Catchment Erosion Research Articles

Estimation of Soil Erosion and Identification of Critical Areas for Soil Conservation Measures using RS and GIS-based Universal Soil Loss Equation

Present study aimed at estimating soil erosion potential and identification of critical areas for soil conservation measures in an ungauged catchment situated in Aravalli hills of Udaipur district, Rajasthan (India). Also, impact of rainfall on soil erosion is evaluated. The soil erosion is estimated for 10 year period (2001–2010) by Universal Soil Loss Equation (USLE) model using Geographical information system (GIS) and remote sensing techniques for every 12 m × 12 m area. Thematic maps of six USLE model parameters, i.e., rainfall erosivity (R-factor), soil erodibility (K-factor), slope length (L-factor), slope steepness (S-factor), crop and management (C-factor), and support practice (P-factor), were prepared in GIS platform. The R-factor ranged from 1,522.93 to 10,225.88 MJ mm ha−1 h−1 year−1 in the years 2006 and 2008, respectively, when the annual rainfall was 984.3 and 572.2 mm, and number of rainy days were 58 and 47, respectively. The K-factor was highest for fine loam soil covering 56 % area, while the lowest value was for coarse loamy sand in 22 % area. The lowest value of the L-factor (0.736) was in accordance with the high slopes nearby catchment boundary; whereas the highest value (0.832) was for almost zero slopes in 34 % area nearby waterbodies. Opposite to the L-factor, the S-factor values were high (>4) for the higher slopes nearby catchment boundary and the lowest values for the zero slopes. The C-factor value in 170.36 km2 or 48.91 % of the area is 0.1 while the value is zero for waterbodies and builtup lands. The P-factor value in 250.36 km2 or 71.87 % of the area is 0.8. The mean annual soil erosion in the major portion of the catchment (231.13 km2 or 66.38 %) exceeds 10 t ha−1 year−1 indicating high to very severe soil erosion conditions prevailing in the catchment. It is apparent that vast quantities of the soil are getting eroded from the catchment, and the annual rainfall amount and rainfall intensity have the profound effect on the soil erosion potential. This study emphasizes that USLE model coupled with GIS and remote sensing techniques are promising and cost-effective tools for mapping critical areas of soil erosion in ungauged catchments especially in developing countries.

Organic lacustrine sediment varves as indicators of past precipitation changes: a 3,000-year climate record from Central Finland

Annually laminated (varved) sediments from Lake Kallio-Kourujarvi, Central Finland, provide high-resolution sedimentological data for the last three millennia. These varves consist of two laminae that represent (1) deposition during the spring-to-autumn growing season, composed of degraded organic matter and a variety of microfossils, and (2) deposition during winter, composed of fine-grained homogenous organic matter. Because of the absence of a clastic lamina, these varves differ from the typical, well-described, clastic-organic varve sequences in Fennoscandian lakes. Such organic varves in Finnish lakes have not been studied in detail before. Three thousand varves were counted and their seasonal deposition was distinguished. Comparison of varve thickness with meteorological data revealed a positive correlation between organic varve thickness and precipitation. This suggests that catchment erosion processes and consequent organic matter and nutrient inputs are important factors in organic varve formation. The correlation between temperature and growing-season lamina thickness varied from insignificant, to positive, to negative during different time spans. This suggests that organic matter accumulation can sometimes have a significant, but unpredictable role in organic varve formation, via organic matter production and degradation, processes that are influenced strongly by water column temperature. The organic varves of Lake Kallio-Kourujarvi enable a unique, high-resolution approach for the study of past climate and environment. Our results suggest that decadal periods of increased precipitation occurred during BP 2150–2090, 1710–1620, 1410–1360, 920–870 (1030–1080 AD), and after 370 BP (1580 AD). Dryer intervals occurred during BP 2750–2720, 1900–1850, 1800–1740, 1600–1500, and 780–700 (1170–1250 AD), 590–520 (1360–1430 AD).

Evidence of global pollution and recent environmental change in Kamchatka, Russia

Kamchatka is a remote, isolated and understudied area and is presumed to be pristine. Here we present the first high-resolution palaeolimnological investigation of the recent past. A short core representing the last 250years was taken from Olive-backed Lake situated in central Kamchatka. Lead-210 dating revealed that sediment accumulation has increased at the site since the 1960s and may be related to greater rates of catchment erosion associated with wetter winters in the region. Mercury and spheroidal carbonaceous particle (an unambiguous indicator of fossil fuel combustion) concentrations are low but clearly detectable indicating that both regional and global pollution sources are observed at this site. The recent increase in the flux of mercury is more related to catchment sources and catchment erosion than increases from regional or global sources. The diatom and chironomid populations are stable and do not show any statistically significant changes related to either the low levels of pollution, or to temperature and precipitation changes. The lake is not pristine since anthropogenic contamination has occurred but since there have been no significant effects on the flora and fauna the lake can be considered to be unimpacted. Olive-backed Lake may be a suitable reference site to benchmark the natural variability of a lake ecosystem.

Sediment lipid biomarkers record increased eutrophication in Lake Fuxian (China) during the past 150 years

Organic geochemical analysis of a sediment core from Lake Fuxian was carried out to reconstruct the progression of eutrophication in this lake from ca. 1857 to 2012. The results indicate that two episodes can be identified during the eutrophication of the lake water. Both bulk geochemical proxies and sediment lipid biomarkers recorded an enhanced eutrophication in this lake since 1986. Total organic carbon (TOC), total nitrogen (TN) and phytoplankton biomarkers were most abundant in this period, indicating high phytoplankton productivity. Phytoplankton composition showed an elevated proportion of dinoflagellate and green algae relative to diatoms, in response to the anthropogenic nutrient inputs (e.g., sewage, fertilizers) since the 1980s. On the other hand, only the sediment lipid biomarker detected a minor eutrophication of the lake water between 1951 and 1986. During this period, phytoplankton biomarkers, regardless of their labile or refractory nature, exhibited a little enhanced concentration in the sediment, indicating a minor increase of phytoplankton productivity. Phytoplankton composition was characterized by a slight increase of diatoms relative to dinoflagellates and green algae. Variations of algal productivity and community structure in this period could be attributed to the nutrients (e.g., soil Si) input from catchment erosion. A causal relationship might be present between the eutrophication process of Lake Fuxian and the anthropogenic activities within the catchment.

Tracing Sediment Sources Using Strontium Isotopes in a Pond Draining an Agricultural Catchment (Loire River Basin, France)

Soil erosion in agricultural catchments leads to the accelerate supply of sediment to rivers. Here, we determine the sources contributing to the siltation of a pond in the Louroux catchment (24km2, French Loire River basin), representative of lowland agricultural environments of Northwestern Europe. Strontium isotopic ratios weredetermined in potential sources and sediment core samples collected in the Louroux pond, at the outlet of the catchment. These ratios were shown to provide good discrimination between differentsediment sources (soil, suspended particulate matter, calcareous rock samples), and their respective contributions were calculated using mass balance equations. These results should be used to guide the implementation of soil conservation measures to limit sediment supply to the Louroux River.

Skład Chemiczny Osadów Bagiennych z Doliny Luciąży (Torfowisko Bęczkowice na Równinie Piotrkowskiej)

AbstraktAnalizę geochemiczną przeprowadzono dla osadów organicznych z torfowiska Bęczkowice w środkowym biegu doliny rzeki Luciąża. Z profilu B-1, zlokalizowanego w południowej części doliny, pobrano do analiz geochemicznych 60 próbek osadów (do głębokości 3,00 m), reprezentujących 5-cm segmenty rdzenia. W próbkach tych oznaczono straty na prażeniu w temperaturze 550°C, informujące o zawartości materii organicznej oraz oznaczono pierwiastki o wymowie paleogeograficznej: Na, K, Mg, Ca, Fe, Mn i metale ciężkie: Cu, Zn i Pb. Stosując analizę skupień wydzielono sześć lokalnych poziomów geochemicznych (LGZ), różniących się istotnie pod względem składu chemicznego osadów. Dla serii osadów torfowych określono również związki korelacyjne pomiędzy analizowanymi metalami, a także materią organiczną oraz obliczonymi wskaźnikami geochemicznymi: Ca/Mg, Na/K, Na+K+Mg/ Ca, Fe/Ca, Fe/Mn, Cu/Zn, pozwalającymi na rekonstrukcję warunków akumulacji analizowanych osadów. W toku analizy składowych głównych zidentyfikowano 10 głównych czynników, z których wybrano trzy pierwsze PC1, PC2 i PC3, wyjaśniające łącznie 84,4% wariancji. Do czynników kształtujących skład chemiczny holoceńskich osadów organicznych w środkowej części doliny Luciąży zaliczono produktywność biologiczną ekosystemu torfowego, procesy denudacji chemicznej i mechanicznej w zlewni (zwłaszcza dostawę allochtonicznej materii mineralnej), wzrost udziału wód opadowych w bilansie wodnym torfowiska oraz działalność człowieka.

Sources and Deposition Fluxes of PCDD/Fs in the Largest Reservoir System in Taiwan before and after Typhoon Morakot

The Tsengwen Reservoir (TWR) is located at 23°16′N, 120°34′E and is the largest reservoir in Taiwan. During 2009 to 2010, deposition of polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) in ambient air, water bodies and sediments were measured in the TWR area for this study. The monthly atmospheric PCDD/F deposition in the vicinity of the TWR was found to range from 0.206 to 2.43 pg I-TEQ/m2/day. In addition, the PCDD/F levels (11 to 12 fg I-TEQ/L) in the surface water (water depth: 0.5 m) were found to be uniform and well mixed within the upstream, middle and lower reaches of the TWR. The PCDD/F input flux into the reservoir was investigated together with the PCDD/F concentrations at 1–2 cm intervals in two sediment cores collected at the TWR. The year dating within the sediment cores at different depths was estimated from the sedimentation rate (> 17 cm/y) and calculated by 210Pb analysis. The PCDD/F content at different depth within the sediment cores ranged from 0.136 ng I-TEQ/kg d. w. to 0.413 ng I-TEQ/kg d. w. Before the Morakot Typhoon event (June, 2009), the PCDD/F content of the surface sediments ranged from 0.091 to 0.187 ng I-TEQ/kg d. w. as measured at the upstream, middle and lower reaches of the TWR. Interestingly, the PCDD/F content (0.132 to 0.222 ng I-TEQ/kg d. w.) of surface sediments increased somewhat after typhoon event (January, 2010). The PCDD/F input contributed by the enhanced catchment erosion significantly increased to 99% during intensive typhoon periods.

Skład chemiczny osadów bagiennych z doliny Luciąży (torfowisko Bęczkowice na Równinie Piotrkowskiej)

Beczkowice peatland is located in central part of the river valley Luciąza, near Kamiensk. In order to reconstruct the main stages of sedimentation of organic deposits, taken from the southern part of the Beczkowice peatlands, used stratigraphic variability of concentration marked lithogeochemistry elements (organic matter, mineral matter, macro- and microelements) and changes in the proportions of the participation of selected elements, i.e., catchment erosion indicator (Na+Mg+K/Ca), eutrophication indicator (Fe/Ca), type and rate of denudation in the catchment indicator (Na/K, Ca/Mg) and conditions of oxidation-reduction indicator (Cu/Zn, Fe/Mn). Peat sedentation was interrupted at the beginning of the Atlantic. The peatland developed again in the Subatlantic (Forysiak 2012). As a results of the hierarchical cluster analysis was collected in the study profile of six geochemical level (B-1/I–VI), which differ significantly of chemical composition. The main lithogeochemistry component of studied sediment is organic matter (42.4–93.2%), which indicates the relative changes in the primary of biological production in wetlands. The average content of the several tested components (for example: mineral matter, lithophilic and sulfophilic elements) have the low differentiation between all geochemical levels, constitute a record stable of environmental conditions (mainly geomorphological and hydrological). Sediments of geochemical level B-1/I represents the phase of mineral and mineral with a small amount organic matter layer in reduced conditions (increase Fe/Mn ratio to 313) and increased mechanical denudation (catchment erosion indicator ranges from 2.58 to 3.1 and type of denudation in the catchment indicator – Na/K ranges from 0.07 to 0.10). Geochemical level B-1/II are characterized by gradual increase of organic matter content (from 18.3 to 66.2%) and slow increase type of denudation in the catchment indicator: Na/K (from 0.07 to 0.09) in the clay limnic deposits. Geochemical levels B-1/III are the record of sedentation autochthonous rock-forming matter of autogenic origin (average content of organic matter is 72.2%) and rapid change of redox conditions (decrease of Fe/Mn ratio from 1036 to 358). Geochemical levels B-1/IV represents the phase mainly of sedge-moss peat and reed peat layers deposition in oxygenated conditions in sedimentary environment (average Fe/Mn ratio is 63.1) and gradual increase rate of chemical denudation (Na/K ratio ranges from 0.6 to 1.95). Geochemical level B-1/V is record of sedentation of autochthonous rock-forming matter of autogenic origin (organic matter ranges from 69.3% to 91.9%) and significant increase of mechanical denudation in the catchment (average of catchment erosion indicator increased to 0,36). Geochemical level B-1/VI is the record change type of sedentation of peat on muck, as is also indicated by abrupt decrease of organic matter (to 80%), decrease of Fe/Mn ratio (to 51) and increase catchment erosion indicator (from 0.14 to 0.67). The most important factors (distinguished on the basis of principal components analysis) that affect the chemical composition of sediments from the site Beczkowice are: biological productivity in wetland’s ecosystem, mechanical and chemical denudation processes in the catchment (eg. the supply of allochtonous mineral matter), sorption of organic deposits, increase of the rainwater in the water balance and anthropogenic activity.

Skład Chemiczny Osadów Bagiennych Z Doliny Rawki (Torfowisko Kopanicha, Równina Łowicko-Błońska)

AbstraktAnalizę geochemiczną przeprowadzono dla osadów organicznych z torfowiska Kopanicha w dolinie środkowej Rawki. Z profilu Kop-1, zlokalizowanego w centralnej części mokradła, pobrano do analiz geochemicznych 59 próbek osadów (do głębokości 3,1 m), reprezentujących 2,5-cm segmenty rdzenia. W próbkach tych oznaczono straty na prażeniu w temperaturze 550°C, informujące o zawartości materii organicznej oraz oznaczono pierwiastki o wymowie paleogeograficznej: Na, K, Mg, Ca, Fe, Mn i metale ciężkie: Cu, Zn i Pb. Stosując analizę skupień wydzielono siedem lokalnych poziomów geochemicznych (LGZ ), różniących się istotnie pod względem składu chemicznego osadów. Dla serii osadów torfowych określono również związki korelacyjne pomiędzy analizowanymi metalami, a także materią organiczną oraz obliczonymi wskaźnikami geochemicznymi: Ca/Mg, Na/K, Na+K+Mg/ Ca, Fe/Ca, Fe/Mn, Cu/Zn, pozwalającymi na rekonstrukcję warunków akumulacji analizowanych osadów. W toku analizy składowych głównych zidentyfikowano 10 głównych czynników, z których wybrano trzy pierwsze - PC 1, PC 2 i PC 3, wyjaśniające łącznie 78,98% wariancji. Do głównych czynników kształtujących skład chemiczny holoceńskich osadów organicznych w dolinie Rawki zaliczono sorpcję metali przez materię organiczną i uwodnione tlenki żelaza, zmienność akumulacji minerałów ilastych w środowisku sedymentacyjnym, warunki oksydacyjno-redukcyjne oraz działalność człowieka.

Landscape hydrology. The hydrological legacy of deforestation on global wetlands.

Increased catchment erosion and nutrient loading are commonly recognized impacts of deforestation on global wetlands. In contrast, an increase in water availability in deforested catchments is well known in modern studies but is rarely considered when evaluating past human impacts. We used a Budyko water balance approach, a meta-analysis of global wetland response to deforestation, and paleoecological studies from Australasia to explore this issue. After complete deforestation, we demonstrated that water available to wetlands increases by up to 15% of annual precipitation. This can convert ephemeral swamps to permanent lakes or even create new wetlands. This effect is globally significant, with 9 to 12% of wetlands affected, including 20 to 40% of Ramsar wetlands, but is widely unrecognized because human impact studies rarely test for it.

Process-based modelling of erosion, sediment transport and reservoir siltation in mesoscale semi-arid catchments

To support scientifically sound water management in dryland environments a modelling system has been developed for the quantitative assessment of water and sediment fluxes in catchments, transport in the river system, and retention in reservoirs. The spatial scale of interest is the mesoscale because this is the scale most relevant for management of water and land resources. This modelling system comprises process-oriented hydrological components tailored for dryland characteristics coupled with components comprising hillslope erosion, sediment transport and reservoir deposition processes. The spatial discretization is hierarchically designed according to a multi-scale concept to account for particular relevant process scales. The non-linear and partly intermittent run-off generation and sediment dynamics are dealt with by accounting for connectivity phenomena at the intersections of landscape compartments. The modelling system has been developed by means of data from nested research catchments in NE-Spain and in NE-Brazil. In the semi-arid NE of Brazil sediment retention along the topography is the main process for sediment retention at all scales, i.e. the sediment delivery is transport limited. This kind of deposition retains roughly 50 to 60 % of eroded sediment, maintaining a similar deposition proportion in all spatial scales investigated. On the other hand, the sediment retained in reservoirs is clearly related to the scale, increasing with catchment area. With increasing area, there are more reservoirs, increasing the possibility of deposition. Furthermore, the area increase also promotes an increase in flow volume, favouring the construction of larger reservoirs, which generally overflow less frequently and retain higher sediment fractions. The second example comprises a highly dynamic Mediterranean catchment in NE-Spain with nested sub-catchments and reveals the full dynamics of hydrological, erosion and deposition features. The run-off modelling performed well with only some overestimation during low-flow periods due to the neglect of water losses along the river. The simulated peaks in sediment flux are reproduced well, while low-flow sediment transport is less well captured, due to the disregard of sediment remobilization in the riverbed during low flow. This combined observation and modelling study deepened the understanding of hydro-sedimentological systems characterized by flashy run-off generation and by erosion and sediment transport pulses through the different landscape compartments. The connectivity between the different landscape compartments plays a very relevant role, regarding both the total mass of water and sediment transport and the transport time through the catchment.

The effect of cultivation method on erosion in agricultural catchments: integrating AHP in GIS environments

AbstractThe cultivation method used in agricultural catchments can have a great effect on erosion processes; as such, determining the effects on form and degree is crucial. One commonly held hypothesis is that a shift to minimum tillage methods should reduce the rate of erosion. Here, we examine the effect of cultivation methods and environmental conditions on soil erosion risks in field crops and orchards in an agricultural catchment in northern Israel. The examination was conducted using AHP (analytic hierarchy process) and GIS (geographic information system)‐based computer simulations. Field validation of the simulations was conducted during the 2009–2010 winter season. The spatially explicit data on cultivation method, combined with environmental and climatic data, yielded an explanation of most of the variation in erosion risks in the catchment (kappa =0·93). Of the 10 criteria examined, the cultivation method and slope were the two variables with the greatest effect on increased soil erosion. Furthermore, soil loss risks were reduced substantially as a result of substituting conventional tillage with reduced tillage; substituting reduced tillage with conservation tillage; and changing the tillage direction to perpendicular to the direction of the slope. These results are reasonable in light of the modifications that mechanical tools cause in the soil structure, as observed in the penetration depth and the aggregate stability measurements used in this study. Despite the difficulty in collecting spatially explicit data on cultivation methods, we believe that it is of utmost importance to use such data to study erosion risks in agricultural catchments. Copyright © 2014 John Wiley & Sons, Ltd.

Assessment of sediment connectivity from vegetation cover and topography using remotely sensed data in a dryland catchment in the Spanish Pyrenees

Many Mediterranean drylands are characterized by strong erosion in headwater catchments, where connectivity processes play an important role in the redistribution of water and sediments. Sediment connectivity describes the ease with which sediment can move through a catchment. The spatial and temporal characterization of connectivity patterns in a catchment enables the estimation of sediment contribution and transfer paths. Apart from topography, vegetation cover is one of the main factors driving sediment connectivity. This is particularly true for the patchy vegetation cover typical of many dryland environments. Several connectivity measures have been developed in the last few years. At the same time, advances in remote sensing have enabled an improved catchment-wide estimation of ground cover at the subpixel level using hyperspectral imagery. The objective of this study was to assess the sediment connectivity for two adjacent subcatchments (~70 km2) of the Isabena River in the Spanish Pyrenees in contrasting seasons using a quantitative connectivity index based on fractional vegetation cover and topography data. The fractional cover of green vegetation, non-photosynthetic vegetation, bare soil and rock were derived by applying a multiple endmember spectral mixture analysis approach to the hyperspectral image data. Sediment connectivity was mapped using the index of connectivity, in which the effect of land cover on runoff and sediment fluxes is expressed by a spatially distributed weighting factor. In this study, the cover and management factor (C factor) of the Revised Universal Soil Loss Equation (RUSLE) was used as a weighting factor. Bi-temporal C factor maps were derived by linking the spatially explicit fractional ground cover and vegetation height obtained from the airborne data to the variables of the RUSLE subfactors. The resulting connectivity maps show that areas behave very differently with regard to connectivity, depending on the land cover and on the spatial distribution of vegetation abundances and topographic barriers. Most parts of the catchment show higher connectivity values in August as compared to April. The two subcatchments show a slightly different connectivity behaviour that reflects the different land cover proportions and their spatial configuration. The connectivity estimation can support a better understanding of processes controlling the redistribution of water and sediments from the hillslopes to the channel network at a scale appropriate for land management. It allows hot spot areas of erosion to be identified and the effects of erosion control measures, as well as different land management scenarios, to be studied.

Multi‐temporal Digital Photogrammetric Analysis for Quantitative Assessment of Soil Erosion Rates in the Landola Catchment of the Upper Orcia Valley (Tuscany, Italy)

ABSTRACTThis paper focuses on the analysis of the slope and catchment erosion dynamics in a typical Mediterranean context and its sensitivity to recent climatic and socio‐economic changes. The main objective of the present study is to test the reliability of digital photogrammetric analysis to evaluate the time and space evolution of erosion processes mainly triggered by surface running waters and landslides for about the last 60 years. The selected test area is the Landola catchment, a minor tributary of the Upper Orcia River Valley (Southern Tuscany). The Upper Orcia valley is a key site for the comprehension of denudation processes typically acting in Mediterranean badland areas thanks to the following: (i) the availability of long‐lasting erosion monitoring datasets (20 years' long direct measurements at erosion ‘hot spots’); (ii) its representativeness as a sub‐humid Mediterranean badland area; and (iii) the rapidity of development of erosion processes, which makes it suitable as an open‐air laboratory for the study of badland dynamics. The outputs of this work highlight that the application of high‐resolution digital photogrammetric methodologies can represent a powerful and low‐cost tool to evaluate rates and spatial–temporal distribution of denudation processes, as confirmed by the validation through field point monitoring in areas close to the study area. The results obtained for the study area confirm that high erosion rates are a persistent environmental problem for the Upper Orcia Valley, which is not solved up to now, despite various land conservation interventions. The performed multi‐temporal analysis shows a slight decrease in the average water erosion rate during the last 60 years and a parallel increase in the frequency of mass wasting events. These variations are most likely related to a complex interplay between land use changes that have affected the study area during the studied period, revealing a very delicate equilibrium between farming activities and erosion processes. Copyright © 2014 John Wiley & Sons, Ltd.

Identifying subsoil sediment sources with carbon and nitrogen stable isotope ratios

AbstractIncreased sediment loads from accelerated catchment erosion significantly degrade waterways worldwide. In the South East Queensland region of Australia, sediment loads are degrading Moreton Bay, a Ramsar listed wetland of international significance. In this region, like most parts of coastal Australia, sediment is predominantly derived from gully and channel bank erosion processes. A novel approach is presented that uses carbon and nitrogen stable isotope ratios and elemental composition to discriminate between these often indistinguishable subsoil sediment sources. The conservativeness of these sediment properties is first tested by examining the effect of particle size separation (testing for consistency during transport) and the effect of sampling at different times (testing for temporal source consistency). The discrimination potential of these sediment properties is then assessed with the conservative properties, based on the particle size and temporal analyses, modelled to determine sediment provenance in three catchments. Nitrogen sediment properties were found to have significant particle size enrichment and high temporal variance indicative of non‐conservative behaviour. Conversely, carbon stable isotopes had very limited particle size and temporal variability highlighting their suitability for sediment tracing. Channel erosion was modelled to be a significant source of sediment (μ 51%, σ 9%) contrasting desktop modelling research that estimated gully erosion is the predominant sediment source. To limit the supply of sediment to Moreton Bay, channel bank and gully erosion must both be targeted by sediment management programs. By distinguishing between subsoil sediment sources, this approach has the potential to enhance the management of sediment loads degrading waterways worldwide. Copyright © 2014 John Wiley & Sons, Ltd.

Soil erosion and tolerable soil loss: Insights into erosion rates for a well-managed grassland catchment

Soil erosion and its spatial and temporal variabilities are rarely placed in the context of soil production and soil depth. This study examines the question of sustainable soil erosion and soil loss in a conservatively managed grassland catchment in South East Australia in what at first appears to be a catchment with a tolerable soil loss. Catchment erosion rates are determined using the environmental tracer 137Cs. A thorough examination of the accuracy and reliability of this method is conducted across a number of spatial scales and years providing confidence in the method and results. Soil depth is measured across the study catchment providing the first bedrock map of a basalt derived soil catchment. Both soil erosion and soil depth are topographically assessed using a high resolution digital elevation model. Results show that soil depth was strongly correlated with elevation and also wetness indices indicating a strong relationship with soil moisture in soil production. Interestingly bedrock topography was decoupled from surface topography. Erosion rates using the 137Cs method and calibrated against independent field data produced a maximum erosion rate of between 0.8 and 2.9tha−1yr−1 using two different modelling approaches. Even though the erosion rates are low, given a mean soil depth of 0.44m for the catchment this suggests that soil is being lost at rates greater than production. This highlights the significance of assessing erosion loss in the context of overall soil depth and production rates and that even in areas with what appears to be low soil loss rates, the loss can be higher than production. The findings provide a rationale to examine soil erosion in the context of whole catchment processes, not simply soil loss in isolation to other hillslope and catchment data.

Use of the mercury record in Red Tarn sediments to reveal air pollution history and the implications of catchment erosion

Red Tarn is a cirque lake with a small ratio of terrestrial area to lake area, surrounded by glacial edges with little soil in the catchment. A sediment core taken from the deepest area of the lake was (210)Pb dated and validated by (137)Cs and (241)Am stratigraphic records. The core was analysed for mercury (Hg) and other elements. The results show Hg pollution before the mid-19th century, and thereafter, a rapid increase in Hg pollution into modern time, followed by a decline in pollution since 1968-1970. This agrees well with the decline in UK Hg emissions since the Clear Air Act of 1968. The results suggest that the core has recorded Hg air pollution history, and it can be used to benchmark Hg changes in the sediments from other lakes in the region up to the late 1980s. However, increased (210)Pb fluxes after the late 1980s indicate enhanced catchment erosion, which has brought more legacy Hg in the catchment into the lake. As a consequence, since 2000, the Hg in the sediment record no longer reflects the atmospheric Hg deposition. The core shows how dominant Hg sources for the lake changed from atmospheric deposition to the catchment inputs, and demonstrates that contaminated catchment inputs have not only increased Hg fluxes to the lake sediments but have also increased Hg concentrations in the sediments.

Very long hillslope transport timescales determined from uranium-series isotopes in river sediments from a large, tectonically stable catchment

The uranium-series isotopic compositions of soils and sediments evolve in response to time and weathering conditions. Therefore, these isotopes can be used to constrain the timescales of river sediment transport. Catchment evolution depends on the sediment dynamic timescales, on which erosion imparts a major control. Erosion rates in tectonically stable catchments are expected to be lower than those in tectonically active catchments, implying longer sediment residence times in tectonically stable catchments. Mineralogical, elemental and isotopic data are presented for modern channel sediments, alluvial and colluvial deposits from the Murrumbidgee River, a large catchment in the passive margin highlands of south-eastern Australia and three of its tributaries from the headwaters to the alluvial plain. Low variability in Si-based Weathering Index indicates that there is little chemical weathering occurring in the Murrumbidgee River during sediment transport. However, quartz content increases and plagioclase content decreases downstream, indicating progressive mineralogical sorting and/or physical comminution with increasing transport distance. U-series isotopic ratios in the Murrumbidgee River trunk stream sediments show no systematic downstream variation. The weathering ages of sediments within the catchment were determined using a loss–gain model of U-series isotopes. Modern sediments from a headwater tributary, the Bredbo River at Frogs Hollow, have a weathering age of 76±30kyr but all other modern channel sediments from the length of the Murrumbidgee River and its main tributaries have weathering ages ∼400±180kyr. The two headwater colluvial deposits have weathering ages of 57±13 and 47±11kyr, respectively. All the alluvial deposits have weathering ages similar to those of modern sediments. No downstream trend in weathering age is observed. Together with the soil residence time of up to 30kyr for ridge-top soils at Frogs Hollow in the upper catchment area of the Murrumbidgee River (Suresh et al., 2013), the current results indicate, for the first time, that sediments in the Murrumbidgee catchment are stored in hill slope for long time (∼200kyr) before carried by the river. The long residence times of sediments indicate a low erosion rate from the catchment. The sediment transport timescales estimated are up to two orders of magnitude higher than those reported for tectonically active catchments in Iceland (Vigier et al., 2006) and in the Himalayas (Granet et al., 2007), indicating the influence of tectonism on catchment erosion.

Sedimentary geochemical record of human-induced environmental changes in Huanggaihu Lake in the middle reach of the Yangtze River, China

Total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP) and heavy metals were determined along with stable isotope ratios of carbon and nitrogen (δ13Corg and δ15N) in an 86 cm sediment core from Huanggaihu Lake, located in the middle reach of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and the trophic state of Huanggaihu Lake over the past 150 years. Before the 1960s, organic matter, nutrients and heavy metal contents were low, revealing the low trophic state and pollution inputs at Huanggaihu Lake. High sediment mass accumulation rates and low clay content suggested increased catchment erosion related to human activities such as railway construction and forest clearing during the 1910s-1940s in the catchment area. Since the 1960s, TOC, TN and δ15N have increased along with decreased δ13Corg, indicating an increase in the primary productivity of Huanggaihu Lake. After the year 2000, the TOC, TN and TP decreased or remained stable, and the δ13Corg and δ15N dropped slightly, indicating minor improvement in the lake’s water quality.

Holocene versus modern catchment erosion rates at 300 MW Baspa II hydroelectric power plant (India, NW Himalaya)

300MW Baspa II is India’s largest private hydroelectric facility, located at the Baspa River which is an important left-hand tributary to the Sutlej River in the NW Himalaya (India). In this valley the Sangla palaeo-lake has been dammed around 8200yr BP behind a rock-avalanche dam and Baspa II is located exactly on top of this palaeo-lake. This special location represents a very rare possibility to evaluate the short-term, river load and hydrological parameters measured during the planning and operational stages of Baspa II with the long-term parameters gained from the palaeo-lake sediments from the catchment. Sedimentological and geomorphological investigations of the lacustrine sediments have been used to reconstruct environmental changes during >2500years of its existence. The Mid-Holocene erosion rates of the Baspa catchment estimated from the volume and duration of deposition of the exposed lake sediments are at 0.7–1.0mmyr−1, almost identical with the modern erosion rates calculated from river gauge data from Baspa II. Several charcoal layers and charcoal pieces from the uppermost palaeo-lake levels around 5000calyr BP might be related to woodland clearance and they possibly represent one of the oldest evidences for human presence in the Baspa Valley during Neolithic time.