Catchment Soils Research Articles

Challenges in flood modeling over data-scarce regions: how to exploit globally available soil moisture products to estimate antecedent soil wetness conditions in Morocco

Abstract. The Mediterranean region is characterized by intense rainfall events giving rise to devastating floods. In Maghreb countries such as Morocco, there is a strong need for forecasting systems to reduce the impacts of floods. The development of such a system in the case of ungauged catchments is complicated, but remote-sensing products could overcome the lack of in situ measurements. The soil moisture content can strongly modulate the magnitude of flood events and consequently is a crucial parameter to take into account for flood modeling. In this study, different soil moisture products (European Space Agency Climate Change Initiative, ESA-CCI; Soil Moisture and Ocean Salinity, SMOS; Soil Moisture and Ocean Salinity by the Institut National de la Recherche Agronomique and Centre d'Etudes Spatiales de la Biosphère, SMOS-IC; Advanced Scatterometer, ASCAT; and ERA5 reanalysis) are compared to in situ measurements and one continuous soil-moisture-accounting (SMA) model for basins located in the High Atlas Mountains, upstream of the city of Marrakech. The results show that the SMOS-IC satellite product and the ERA5 reanalysis are best correlated with observed soil moisture and with the SMA model outputs. The different soil moisture datasets were also compared to estimate the initial soil moisture condition for an event-based hydrological model based on the Soil Conservation Service curve number (SCS-CN). The ASCAT, SMOS-IC, and ERA5 products performed equally well in validation to simulate floods, outperforming daily in situ soil moisture measurements that may not be representative of the whole catchment soil moisture conditions. The results also indicated that the daily time step may not fully represent the saturation state before a flood event due to the rapid decay of soil moisture after rainfall in these semiarid environments. Indeed, at the hourly time step, ERA5 and in situ measurements were found to better represent the initial soil moisture conditions of the SCS-CN model by comparison with the daily time step. The results of this work could be used to implement efficient flood modeling and forecasting systems in semiarid regions where soil moisture measurements are lacking.

Susceptibility Analysis of the Mt. Umyeon Landslide Area Using a Physical Slope Model and Probabilistic Method

Every year, many countries carry out landslide susceptibility analyses to establish and manage countermeasures and reduce the damage caused by landslides. Because increases in the areas of landslides lead to new landslides, there is a growing need for landslide prediction to reduce such damage. Among the various methods for landslide susceptibility analysis, statistical methods require information about the landslide occurrence point. Meanwhile, analysis based on physical slope models can estimate stability by considering the slope characteristics, which can be applied based on information about the locations of landslides. Therefore, in this study, a probabilistic method based on a physical slope model was developed to analyze landslide susceptibility. To this end, an infinite slope model was used as the physical slope model, and Monte Carlo simulation was applied based on landslide inventory including landslide locations, elevation, slope gradient, specific catchment area (SCA), soil thickness, unit weight, cohesion, friction angle, hydraulic conductivity, and rainfall intensity; deterministic analysis was also performed for the comparison. The Mt. Umyeon area, a representative case for urban landslides in South Korea where large scale human damage occurred in 2011, was selected for a case study. The landslide prediction rate and receiver operating characteristic (ROC) curve were used to estimate the prediction accuracy so that we could compare our approach to the deterministic analysis. The landslide prediction rate of the deterministic analysis was 81.55%; in the case of the Monte Carlo simulation, when the failure probabilities were set to 1%, 5%, and 10%, the landslide prediction rates were 95.15%, 91.26%, and 90.29%, respectively, which were higher than the rate of the deterministic analysis. Finally, according to the area under the curve of the ROC curve, the prediction accuracy of the probabilistic model was 73.32%, likely due to the variability and uncertainty in the input variables.

Speciation of Phosphorus from Suspended Sediment Studied by Bulk and Micro-XANES

Mobilization, transformation, and bioavailability of fluvial suspended sediment-associated particulate phosphorus (PP) plays a key role in governing the surface water quality of agricultural catchment streams. Knowledge on the molecular P speciation of suspended sediment is valuable in understanding in-stream PP cycling processes. Such information enables the design of appropriate catchment management strategies in order to protect surface water quality and mitigate eutrophication. In this study, we investigated P speciation associated with fluvial suspended sediments from two geologically contrasting agricultural catchments. Sequential chemical P extractions revealed the operationally defined P fractions for the fluvial suspended sediments, with Tintern Abbey (TA) dominated by redox-sensitive P (PCBD), Al, and Fe oxyhydroxides P (PNaOH) and organic P (POrg) while Ballyboughal (BB) primarily composed of acid soluble P (PDetr), redox-sensitive P (PCBD), and loosely sorbed P (PNH4Cl). The dominant calcareous (Ca) elemental characteristic of BB suspended sediment with some concurrent iron (Fe) influences was confirmed by XRF which is consistent with the catchment soil types. Ca-P sedimentary compounds were not detected using bulk P K-edge XANES, and only P K-edge µ-XANES could confirm their presence in BB sediment. Bulk P K-edge XANES is only capable of probing the average speciation and unable to resolve Ca-P as BB spectra is dominated by organic P, which may suggest the underestimation of this P fraction by sequential chemical P extractions. Notably, µ-XANES of Ca K-edge showed consistent results with P K-edge and soil geochemical characteristics of both catchments where Ca-P bonds were detected, together with calcite in BB, while in TA, Ca-P bonds were detected but mostly as organic complexed Ca. For the TA site, Fe-P is detected using bulk P K-edge, which corresponds with its soil geochemical characteristics and sequential chemical P extraction data. Overall, P concentrations were generally lower in TA, which led to difficulties in Fe-P compound detection using µ-XANES of TA. Overall, our study showed that coupling sequential chemical P extractions with progressively more advanced spectroscopic techniques provided more detailed information on P speciation, which can play a role in mobilization, transformation, and bioavailability of fluvial sediment-associated P.

Quantifying the impact of vegetation changes on global terrestrial runoff using the Budyko framework

The global hydrological cycle is undergoing unprecedented changes with the intensifications of climate change and human activities. In particular, the impact of vegetation changes on the catchment hydrological processes has recently attracted widespread scientific attention. Although such impact in different regions of the globe has been extensively investigated, a comprehensive understanding of the impact across the entire globe remains elusive. In this study, we quantify the impact of vegetation change on surface runoff (Q) based on the Budyko framework over the global terrestrial environments (excluding Antarctica and northern high latitudes) during 1982–2014. Using long-term hydrometeorological observations and satellite-derived leaf area index (L) from 663 strictly selected catchments globally, we quantify the sensitivity of the Budyko parameter n to changes in L (Sn_L) at individual catchments. We find that Sn_L is closely correlated with catchment aridity index (φ) and soil water holding capacity (θp), with higher (lower) Sn_L found in more arid (humid) catchments and catchments having smaller (larger) θp values. Based on this, an empirical model relating Sn_L to φ and θp is developed, which is further combined with the Budyko model to quantify the L and climate elasticities of Q and to attribute global Q changes over 1982–2014. Results show that the L elasticity of Q is higher and can be even greater than the climate elasticities of Q in relatively dry regions. Averaged over the globe, an increase in Q (+15.5%) induced by precipitation increases are largely offset by a decreased Q (−14.9%) caused by L increases over the last three decades. The vegetation greening-induced Q declines are manifested in global drylands. Over the study period, increases in L have led to Q reductions of 48.0%, 33.2% and 20.2% in global arid, semi-arid and sub-humid regions, respectively, which are larger than climate change-induced Q changes in these three regions. Our findings highlight the importance of vegetation change in regulating the long-term Q changes in relatively dry regions and call for cautions when conducting and/or planning future revegetation activities in these areas and needs for developing appropriate vegetation management policies to better inform water resources management in the context of global change.

Integration of dendrochronological and palaeoecological disturbance reconstructions in temperate mountain forests

Disentangling the long-term changes in forest disturbance dynamics provides a basis for predicting the forest responses to changing environmental conditions. The combination of multidisciplinary records can offer more robust reconstructions of past forest disturbance dynamics. Here we link disturbance histories of the central European mountain spruce forest obtained from dendrochronological and palaeoecological records (fossil pollen, sedimentary charcoal, bark beetle remains and geochemistry) using a small glacial lake and the surrounding forest in the Šumava National Park (Czech Republic). Dendrochronological reconstructions of disturbance were created for 300-year-long records from 6 study plots with a minimum of 35 trees analyzed for the abrupt growth increases (releases) and rapid early growth rates, both indicative of disturbance events. High-resolution analysis of lake sediments were used to reconstruct 800-year long changes in forest composition and landscape openness (fossil pollen), past fire events (micro- and macroscopic charcoal), bark beetle occurrence (fossil bark beetle remains), and erosion episodes (geochemical signals in the sediment) potentially resulting from disturbance events.Tree-ring data indicate that disturbances occurred regularly through the last three centuries and identify a most intensive period of disturbances between 1780 and 1830 CE. Geochemical erosion markers (e.g. K, Zr, % inorganic) show greater flux of catchment sediment and soils in the periods 1250–1400 and 1450–1500 CE, before a substantial shift to a more erosive regime 1600–1850 and 1900 CE onwards. Pollen records demonstrate relatively small changes in forest composition during the last 800 years until the beginning of the 20th century, when there was decrease in Picea. Fossil bark beetle remains indicate continuous presence of bark beetles from 1620s to 1800s, and charcoal records suggest that more frequent fires occurred during the 18th century. Each of the dendrochronological, palaeoecological and sedimentological records provide a unique perspective on forest disturbance dynamics, and combined offer a more robust and complete record of disturbance history. We demonstrate that sedimentary proxies originating from the lake catchment mirror the forest disturbance dynamics recorded in the tree-rings. The multidisciplinary records likely record forest disturbances at different spatial and temporal scales revealing different disturbance characteristics. Integrating these multidisciplinary datasets demonstrates a promising way to obtain more complete understanding of long-term disturbance dynamics. However, integrating datasets with variable spatial and temporal influence remains challenging. Our results indicated that multiple disturbance factors, such as windstorms, bark beetle outbeaks and fires, may occur simultaneously creating a complex disturbance regime in mountain forests, which should be considered in forest management and conservation strategies.

Using a multi-dimensional approach for catchment scale herbicide pollution assessments

Worldwide herbicide use in agriculture, whilst safeguarding yields also presents water quality issues. Controlling factors in agricultural catchments include both static and dynamic parameters. The present study investigated the occurrence of herbicides in streams and groundwater in two meso-scale catchments with contrasting flow controls and agricultural landuse (grassland and arable land). Using a multi-dimensional approach, streams were monitored from November 2018 to November 2019 using Chemcatcher® passive sampling devices and groundwater was sampled in 95 private drinking water wells. The concentrations of herbicides were larger in the stream of the Grassland catchment (8.9–472.6 ng L−1) dominated by poorly drained soils than in the Arable catchment (0.9–169.1 ng L−1) dominated by well-drained soils. Incidental losses of herbicides during time of application and low flows in summer caused concentrations of MCPA, Fluroxypyr, Trichlorpyr, Clopyralid and Mecoprop to exceeded the European Union (EU) drinking water standard due to a lack of dilution. Herbicides were present in the stream throughout the year and the total mass load was higher in winter flows, suggesting a persistence of primary chemical residues in soil and sub-surface environments and restricted degradation. Losses of herbicides to the streams were source limited and influenced by hydrological conditions. Herbicides were detected in 38% of surveyed drinking water wells. While most areas had concentrations below the EU drinking water standard some areas with well-drained soils in the Grassland catchment, had concentrations exceeding recommendations. Individual wells had concentrations of Clopyralid (619 ng L−1) and Trichlorpyr (650 ng L−1). Despite the study areas not usually associated with herbicide pollution, and annual mass loads being comparatively low, many herbicides were present in both surface and groundwater, sometimes above the recommendations for drinking water. This whole catchment assessment provides a basis to develop collaborative measures to mitigate pollution of water by herbicides.

Li and δ7Li as proxies for weathering and anthropogenic activities: Application to the Dommel River (meuse basin)

Lithium (Li) contents and isotopes were studied in the Dommel catchment, a small riverine system in northern Belgium and the southern part of the Netherlands discharging into the Meuse River. The study covered surface- and groundwaters in and on sand and gravel in the catchment; it aimed at evaluating the potential of Li isotopes as effective tracers of anthropogenic activities in addition to tracing water/substratum interaction processes within a sandy environment. Dissolved lithium concentrations in the Dommel catchment span one order of magnitude, ranging from 1.55 to 39.20 μg/L with a mean concentration of 6.58 μg/L, higher than the worldwide riverine average of 1.9 μg/L. Dissolved δ7Li varies from +5.4‰ to +27.8‰. Part of the catchment is impacted by smelter effluents with Li concentrations in the range 91–526 μg/L (mean value 288.36 μg/L) and a δ7Li of around +25.6‰ before dilution along the flowpaths of the catchment basin. To interpret the dataset in terms of using Li isotopes as a proxy for anthropogenic activity, we first applied an atmospheric-input correction to waters for both Li concentration and δ7Li, as rain contributes an important fraction of the dissolved elements in Dommel waters (8–100% of Li in waters is of atmospheric origin; 100% means no interaction with rocks, no anthropogenic disturbance). Then, we determined and quantified the anthropogenic influence using δ7Li and mixing equations for the impacted parts of the catchment. Finally, we added constraints on the water/substratum interaction in this mostly sandy-feldspar soil catchment through the δ7Li and Li/Na ratios, highlighting the role of water-residence time in the system on weathering intensity.

Heterogeneous CO2 and CH4 patterns across space and time in a small boreal lake

ABSTRACT Small boreal lakes emit large amounts of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Yet emissions of these greenhouse gases are variable in space and time, in part due to variable within-lake CO2 and CH4 concentrations. To determine the extent and the underlying drivers of this variation, we measured lake water CO2 and CH4 concentrations and estimated associated emissions using spatially discrete water samples collected every 2 weeks from a small boreal lake. On select dates, we also collected groundwater samples from the surrounding catchment. On average, groundwater draining a connected peat mire complex had significantly higher CO2 and CH4 concentrations compared to waters draining forest on mineral soils. However, within the lake, only CH4 concentrations nearshore from the mire complex were significantly elevated. We observed little spatial variability in surface water CO2; however, bottom water CO2 in the pelagic zone was significantly higher than bottom waters at nearshore locations. Overall, temperature, precipitation, and thermal stratification explained temporal patterns of CO2 concentration, whereas hydrology (discharge and precipitation) best predicted the variation in CH4 concentration. Consistent with these different controls, the highest CO2 emission was related to lake turnover at the end of August while the highest CH4 emission was associated with precipitation events at the end of June. These results suggest that annual carbon emissions from small boreal lakes are influenced by temporal variation in weather conditions that regulate thermal stratification and trigger hydrologic land–water connections that supply gases from catchment soils to the lake.

Runoff and sediment yield modeling in data-sparse catchments in the Garehsoo River basin, northern Iran

To improve management and conservation in ungauged catchments in northern Iran, we developed regional models to predict annual runoff and discharge using multi-year time series from 20 gauged catchments in Ardabil, Province. We employed correlation, Ward cluster analysis, and principal component analysis to reduce the total parameter set (69 parameters represent catchment geometry, geology, soil, rainfall, and climate) to robust sets for regression analysis. The reduction method based on PCA using non-log-transformed data produced the best results, based on several error metrics (modified Nash Sutcliffe efficiency, R2, root mean squared error, and bias). Important parameters included area and mean annual precipitation, as well as those indicative of catchment shape and geology. Despite excellent ability to simulate mean annual discharge and sediment yield in the 20 catchments, the models performed poorly when applied to estimating year-to-year discharge and sediment yield in individual catchments. Poor prediction likely resulted from (a) rainfall data of insufficient spatial resolution in the large area with greatly varying elevation including mountains; and (b) inability to include data reflecting yearly changes in land-cover/land-use. Nevertheless, the regional models are likely useful for estimating mean runoff and sediment yields in other catchments in the region with similar geology, geomorphology, and climate.

Comparison of spatial interpolation methods for the estimation of precipitation patterns at different time scales to improve the accuracy of discharge simulations

Abstract Interpolating precipitation data is of prime importance to hydrological design, modeling, and water resource management. Various models have been developed that estimate spatial precipitation patterns. The purpose of this study is to analyze different precipitation interpolation schemes at different time scales in order to improve the accuracy of discharge simulations. The study was carried out in the upstream area of the Changjiang River basin. The performance of all selected methods was assessed using cross-validation schemes, with the mixed methods ultimately displaying the best performance at all three time scales. However, the differences in performance between the spatial interpolation methods decreased with increasing time scales. The unifying catchment Soil and Water Assessment Tool (SWAT), ‘abcd’, and the Budyko equation were employed at the daily, monthly, and annual scales, respectively, to simulate discharge. The performance of the discharge simulation at the monthly and annual time scales was consistent with their ranks of spatial precipitation estimation. For coarse, or long period, precipitation, there were no significant differences. However, the mixed methods performed better than the single model for the daily, or short, time scale with respect to the accuracy of the discharge simulation.

Assessing branched tetraether lipids as tracers of soil organic carbon transport through the Carminowe Creek catchment (southwest England)

Abstract. Soils represent the largest reservoir of organic carbon (OC) on land. Upon mobilization, this OC is either returned to the atmosphere as carbon dioxide (CO2) or transported and ultimately locked into (marine) sediments, where it will act as a long-term sink of atmospheric CO2. These fluxes of soil OC are, however, difficult to evaluate, mostly due to the lack of a soil-specific tracer. In this study, a suite of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane lipids of soil bacteria, is tested as specific tracers for soil OC from source (soils under arable land, ley, grassland, and woodland) to sink (Loe Pool sediments) in a small catchment located in southwest England (i.e. Carminowe Creek draining into Loe Pool). The analysis of brGDGTs in catchment soils reveals that their distribution is not significantly different across different land use types (p>0.05) and thus does not allow land-use-specific soil contributions to Loe Pool sediments to be traced. Furthermore, the significantly higher contribution of 6-methyl brGDGT isomers in creek sediments (isomerization ratio (IR) = 0.48±0.10, mean ± standard deviation (SD); p<0.05) compared to that in catchment soils (IR = 0.28±0.11) indicates that the initial soil signal is substantially altered by brGDGT produced in situ. Similarly, the riverine brGDGT signal appears to be overwritten by lacustrine brGDGTs in the lake sedimentary record, indicated by remarkably lower methylation of branched tetraethers (MBT5ME′=0.46±0.02 in creek bed sediments and 0.38±0.01 in lake core sediments; p<0.05) and a higher degree of cyclization (DC = 0.23±0.02 in creek bed sediments and 0.32±0.08 in lake core sediments). Thus, in this small catchment, brGDGTs do not allow us to trace soil OC transport. Nevertheless, the downcore changes in the degree of cyclization and the abundance of isoprenoid GDGTs produced by methanogens in the Loe Pool sediment do reflect local environmental conditions over the past 100 years and have recorded the eutrophication history of the lake.

Quantifying farmland shelterbelt impacts on catchment soil erosion and sediment yield for the black soil region, northeastern China

AbstractAs one part of the ‘Three Norths’ forest protection system, dense farmland shelterbelt networks in northeastern China could greatly modify water and sediment flows. In this paper, catchment soil erosion rate and sediment yield (SY) that are impacted by farmland shelterbelts were estimated using WaTEM/SEDEM model. The shelterbelts reduced catchment soil erosion and SY to some extent. The mean soil erosion rate and specific sediment yield (SSY; defined as the ratio of SY to catchment area; t km−2 yr−1) of the 25 reservoir catchments decreased from 351.6 and 93.9 t km−2 yr−1 under the supposed scenario without shelterbelts to 331.1 t km−2 yr−1 and 86.3% t km−2 yr−1 under the current situation with shelterbelts. The sediment trap efficiencies (STEs) varied from 0.01% to 23.6% with an average value of 7.6%. The STEs were significantly correlated with shelterbelt density, catchment perimeter, topographic factors, RUSLEP‐factor and land use patterns including patch density (PD), patch cohesion index (COHESION), Shannon's diversity index (SHDI) and aggregation index (AI). The multiple regression equation involving factors of catchment's topography and morphology and land use pattern has a satisfactory performance, and mean slope gradient (MSG) and AI explained most of the variability of shelterbelts’ STE. This information can help land managers to better design shelterbelts and to reduce water‐derived soil loss at catchment scale.

Ice formation on lake surfaces in winter causes warm-season bias of lacustrine brGDGT temperature estimates

Abstract. It has been frequently found that lacustrine branched glycerol dialkyl glycerol tetraethers (brGDGT)-derived temperatures are warm-season-biased relative to measured mean annual air temperature (AT) in the middle to high latitudes, the mechanism of which, however, is not very clear. Here, we investigated the brGDGTs from catchment soils, suspended particulate matter (SPM) and surface sediments in different water depths in Gonghai Lake in northern China to explore this question. Our results showed that the brGDGT distribution in sediments resembled that in the SPM but differed from the surrounding soils, suggesting a substantial aquatic origin of the brGDGTs in the lake. Moreover, the increase in brGDGT content and decrease in methylation index with water depth in sediments suggested more contribution of aquatic brGDGTs produced from deep or bottom waters. Therefore, established lake-specific calibrations were applied to estimate local mean annual AT. As usual, the estimates were significantly higher than the measured mean annual AT. However, they were similar to (and thus actually reflected) the mean annual lake water temperature (LWT). Interestingly, the mean annual LWT is close to the measured mean warm-season AT, thus suggesting that the apparent warm-season bias of lacustrine brGDGT-derived temperatures could be caused by the discrepancy between AT and LWT. In our study region, ice forms at the lake surface during winter, leading to isolation of the underlying lake water from air and hence higher LWT than AT, while LWT basically follows AT during warm seasons when ice disappears. Therefore, we think that lacustrine brGDGTs actually reflected the mean annual LWT, which is higher than the mean annual AT in our study location. Since the decoupling between LWT and AT in winter due to ice formation is a universal physical phenomenon in the middle to high latitudes, we propose this phenomenon could be also the reason for the widely observed warm-season bias of brGDGT-derived temperatures in other seasonally surface ice-forming lakes, especially in shallow lakes.

Age and origin of leaf wax <i>n</i>-alkanes in fluvial sediment–paleosol sequences and implications for paleoenvironmental reconstructions

Abstract. Leaf wax n-alkanes are increasingly used for quantitative paleoenvironmental reconstructions. However, this is complicated in sediment archives with associated hydrological catchments since the stored n-alkanes can have different ages and origins. 14C dating of the n-alkanes yields independent age information for these proxies, allowing their correct paleoenvironmental interpretation. This also holds true for fluvial sediment–paleosol sequences (FSPSs) that integrate two different n-alkane signals: (i) a catchment signal in fluvial sediments and (ii) an on-site signal from local biomass that increasingly dominates (paleo)soils with time. Therefore, the age and origin of n-alkanes in FSPSs are complex: in fluvial sediment layers they can be pre-aged and reworked when originating from eroded catchment soils or from organic-rich sediment rocks in the catchment. In (paleo)soils, besides an inherited contribution from the catchment, they were formed on-site by local biomass during pedogenesis. Depending on the different relative contributions from these sources, the n-alkane signal from an FSPS shows variable age offsets between its formation and final deposition. During this study, we applied compound-class 14C dating to n-alkanes from an FSPS along the upper Alazani in eastern Georgia. Our results show that preheating the n-alkanes with 120 ∘C for 8 h before 14C dating effectively removed the shorter chains (<C25) that partly originate from n-alkanes from Jurassic black clay shales in the upper catchment. The remaining petrogenic contributions on the longer chains (≥C25) were corrected for by using a constant correction factor that was based on the n-alkane concentrations in a black clay shale sample from the upper catchment. Due to different degrees of pre-aging and reworking, the corrected leaf wax n-alkane ages still indicate relatively large age offsets between n-alkane formation and deposition: while intensively developed (paleo)soils showed no age offsets due to a dominance of leaf wax n-alkanes produced on-site, less intensively developed paleosols showed much larger age offsets due to larger proportions of inherited leaf wax n-alkanes from the fluvial parent material. Accordingly, age offsets in nonpedogenic fluvial sediments were largest and strongly increased after ∼4 ka cal BP. The leaf wax n-alkane homolog distribution from intensively developed (paleo)soils indicates a local dominance of grasses and herbs throughout the Holocene, which was most likely caused by anthropogenic activity. The leaf wax n-alkanes from fluvial sediments show a dominance of deciduous trees and shrubs as well as grasses and herbs in different parts of the catchment between ∼8 and ∼5.6 ka cal BP. Since no older deciduous tree- or shrub-derived n-alkanes were dated, this seems to confirm a delayed regional postglacial reforestation of parts of the catchment compared with western and central Europe.

Impact of land use changes on catchment soil erosion and sediment yield in the northeastern China: A panel data model application

Land use is an important factor influencing soil erosion and sediment yield (SESY). Regressing soil erosion intensity (SEI) and sediment yield (SY) to land use characteristics can provide necessary information for controlling soil loss. However, current simple regression methods emphasize cross sectional parameters, with less emphasis on temporal variability of relevant land use parameters so that the derived effects of land use change on SESY can be biased. Here, a panel data method was applied to quantify the impact of land use change on SESY in 1954, 1975, and 2015, based on the WaTEM/SEDEM model and seven landscape metrics for 25 reservoir catchments in northeastern China. The results indicate that SEI and area-specific SY (SSY) continuously decreased from 1954 to 2015, which were significantly correlated with landscape metrics such as area-edge metrics of mean patch area (AREA_MN), shape index of the mean related circumscribing circle (CIRCLE_MN), aggregation index of effective mesh size (MESH), patch cohesion index (COHESION), and diversity metrics such as Shannon's diversity index (SHDI), patch richness density (PRD), and modified Simpson's evenness index (MSIEI). The results suggested that catchment SESY can be reduced through decreasing mean patch area, patch mesh size, and physical connectivity of patches, enriching landscape types, and elongating land use patches. These findings are helpful to effectively implement soil conservation measures in northeastern China and similar regions worldwide. The current study also implies that the panel data approach will have beneficial potential applications in earth-science research fields.

Dynamic transfer of soil bacteria and dissolved organic carbon into small streams during hydrological events

Small headwater streams interlink catchment soils with the river network. As water makes its way from the hillslopes to the stream, it may transport dissolved organic carbon (DOC) and potentially soil microbes into stream water. In this study, we aimed at quantifying the dynamic transfer of DOC and microbial life, namely bacteria from catchment soils into streams. We hypothesized that increased soil saturation enhances the lateral inflow of bacteria and DOC into streams. To address this hypothesis, we sampled six first order streams and three soil transects at two different depths located within the pre-alpine Oberer Seebach (OSB) catchment in Austria over a duration of 2 years. We found a strong variation in DOC concentrations (range 0.4–5.6 mg L−1) and bacterial abundances (range < 500,000–3,863,000 cells mL−1) measured by flow-cytometry. The highest values of DOC and bacterial cells occurred during high flow events. DOC concentration and bacterial abundance were correlated across all streams and seasons. In soils, DOC ranges were higher and were also correlated with bacterial abundance, while DOC concentrations were ∼ 10 times higher per bacterial cell than in streams. Overall we show that soils provide a dynamic inflow of bacteria and DOC to first order streams. Most probably, this results in a dynamic and reoccurring inoculation of small streams from catchment soils during runoff events. We propose that this dynamic microbial inoculation of small streams is potentially relevant for microbial community dynamics of downstream receiving waters.

Continuous measurements of riverine chemical constituents reveal the environmental effect of the 2014–2015 Bárðarbunga eruption in Iceland

The 2014–2015 Bárðarbunga eruption caused one of the most intense and widespread volcanic derived air pollution events in centuries. The volcanic plume was tracked all the way to central Europe. Volcanic gas and aerosol dissolved in precipitation and accumulated as aerosol and snow on land in Iceland. Here we report on the continuous monitoring of discharge and dissolved constituents of a direct runoff river located 85 km from the eruption site during the entire snowmelt period in 2015. Osmotic samplers collected continuous water samples so short-lived flood peaks would not be missed. Comparison between results from this 2015 study and data from 1998 to 2013 from the same river showed that average pH was 0.2 units lower in 2015 and the Cl and SO4 average concentration were 36% and 38% higher than prior to the eruption, respectively. Integrated riverine fluxes of H+ and SO4 were two to three times higher during summer 2015 than in the control years, and the Cl flux was 37% higher. The measured excess H+ flux was however only a small fraction of the calculated excess H+ flux stemming from excess SO4 and Cl fluxes in 2015 which can be explained by ion-exchange in the catchment soil, water-rock interaction consuming protons within the catchment, and by elevated sea spray derived Cl and SO4 due to relatively high wind speed during the 2014–2015 winter. In addition, the small H+ increase in the river water can be the cause of the less than expected excess SO4 riverine flux, normalised to the total measured volcanogenic SO2 and HCl emission fluxes (11 Tg/0.1 Tg), which underscores the slow oxidation of SO2 to SO4 in relatively dry weather conditions and reduced sunlight during winter in Iceland. Despite being sensitive to riverine pH and aluminium changes, freshwater ecosystem was not threatened by the eruption, since it did not cause large pH changes and the dissolved aluminium concentration was always low during the study period. This study provided the means to quantify the integrated chemical fluxes and the impact of volcanic emissions on a riverine system in the vicinity of the eruption site and the implication of increased chemical fluxes of solutes of volcanic origin on the system.

Multivariate statistics and geochemical approaches for understanding source-sink relationship - a case study from close-basin lakes in Southeast Amazon

Multivariate statistical approaches were applied on geochemical data of lake sediments and the catchment basin materials (Al-enriched crusts and soils) from four upland lakes (namely LB1, LB2, LB3, and LB4) in the Serra da Bocaína, Southeast Amazon, to recognize the geochemical signatures of lake sediments and their link with source rocks. This is also compared with Fe-enriched crusts and soils, which collected from Serra Sul de Carajás. The major and trace elements were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and X-ray Fluorescence (XRF), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), respectively. The data closure effect was reduced by centred log ratio (clr) transformation. The upper continental crust (UCC) normalized patterns show that sediments are mainly enriched with Ti, Fe, P, Se, V and Cr, similar to catchment basin crusts and soils. The distribution of elements varied between lakes, which is mainly influenced by the morphology and lithology of the basin. The chemical index of alternation (CIA) values were very high (>90) for both lake sediments and catchment materials, which indicates intense source area weathering. Index of lateritization (IOL) shows that the catchment crusts are strongly lateralized and it identifies samples between extremely weathered and reworked samples that do not meet the criteria for strict in situ formation. The principal components analysis (PCA) and Spearman correlation matrices based on clr-transformed data revealed four geochemical groups in sediments. The major detritic geochemical association of sediments (Group 1: Al–Ti-Ga-Cr-V-Ti-Nb-Sc, which indicative of mafic/metavolcanic rock; and Group 2: Zr-Hf-HREEs, which refers to the heavy mineral fractions that remain stable during lateritization) had a close relationship with catchment lateritic crust, except a few alterations, which possibly influenced by weathering and transport processes. REE pattern normalized to chondrite and the geochemical ratios show the analogies between the lake sediments and the catchment lateritic crusts. The linear discriminant analysis (LD) and PCA distinguished Al-rich laterites from Fe-rich laterites and indicates that the former (which likely derived from mafic/metavolcanic rocks) is the main source of lake sediments.

Trade-off between surface runoff and soil erosion during the implementation of ecological restoration programs in semiarid regions: A meta-analysis

The application of ecological restoration programs, related to water resources protection and soil erosion control, may have some undesirable outcomes. An important example is the effect that vegetation restoration may have in reducing surface water resources. After searching peer-reviewed articles, we selected 38 publications from 16 countries in comparable areas - semiarid conditions (aridity index <0.5), surface coverage >50% and fine soil texture - to evaluate the effectiveness of different types of vegetation (i.e., forestland, scrubland and grassland) in regulating runoff and sediment transport. In particular, we used three indices: the runoff reduction effect, the sediment reduction effect and the ratio between runoff and sediment reduction. These indices were calculated from measured data reported in the original articles. Results showed that scrubland had higher runoff reduction effect (59% in gentle slopes; 65% in steep slopes) than in grassland (39% on gentle slopes; 43% on steep slopes) and forestland (33% on gentle slopes; 51% on steep slopes). For the three types of vegetation, the sediment reduction effect was >70%. Concerning the ratios between runoff and sediment reduction, grassland showed the lowest ratios (56% on gentle slopes; 53% on steep slopes) compared to forestland (63% on gentle slopes; 65% on steep slopes) and scrubland (93% on gentle slopes; 81% on steep slopes). Our results indicate that low values of ratios between runoff and sediment reduction are the most suitable because they indicate an effective soil erosion and sediment delivery reduction but maintaining surface runoff. Overall, our study demonstrates that grassland may be the best choice for optimizing the trade-off between catchment water yield and soil conservation during the implementation of ecological restoration programs in semi-arid regions.

Modern Analogue Approach Applied to High-Resolution Varved Sediments—A Synthesis for Lake Montcortès (Central Pyrenees)

In Quaternary paleosciences, the rationale behind analogical inference presupposes that former processes can be explained by causes operating now, although their intensity and rates can vary through time. In this paper we synthesised the results of different modern analogue studies performed in a varved lake. We discuss their potential value to obtain best results from high resolution past records. Different biogeochemical contemporary processes revealed seasonality and year-to-year variability, e.g., calcite precipitation, lake oxygenation, production and deposition of pollen and phytoplankton growth. Fingerprints of the first two of these processes were clearly evidenced in the varve-sublayers and allow understanding related to past events. Pollen studies suggested the possibility of identifying and characterizing seasonal layers even in the absence of varves. Marker pigments in the water column were tightly associated with phytoplankton groups living today; most of them were identified in the sediment record as well. We observed that 50% of these marker pigments were destroyed between deposition and permanent burying. In another study, seasonality in the production/distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) and derived temperature estimates were investigated in catchment soils and particles settling in the lake. The signatures of brGDGTs in depositional environments mainly were representative of stable conditions of soils in the catchment that last over decades; no brGDGTs seemed to be produced within the lake. The main contribution of this review is to show the advantages and limitations of a multiproxy modern-analogue approach in Lake Montcortès as a case study and proposing new working hypotheses for future research.