring’s discharge (∼35%) originates from a nearby dam reservoir, which can leak into the host spring aquifer through joints and fractures. This type of recharge is described as “local (or internal) allogenic recharge” and is primarily diffuse. The results of this study demonstrate that the karst aquifers in the downstream catchments can supply a stable and continuous source of potable groundwater that can meet the region’s annual consumption, agricultural, and industrial demands. This study will provide insights into the hydrogeological regimes of fault-influenced aquifers, contributing to the sustainable development and proper management of water resources in tectonically complex karst catchments.

Estimation of root zone soil moisture at point scale based on soil water measurements from cosmic-ray neutron sensing in a karst catchment

The cosmic-ray neutron sensing (CRNS) is an emerging method for continuously monitoring soil water content (SWC) at an intermediate scale. However, when multiple hydrologic units are present within its footprint, the potential application of CRNS in water resources management is restricted. Here we propose a new strategy to predict point-scale SWC in root zone established on CRNS-based soil moisture and improved relative difference method. A total of 768 days of soil moisture data were collected by CRNS at the intermediate scale and EC-TM sensors at the point scale in a karst catchment. The original and improved mean relative difference methods predicted point-scale SWCs within and without the effective measuring depth, respectively. The mean effective measuring depth was 13.16 cm, ranging from 10.13 to 19.23 cm. Both land use type and soil structure played essential roles in regulating point scale SWC in the soil profile. Point-scale SWC in root zone can be predicted accurately (P < 0.001) based on SWC data derived from the CRNS system. The prediction accuracy of point scale SWC can be improved by increasing the averaging time of the soil moisture values. Our results demonstrated that the proposed strategy was reliable for CRNS to predict SWC beyond the effective measurement depth. This study provides a good perspective for effectively managing of water resources in areas with complex hydrological processes.

Groundwater Recharge Assessment for Small Karstic Catchment Basins with Different Extents of Anthropogenic Development

Climate change and anthropogenic development considerably influence groundwater resource distribution and conditions. Catchment basin groundwater recharge—discharge computation reliability is needed for effective groundwater management policy formulation and implementation and also for resolving environmental challenges in such a watershed. This paper compares groundwater recharge patterns between urbanized and nearly natural small catchment basins of Israel’s Western Mountain Aquifer (WMA). The correlation between precipitation volumes and surface runoff shows that surface runoff volume constitutes 3–4% of the precipitation volume in the Natuf catchment and 1–2% in the Te’enim catchment. These assessments reflect the differences in the land use, outcrop lithology, topography and hydrodynamic properties of the WMA within the model basins. A groundwater recharge assessment based on water balance and water table fluctuation methods was performed for the mountainous karstic Te’enim and Natuf catchment basins for all the available data from 2000 to 2020. The water balance method provided reliable estimates. The groundwater recharge assessment considered land use classification and climate changes during this period. The average multiannual groundwater recharge values for the 2000–2021 period varied from 17.6 × 106–24.8 × 106 m3 to 24.5–29.2 × 106 m3 for the Te’enim and Natuf catchment basins, respectively. For the relatively dry period of the 2013/2014–2017/2018 hydrological years when detailed measurements of the surface runoff were available, the corresponding groundwater recharge volumes were 17.6 × 106 m3 and 24.5 × 106 m3. The corresponding local groundwater recharge coefficients constitute 0.46–0.57 for the mostly agricultural Te’enim basin and 0.29–0.32 for the urbanized Natuf basin. A significant difference in the groundwater recharge coefficients between the studied catchments is caused mostly by the differences in land use. It is suggested that applying such a groundwater recharge estimation for small hydrological sub-basins can improve one’s understanding of the groundwater recharge distribution within a major basin, enabling the application of an accurate regional hydrogeological model that may be extrapolated to other similar regions.

Insight into heterogeneous karst catchment by the dynamical system approach

The hydrological process heterogeneity in karst catchments makes collecting sufficient information about the karst system properties extremely difficult. Consequently, the parameterization and application of currently available modeling approaches is highly uncertain in karst. We implemented a modified dynamical system approach and explored the possibilities to analyze, simulate and characterize karst springs and sinking streams in the heterogeneous Ljubljanica river catchment in Slovenia. The discharge sensitivity functions were used to simulate the hydrographs and identify the differences in the temporal dynamics of the discharge recession. Most of the derived karst springs and sinking streams discharge sensitivity functions express dual discharge recession behavior most likely controlled by the hydraulic conveyance characteristics of karst underground conduits. The implemented approach offers a high potential for advancement in karst hydrology by using basic discharge data for analyses of karst catchment characteristics and future planning of karst water resource management.

Contribution of natural and agricultural activities on Fe dynamics: Insights from Fe isotope in soils under different land-use types

Evaluating the contribution of natural and agricultural factors on Fe dynamics in soils is crucial to the study of material cycling in critical zone observation. Five soil profiles under cultivated land, abandoned cultivated land, grassland, shrub land, and secondary forest from Puding karst catchment in Guizhou Province, China, were analyzed to identify natural and agricultural contributions to Fe processes. Vertical distribution changes of Fe content and Fe isotopic compositions (δ56Fe) were investigated. The results presented that natural chemical weathering accompanied by soil erosion occurs primarily in the upper layer, leading to the loss of Fe from the surface to the bottom during the eluviation-illuviation process. Fe isotopic fractionation occurred in soils rich in natural organic matter (i.e., secondary forest and shrub land), resulting in the accumulation of lighter Fe isotopes. Soil impacted by anthropogenic activity have higher Fe content (e.g., cultivated land or abandoned cultivated land). The reduction of Fe is the principal cause of light Fe isotopes in the upper layer, which is caused by periodic artificial irrigation on cultivated land. Compared to worldwide soils, Puding soils are impacted by natural organic matter, eluviation-illuviation, and agricultural activities, which tend to modify Fe isotopic signatures on a local scale within soil profiles. Fe isotopic fractionations in bulk soils among different land-use soils are very limited. This study provides insightful analysis based on Fe isotopic to recognize natural and anthropogenic activities on Fe dynamic, and offers information on material cycling in the Puding karst catchment.

Exceeding Turbidity versus Karst Spring Discharge during Single Rainfall Events: The Case of the Jadro Spring

The relationship between karst spring discharge and turbidity during periods where the turbidity exceeds the permitted value of 4 NTU is analyzed by means of classical hydrological methods including hysteresis loops. The Williams method (1989) is adjusted by replacing river discharge with karst spring discharge, and sediment concentration with exceeding turbidity. The study’s first part focuses on the general hydrological characteristics of cumulative rainfall, turbidity, and discharge during each period. In the second part, four major types of hysteresis loops are identified, classified, and hydrologically explained. The study area is the karst catchment of the Jadro Spring in Croatia. The obtained results show that hysteresis loops can be very useful in analyses of water turbidity from karst springs. A relationship between the type of loop, intensity of discharge, and size of the secondary peak in turbidity is identified. The major types of hysteresis loops were clockwise, counterclockwise, figure-eight and complex loops, and their main characteristics were summarized. The results gave a new insight into the behavior of the turbidity of the Jadro Spring.

Base flow separation method in conditions of the karst catchment

AbstractGeological conditions are an important parameter on hydrological modeling that strongly affects runoff generation processes. They are not usually taken into account in the parameters of hydrological models, especially karst catchments, where water transfers through fissures, cavities, caves, and phreatic channels are regularly discarded in studies because of their nonlinear behavior and the variability of their typology. The study compared using of base flow separation methods in the conditions of karst catchments, where the reference base flow values measured are not available. The base flow separation was realized by various methods and the models available, including BFI+ and WHAT

Uloga nagiba padina, gustoće vrtača i analize vodne bilance u izdvajanju kompleksnog slivnog područja rijeke Slunjčice (Hrvatska)

Due to the high vulnerability of the karst aquifer to the surface contaminants, a precisely defined catchment area has the highest priority. In this study, the influence of slope inclination, the doline density analysis, and the water budget analysis in the delineation process of a complex karst catchment area is discussed. To define hydrogeological role of lithological units, cross sections of slope inclination and doline density were combined with hydrogeological cross sections, while the degree of karstification was used to describe the permeability of rock units. The verification of karst catchment delineation area was performed with water budget analysis. The methodology used for the determination of hydrogeological behavior and delineation of a complex karst catchment area (Slunjčica River basin, Croatia) is shown with the flow diagram. It has been found that the highest doline density appears in the range from 0 to 1° of the slope inclinations, and that it decreases with a higher slope degree. Although the results of this study confirm that even with the relatively small number of input data it is possible to define the karst catchment area, it must be emphasized that the doline density analysis presents an indispensable tool in the research related to the definition of karst catchment areas.

Key influence of hydrogeological, geochemical, and geological structure factors on runoff characteristics in karst catchments

Lithology, dissolution degree, and other geological factors are essential factors affecting karst environments. However, the complex effects of multiple geologic factors on runoff characteristics have not been well quantified in karst catchments. Thus, 30 catchments and four runoff characteristics (runoff coefficient, runoff modulus, annual runoff extremes ratio, and annual runoff variation coefficient) were selected in this study to evaluate the influence of geological factors on runoff characteristics in karst catchments. The linear mixed effect model was used to quantitatively evaluate the contribution rate of each factor to the runoff characteristics, and the structural equation model was used to analyze the quantitative relationship between each factor and the runoff characteristics. The results show that among the three aspects of geological factors, hydrogeological factors have the greatest influence on runoff characteristic values. In turn, the density of sinkhole is the most influential factor among the hydrogeological factors. Geological structure factors had both direct and indirect effects on runoff characteristics (path coefficient is 0.27). Among them, the density of faults is the most influential factor. Geochemical factors affected runoff characteristics more by influencing hydrogeological factors (path coefficient −0.51) or geological structure factors (path coefficient −0.43). In conclusion, data from several watersheds directly confirm that in karst watersheds, these geological factors such as hydrogeology, geochemistry, and geological structure have a very fundamental influence on karst watershed runoff characteristics. Our results can help us further understand karst hydrological processes and improve the accuracy of karst hydrological modeling.

Dynamic characteristics of nitrogen transport in various land use in a typical karst catchment during rainfall events

Dynamic characteristics of nitrogen transport in various land use in a typical karst catchment during rainfall events

Using continuous electrical conductivity measurements to derive major solute concentrations in karst systems

AbstractHydrochemical data of karst springs provide valuable insights into the internal hydrodynamical functioning of karst systems and support model structure identification. However, the collection of high‐frequency time series of major solute species is limited by analysis costs. In this study, we develop a method to retrieve the individual solute concentration time series and their uncertainty at high temporal resolution for karst springs by using continuous observations of electrical conductivity () and low‐frequency ionic measurements. Due to the large ion content and non‐negligible concentrations of aqueous complexes in karst systems, the concentration of each solute species occurring as free ion and as part of aqueous complexes are computed separately. The concentration of species occurring as free ions are computed considering their contributions to the total , whereas the concentration of the species as part of complexes are obtained from speciation calculations. The pivotal role of the complexation processes for the reconstruction of solute concentration time series starting from the signal is investigated in two karstic catchments with different geologies and temporal resolution of the available hydrochemical datasets, that is the Kerschbaum dolostone system in Austria and the Baget limestone system in France. The results show that complexation processes are significant and should be considered for the estimation of the total solute concentration in case of SO4, Ca, Mg and HCO3. The signal of a karst spring can be used to interpolate and quantify the dynamics of those solutes characterized by large contribution (approximately &gt;6%) to the total and low relative variability, that is HCO3, Ca and Mg. Moreover, the presented method can be used to estimate concentrations of solutes when applied to karst systems with stationary and hydrogeochemical homogeneous contributing area. On the contrary, the method is affected by large uncertainty in case of dynamic systems characterized by varying contributions of water from different geological areas. This study aims to contribute to the problem of hydrogeochemical data availability and to support future works on karst systems conceptualization.

Paleoclimatic and anthropogenic impacts on the environment of Southwest China since 33 ka based on multiproxy analysis of karst depression deposits

Paleoclimate change and anthropogenic activities have fundamentally modified the Earth's environmental system since the late Quaternary. Southwest China has one of the largest continuous karst areas in the world, yielding geological records from karst depressions that can improve our understanding of changes in the dominant control on regional environmental conditions. In this study, we investigated a sediment sequence formed over 33,000 years in a typical karst catchment by using combined geophysical, geochemical and biological properties of the depression deposits. The results suggest that paleoclimate change was the main modulator of environmental changes prior to the Late Holocene, and that significant vegetation change related to paleotemperature influences played an important role during the Early (ca. 12–8 ka) and Middle (ca. 8–4 ka) Holocene. During the Late Holocene (ca. 4–0 ka), the impact of anthropogenic activities has overwhelmed paleoclimatic influences on the environment. This study demonstrates the utility of dated karst depression deposits for reconstructing Holocene climate change and the timing of anthropogenic impacts on the landscape.

Intra‐event concentration–discharge relationships affected by hydrological connectivity in a karst catchment

AbstractConcentration‐discharge (C‐Q) relationship in streamflow provides insights into hydrological transport at the catchment scale. Changes in hydrological connectivity during runoff events often dominate flood and solute export in the karst catchment. However, only few studies have explored intra‐event C‐Q relationships and how they are affected by hydrological connectivity in karst catchment. In this study, we explored the intra‐event C‐Q relationships in underground channel flows by integrating solute concentration, discharge and modelled flow age in a 1.25 km2 karst catchment in southwest China. We apply piecewise functions to characterize the C‐Q relationships on rising and falling limbs of hydrograph. Geogenic solutes exhibited dilution C‐Q patterns during the runoff event, which could be fitted by two power‐law models with different coefficients on rising and falling limbs. Affected by the strong hydrological connectivity between surface and subsurface, hillslope and depression, a steeper C‐Q slope on the rising limb indicated an exhaustible, proximal source, that is, groundwater. In contrast, the C‐Q patterns of the soil enriched solutes changed from enrichment to dilution during the runoff event. The enrichment pattern occurring at the early of rising limb was caused by strong hydrological connectivity between the surface and subsurface, which indicated a distal and plentiful sources of soil water. Whilst the strong hydrological connectivity between hillslope and depression caused a dilution pattern at the latter of rising limb. On the falling limb of hydrograph, a dilution pattern implied that the small fractures could be another source zone of soil‐enriched solutes in addition to soil layer in the depression. The C‐Q behaviours of soil‐enriched solutes can be fitted by a combination of a parabola model and a power law model for rising and falling limbs, respectively. The current study highlights the variations in intra‐event C‐Q relationships of different solutes affected by hydrological connectivity in the karst catchment. Which is crucial for assession of hydrochemical processes and fertilization management in this area.

Soil nitrogen transformation in different land use and implications for karst soil nitrogen loss controlling

A large amount of cropland in the Southwestern (SW) China karst area has been abandoned since the implementation of the Grain for Green project at the end of the last century, which significantly impacted the biogeochemistry cycle in soil. However, the effect of cropland abandonment on the soil nitrogen (N) cycle and the N loss process is still little understood. N content and isotope analysis were used to assess karst soil’s N fate and explore the N loss process in SW China. Soil samples were collected from four land uses, cropland (CR), abandoned CR (AC), shrubland and grassland (SG), and secondary forest (SF), by seasonal in a typically mixed land-use karst catchment at six soil depths. The results showed that the average soil particulate organic N (PON) content increased from 1.58 ± 0.49 g kg−1 to 2.70 ± 1.82 g kg−1, whereas average dissolved nitrate (NO3–-N) content decreased from 10.77 ± 8.87 mg kg−1 to 1.72 ± 1.84 mg kg−1 during revegetation. There was a decreasing amount of PON and dissolved N (NH4+-N、NO3–-N、TDN) content from surface to the bottom of the soil profile and large variation mainly occurred in 0 – 30 cm in the soil profiles of four different land uses, which suggests N transformation mainly occurred in topsoil (<30 cm). Mineralization was the major nitrogen transformation process that affected PON content and δ15N-PON values. The highest δ15N-PON fractionation (-5.0‰) in the soil profiles of the four types of land use caused by mineralization was detected in SF. Variation of N content and isotope values in soil profiles after cropland abandonment suggested that the loss of NO3–-N in karst catchment mainly came from the topsoil of cropland. Overall, N loss from the topsoil of cropland should pay more attention, while cropland abandonment can effectively reduce the loss of dissolved N from the thin karst soil layer.

The Ratio Distribution of Evapotranspiration to Precipitation well Related With Soil Thickness in Karst Watershed of Southwest China

AbstractEvapotranspiration (ET) is predominant variable for water management in various types of ecosystems, and it has been intensively studied through in situ measuring and modeling methods. However, it is challenging to measure actual ET and upscale it to regional level for karst landscapes, where aquifers typically show remarkable and rapid responses to precipitation events. There is an urgent need to quantify water fluxes to provide reliable evidence for the protection and sustainable management of karst water resources. Therefore, in this study, five plots were built to observe actual ET based on Thermal Dissipation Probes, redesigned ventilated chamber and in‐site microlysimeters in a karst watershed in southwest China. Then, three models (Penman‐Monteith‐Leurning, PML; Remote Sensing‐Priestley and Taylor; and Hargreaves) were constructed to upscale ET estimation to the regional level based on Landsat‐8 images and MODIS data. The results showed that: (a) The PML model performed better than other models (p &lt; 0.01) with higher R2 values (0.72 for MODIS images and 0.87 for Landsat‐8 images); (b) Daily ET exhibited significant seasonal variability and different spatial distribution, and yearly ET exhibited well related with soil thickness (R2 = 0.58); (c) Daily ET had a slightly positive correlation with DEM, however, had a negative correlation with ground temperature. This study is helpful to improve accuracy of measured and estimated ET by combining remote sensing data, and it has important implications for upscaling it to regional level. It suggests that ET is strongly regulated by constant supply of water, vegetation coverage, and available energy in karst catchments.

Estimating karst groundwater recharge from soil moisture observations – a new method tested at the Swabian Alb, southwest Germany

Abstract. Understanding groundwater recharge processes is important for sustainable water resource management. Experimental approaches to study recharge in karst areas often focus on analysing the aquifer response using a disintegration of its outlet signals, but only a few approaches directly investigate the recharge processes that occur at the surface of the system. Soil moisture measurements have a high potential to investigate water infiltration to deeper soil depth or epikarst with an easy and not too intrusive installation. They can yield long-term measurements with high temporal resolution. Using these advantages, we developed and tested a method to estimate recharge based on soil moisture measurements. The method consists of the extraction of linked events in rainfall, soil moisture, and discharge time series, as well as a subsequent fitting of the parameters of a simple drainage model to calculate karst recharge from soil moisture metrics of individual events. The fitted parameters could be interpreted in physically meaningful terms and were related to the properties of the karstic system. The model was tested and validated in a karst catchment located in southwest Germany with hourly precipitation, soil moisture, and discharge data of 8 years duration. The soil moisture measurements were distributed among grassland (n = 8) and woodland areas (n = 7) at 20 cm depth. A threshold of about 35 % (± 8 %) of volumetric water content was necessary to initiate effective infiltration. Soil moisture averaged during the wetting period of each event was the best metric for the prediction of recharge. The model performed reasonably well, estimating recharge during single rainfall events. It was also capable of simulating 88 % of the average annual recharge volume despite considerable differences in the performance between years. The event-based approach is potentially applicable to other karstic systems where soil moisture and precipitation measurements are available to predict karst groundwater recharge.

Source availability and hydrological connectivity determined nitrate-discharge relationships during rainfall events in karst catchment as revealed by high-frequency nitrate sensing

Karst terrain seasonal monsoonal rainfall is often associated with high concentrations of nitrate-N in streams draining agricultural land. Such high concentrations can pose problems for environmental and human health. However, the relationship between rainfall events that mobilize nitrate and resulting nitrate export remains poorly understood in karst terrain. To better understand the processes that drive nitrate dynamics during rainfall events, the characteristics of individual rainfall events were analyzed using sensor technology. Thirty-eight rainfall events were separated from the high-frequency dataset spanning 19 months at a karst spring site. The results revealed that nitrate-discharge (N-Q) hysteresis in 79% of rainfall events showed anticlockwise hysteresis loop patterns, indicating nitrate export from long distances within short event periods. Karstic hydrological connectivity and source availability were considered two major determining factors of N-Q hysteresis. Gradual increase in hydrological connectivity during intensive rainfall period accelerated nitrate transportation by karst aquifer systems. Four principal components (PCs, including antecedent conditions PC1&3 and rainfall characteristics PC2&4 explained 82% of the cumulative variance contribution to the rainfall events. Multiple linear regression of four PCs explained more than 50% of the variation of nitrate loading and amplitude during rainfall events, but poorly described nitrate concentrations and hydro-chemistry parameters, which may be influenced by other factors, e.g., nitrate transformation, fertilization time and water-rock interaction. Although variation of N concentration during event flow is evident, accounting for antecedent conditions and rainfall factors can help to predict rainfall event N loading during rainfall events. Pollution of the karstic catchment occurred by a flush of nitrate input following rainfall events; antecedent and rainfall conditions are therefore important factors to consider for the water quality management. Reducing source availability during the wet season may facilitate to reduction of nitrogen loading in similar karst areas.

Temporal Variations of Sediment Provenance in a Karst Watershed, China

The environmental quality of the sediments in karst areas is a common concern, and it is of great significance to analyze the sources of the sediments. This study investigates the sources and its temporal variations of catchment sediments in a typical small karst watershed area. Toxic metal concentrations in the catchment area were monitored via three geochemical baseline projects in China. The sediment identification fingerprint tool (SIFT) was used to establish a geochemical model for tracing the main source contributions and its temporal variations of catchment sediments over the past 28 years (1992–2019). The catchment sediments in the small karst catchment area were mainly sourced from the background lithologies, among which limestone contributed the most, followed by dolomite, sand-shale, and mudstone; however, the anthropogenic lead–zinc tailings contributed the least. The contributions and temporal variations of each source were closely related to the lithology, topography, and landform, as well as the change in land-use and vegetation cover and the degree of rocky desertification. Moreover, the implementation of vegetation restoration and control of rocky desertification decreased the contributions of the upstream geological bodies, and the toxic metal content of the catchment sediment decreased accordingly. This study is of great significance for environmental governance in karst areas.

Assessment of soil redistribution in a typical karst catchment using 137Cs

An effective assessment of soil erosion and redistribution is a prerequisite for soil erosion control and is critical to achieving sustainable development goals. The most typical landscape in the karst region of Southwest China is found in the peak-cluster depression area, but little attention has been given to the soil redistribution here. A typical karst peak-cluster depression catchment water area in Southwest China was selected, and 137Cs technology was used to evaluate the soil redistribution rate and soil erosion process along a total transect (hillslope, depression and sinkhole) in the catchment. The results showed that the distribution of 137Cs had a high spatial variability on the total transect of the catchment (CV = 60.04%), the middle slope was the most severely eroded (highest erosion rate of 13.49 t ha−1 yr−1), and the area between the bottom slope and the depression was the primary sedimentary area on the surface in the catchment. The distribution of soil properties on the hillslope was affected by the process of soil redistribution. According to the distribution of the 137Cs soil profile, the soil profile of the hillslope was uniform, and signs of historical tillage activities were evident; the historical tillage activities of depressions were in the range of 0–20 cm depth, while the 137Cs in the sinkhole was mostly distributed in the shallow layers and decreased exponentially with depth, reflecting the depositional characteristics of noncultivated soil. In addition, this study found evidence of underground soil loss in sinkholes since the 1960s; the shallow sediment of these sinkholes mainly came from depressions, with an average deposition rate of 11.77 t ha−1 yr−1. Human disturbance and land-use change controlled recent changes in soil redistribution. The soil erosion rate of the hillslopes in catchments was extremely low (average erosion rate of 1.92 t ha−1 yr−1). The rocky desertification of hillslopes occurred before 1960; it was not a short-term contemporary process that occurred only during recent decades. This study showed that underground soil loss mainly occurred through sinkholes for a short period of time (100 years). These research results are of great significance for understanding the evolution of rocky desertification and the process of soil erosion.

Scale-specific variation in daily suspended sediment load in karst catchments

Temporal variation in sediment load is generally scale dependent and affected by many factors operating at different timescales. Due to the complexity of sediment transportation mechanisms and the nonlinear behavior of hydrological processes, identifying multi-timescale effects of variables on daily suspended sediment load (SSL) is challenging. Using multivariate empirical mode decomposition (MEMD), the aims of this study were to investigate scale-dependent relationships between daily SSL and associated variables and to predict daily SSL based on these relationships. Data on daily SSL in four typical karst catchments of southwest China and five potential variables affecting daily SSL (i.e., runoff, precipitation, air temperature, potential evapotranspiration, and Normalized Differential Vegetation Index) were collected during 2009–2012. The results indicated that MEMD decomposed temporal patterns of daily SSL and five potential variables affecting daily SSL into eight or nine intrinsic mode functions (IMFs) and a residue. Short timescale oscillations of IMF1 to IMF4 contributed more than 72 % of the total variance in daily SSL in the four catchments, indicating that temporal variation in SSL was dominated by short intervals of 4–23 days. Runoff was the dominant explanatory variable for the overall daily suspended sediment load prediction at the observation scale. The predictions of daily SSL based on the IMFs and their residuals greatly outperformed those based on the original time series data. This result implies that a single scale (e.g., observation scale) is not sufficient to capture the complex relationships that exist between daily SSL and associated variables at all timescales. The results demonstrate the superiority of MEMD in characterizing scale-specific relationships between daily SSL and associated variables and improving daily SSL predictions. Given the complexity, nonlinearity, and nonstationarity inherent in many hydrological and sediment transport processes, MEMD is recommended for identifying scale-specific variation in daily SSL in future studies.