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International audience; Large proportions of rain water and snowmelt infiltrate into the subsurface before contributing to stream flow and stream water quality. Subsurface flow dynamics steer the transport and transformation of contaminants, carbon, weathering products and other biogeochemistry. The distribution of groundwater ages with depth is a key feature of these flow dynamics. Predicting these ages are a strong test of hypotheses about subsurface structures and time varying processes. CFC‐based groundwater ages revealed an unexpected groundwater age stratification in a 0.47 km2 forested catchment called Svartberget in northern Sweden. An overall groundwater age stratification, representative for the Svartberget site, was derived by measuring CFCs from 9 different wells with depths of 2 m to 18 m close to the stream network. Immediately below the water table, CFC‐based groundwater ages of already 30 years that increased with depth were found. Using complementary groundwater flow models, we could reproduce the observed groundwater age stratification and show that the 30 year lag in rejuvenation comes from return flow of groundwater at a subsurface discharge zone that evolves along the interface between two soil types. By comparing the observed groundwater age stratification with a simple analytical approximation, we show that the observed lag in rejuvenation can be a powerful indicator of the extent and structure of the subsurface discharge zone, while the vertical gradient of the age‐depth relationship can still be used as a proxy of the overall aquifer recharge even when sampled in the discharge zone. The single age stratification profile measured in the discharge zone, close to the aquifer outlet, can reveal the main structure of the groundwater flow pattern from recharge to discharge. This groundwater flow pattern provides information on the participation of groundwater in the hydrological cycle and indicates the lower boundary of hydrological connectivity.

Flash floods are the highest sediment transporting agent, but are inaccessible for in-situ sampling and have rarely been analyzed by remote sensing technology. Laboratory and field experiments were done to develop linear spectral unmixing (LSU) remote sensing model and evaluate its performance in simulating the suspended sediment concentration (SSC) in flash floods. The models were developed from continuous monitoring in the laboratory and the onsite spectral signature of river bed sediment deposits and flash floods in the Tekeze River and in its tributary, the Tsirare River. The Pearson correlation coefficient was used to determine the variability of correlations between reflectance and SSCs. The coefficient of determination (R2) and root mean square of error (RMSE) were used to evaluate the performance of the generated models. The results found that the Pearson correlation coefficient between SSCs and reflectance varied based on the level of the SSCs, geological colors, and grain sizes. The performance of the LSU model and empirical remote sensing approaches were computed to be R2 = 0.92, and RMSE = ±0.76 g/l in the Tsirare River and R2 = 0.91, and RMSE = ±0.73 g/l in the Tekeze River and R2 = 0.81, RMSE = ±2.65 g/l in the Tsirare river and R2 = 0.76, RMSE = ±10.87 g/l in the Tekeze River, respectively. Hence, the LSU approach of remote sensing was found to be relatively accurate in monitoring and modeling the variability of SSCs that could be applied to the upper Tekeze River basin.

International audience; The knowledge of the sources of base cations in stream water is a prerequisite to assess potential effects of changing environmental conditions such as changing rainfall, weathering or groundwater flows on cation export with stream water. This study use stable Mg isotopes to identify potential sources in the well-studied catchment of Krycklan located on gneissic bedrock covered by quaternary sediments in Sweden. Samples were collected from open filed rain, throughfall, stream, soil, rock and litterfall. The δ26Mg values of these samples was determined and the contributions of different sources to Mg fluxes in the stream were determined from the variation of the Mg isotope and Sr / Mg ratios.The results show an overall variation of 1.10‰ between all samples. In addition, Magnesium isotope ratios varied little in the streamwater and in soil solution, except during snowmelt periods during which a large portion of the annual runoff occurs. Magnesium in the streamwater is explained as a mixture of three pools (open field rain, soil solution and groundwater) with the latter two influenced by catchment processes. Outside the snowmelt period, Mg in streamwater mainly derived from the groundwater, assumed to be mineral weathering signature in this catchment, with a contribution ranging from 12 to 63% to Mg fluxes. Open field rain dominates Mg fluxes in streamwater during spring flood (0 to 78%) and may contribute significantly during larger summer and autumn rainfall events. Soil solution input to streamwater range from 16 to 59% of Mg fluxes in streamwater. Our results demonstrate that δ26Mg values together with Mg concentrations and Sr/Mg ratios can be used to constrain the Mg sources of stream water and quantify weathering release rates.

13 pages, 4 figures, 3 tables, supporting information https://doi.org/10.1038/s41598-018-37383-z, The formation of the potent neurotoxic methylmercury (MeHg) is a microbially mediated process that has raised much concern because MeHg poses threats to wildlife and human health. Since boreal forest soils can be a source of MeHg in aquatic networks, it is crucial to understand the biogeochemical processes involved in the formation of this pollutant. High-throughput sequencing of 16S rRNA and the mercury methyltransferase, hgcA, combined with geochemical characterisation of soils, were used to determine the microbial populations contributing to MeHg formation in forest soils across Sweden. The hgcA sequences obtained were distributed among diverse clades, including Proteobacteria, Firmicutes, and Methanomicrobia, with Deltaproteobacteria, particularly Geobacteraceae, dominating the libraries across all soils examined. Our results also suggest that MeHg formation is also linked to the composition of non-mercury methylating bacterial communities, likely providing growth substrate (e.g. acetate) for the hgcA-carrying microorganisms responsible for the actual methylation process. While previous research focused on mercury methylating microbial communities of wetlands, this study provides some first insights into the diversity of mercury methylating microorganisms in boreal forest soils, This project was carried out within the Swedish-Sino SMaREF (2013-6978) funded by the Swedish Research Council. This study was also supported by the Swedish Energy Agency (grant number 36155-1) and the Swedish Research Council (Grants 2011-7192 and 2012-3892)

For most catchments, there is insufficient data to determine the location of the groundwater surface. For humid climates, it is, therefore, often assumed that the groundwater-surface follows the surface topography. This assumption allows using digital elevation models (DEMs) to estimate the flow directions and catchment boundaries. However, high-resolution elevation data also include many small-scale features that are unlikely to affect the direction of groundwater flow, or only affect it during specific conditions. Furthermore, flow directions may change during events or depending on the water level.The optimal resolution of the DEM for determining groundwater flow directions is not known yet. Therefore, we studied how much DEM derived flow directions and catchment boundaries are affected by the resolution or smoothing of the elevation data for the Krycklan catchment in northern Sweden. We also measured the groundwater levels in two small sub-catchments to determine what DEM resolution best describes the actual groundwater-surface and flow directions.For the topographic analyses, the LiDAR-based elevation data were first smoothed with various filters (e.g., Gaussian filters) and resampled to obtain lower resolution elevation data. We then determined the flow directions for these different DEMs. The aim was to determine where in the catchment the calculated flow directions are most sensitive to the resolution of the topographic data. The results of the topographic analyses show that for some areas, particularly flat areas, ridges, streambanks and locations where the local slope differs from the general slope, the calculated flow directions depend strongly on the resolution and smoothing of the elevation data.To test how well the DEM based groundwater flow directions represent actual flow directions, we installed a dense (5-20 m spacing) network of shallow (1 to 6 m deep) groundwater wells (75 wells in total) in a 1 ha and a 2 ha gauged sub-catchment. The triangular nested design of the groundwater well network allowed us to determine the smaller (5 m) and larger scale (20 m) groundwater gradients. The recorded water levels were augmented and validated by manual measurements during the summers of 2018 and 2019. The high spatial and temporal resolution data allowed us to study the response of the groundwater level and the flow directions to different meteorological situations (e.g., large precipitation events after dry and wet conditions and during a very dry period in summer 2018). These observations indicate that the degree to which the groundwater-surface is a subdued copy of the surface topography varies throughout the year, and provides information on which DEM resolution most accurately represents the groundwater-surface and flow directions.

Improving our ability to detect changes in terrestrial and aquatic systems is a grand challenge in the environmental sciences. In a world experiencing increasingly rapid rates of climate change and ecosystem transformation, our ability to understand and predict how, when, where, and why changes occur is essential for adapting and mitigating human behaviours. In this context, long-term field research infrastructures have a fundamentally important role to play. For northern boreal landscapes, the Krycklan Catchment Study (KCS) has supported monitoring and research aimed at revealing these changes since it was initiated in 1980. Early studies focused on forest regeneration and microclimatic conditions, nutrient balances and forest hydrology, which included monitoring climate variables, water balance components, and stream water chemistry. The research infrastructure has expanded over the years to encompass a 6790 ha catchment, which currently includes 11 gauged streams, ca. 1000 soil lysimeters, 150 groundwater wells, >500 permanent forest inventory plots, and a 150 m tall tower (a combined ecosystem-atmosphere station of the ICOS, Integrated Carbon Observation System) for measurements of atmospheric gas concentrations and biosphere-atmosphere exchanges of carbon, water, and energy. In addition, the KCS has also been the focus of numerous high resolution multi-spectral LiDAR measurements and large scale experiments. This large collection of equipment and data generation supports a range of disciplinary studies, but more importantly fosters multi-, trans-, and interdisciplinary research opportunities. The KCS attracts a broad collection of scientists, including biogeochemists, ecologists, foresters, geologists, hydrologists, limnologists, soil scientists, and social scientists, all of whom bring their knowledge and experience to the site. The combination of long-term monitoring, shorter-term research projects, and large-scale experiments, including manipulations of climate and various forest management practices, has contributed much to our understanding of boreal landscape functioning, while also supporting the development of models and guidelines for research, policy, and management.

It has been 12 years since Bishop et al. (2008) wrote the Invited Commentary “Aqua Incognita: the unknown headwaters.” They highlighted that “In most regions, the overwhelming majority of stream length lies beyond the frontiers of any systematic documentation and would have to be represented as a blank space on the assessment map. This means that for the majority of streams that support aquatic life, a systematic understanding is lacking on water quality, habitat, biota, specific discharge, or even how many kilometres of such streams are there. This blank space is so vast that it deserves a name to help us at least to remember that it is there. We propose calling it ‘Aqua Incognita’” (Bishop et al., 2008, p. 1239). We continue to agree with this statement and the need to understand headwater streams better. In this commentary, we want to draw attention to a particular type of headwater stream that is even less frequently examined: headwater streams that flow intermittently, that is, the Aqua Temporaria Incognita. Question 3 of the 23 unsolved problems in hydrology (Blöschl et al., 2019) focuses on ephemeral dryland streams. We argue that this focus needs broadening to headwater temporary streams because they are ubiquitous in all climates. Headwater temporary streams feed larger perennial streams and are particularly sensitive to climate change and other human influences (Jaeger, Olden, & Pelland, 2014; Pumo, Caracciolo, Viola, & Noto, 2016; Reynolds, Shafroth, & LeRoy Poff, 2015). Their effective management and protection, therefore, requires an understanding of both natural and artificial causes of intermittence., We thank the SMIRES (Science and Management of Intermittent Rivers & Ephemeral Streams) COST Action CA15113 for facilitating our many discussions on temporary streams, Willy Bertin and Gaela Le Bechec from SR3A and Judy England from the Environment Agency for information on how and why they use the CrowdWater app, and Helena Ramos Ribeiro and Auria Buchs for collecting the data shown in Figure 2.

Peat stored in large wetlands plays an important role in the carbon cycle and strongly influences water quality of terrestrial surface water bodies. At the same time, peat is also stored in the direct vicinity of many boreal forest streams. From this strategic position, peat can receive and chemically reset hillslope water before it reaches the stream network. Yet, in contrast to large wetlands, only little spatial information is available on the lateral extent of near-stream peat and even less about its vertical variation. Here, we present field data on peat depth and lateral extent collected from approximately 200 transects (with 12 soil profiles taken per transect) distributed across the entire stream network of the Krycklan boreal catchment in Northern Sweden. This soil profile data revealed a considerable heterogeneity of peat and organic horizon thicknesses. By combining the field data with morphological and geological maps, we show how parent material, stream order and local topography influence near stream peat structures. Furthermore, we discuss potential consequences on surface water quality by linking the detailed peat data set to estimates of lateral, shallow groundwater inflows derived from hydrometric measurements and digital terrain analysis.

The Earth has entered the Anthropocene epoch and forest managers are facing unprecedented challenges to meet multiple ecosystem service demands from forests. Understanding the complex forest-water relations under a changing environment must add a human dimension, and this is essential in the move towards sustainable forest management in the 21st century. This Special Issue contains 10 papers presented at a joint international forest and water conference in Chile in 2018. These studies provide global examples on new advancements in sciences in forest ecohydrology, watershed management, and ecosystem service assessment under various geographical and socioeconomic settings.

This chapter reviews how global change affects forest-water interactions and water availability to ecosystems and people and synthesises current understanding of the implications of present and anticipated changes to forests and tree cover for local and global hydrology. Forest cover has declined in the past half-century, despite an increase in plantation forestry. Natural and human disturbances affect forest components (e.g. canopy and leaf area, litter and soil surface, rooting depth, and soil porosity) that in turn affect hydrological processes (e.g. interception, evapotranspiration, infiltration, soil moisture storage, and percolation). Many of these changes result from several influential natural disturbance processes including insects and pathogens, wildfire, ice storms, and windthrow, and human disturbances including establishment and harvest of forests, plantations, agroforestry areas, and urban/peri-urban forests. However, each disturbance process affects different components of the forest, producing distinctive hydrologic effects. Climate change will directly alter forest hydrological processes, and social and economic factors will directly alter forest management, via intensive plantations, deforestation, forest degradation, selective logging, loss of riparian forest, and loss of urban trees, and changes in disturbance regimes. Despite extensive knowledge of forest hydrology, forest changes and their effects on hydrology are poorly documented in many areas of the world, and novel combinations of processes and contexts may produce surprising outcomes. Thus, there is a clear need for more geographically extensive and long-term place-based studies of forest and water. In summary, future climate and social changes will alter forests and water, requiring continued research and collaboration with forest managers and forest owners both for improved resilience to such changes, and to better realize multiple benefits.

It is generally assumed that, in humid climates, the groundwater table is a subdued copy of the surface topography. However, the general groundwater table is unlikely to be affected by the microtopography as seen in high-resolution Digital Elevation Models (DEMs). So far, there has been little guidance on the best resolution DEM to use to determine the shape of the water table or the direction of shallow groundwater flow in headwater catchments. We, therefore, looked at the effects of DEM-smoothing and -aggregation on the calculated flow directions and derived catchment boundaries, and identified areas and landscape features for which the calculated flow directions are particularly sensitive to DEM smoothing or aggregation. For > 40 % of the area of the Krycklan study catchment, the calculated flow directions depend strongly on the degree of smoothing or aggregation of the DEM. The four main landscape features for which DEM smoothing or aggregation strongly affected the calculated flow directions were: local slopes in the opposite direction of the general slope, flat areas, ridges, and incised streambanks. To determine the effects of the changing flow directions on the derived catchment boundaries for the smoothed and aggregated DEMs, we calculated the drainage area for 40 locations, representing the outlets of catchments of varying sizes. The shape and size of the catchments of first-order streams were most affected by the processing of the DEM. These streams were often almost completely smoothed out during the DEM preprocessing steps. These shifts in catchment boundaries and drainage area would have a large effect on the water balance. This study thus highlights the need to carefully consider the effects of DEM smoothing or -aggregation on the calculated flow directions and drainage areas.

The boreal ecoregion supports about one-third of the world's forest. Over 90% of boreal forest streams are found in headwaters, where terrestrial-aquatic interfaces are dominated by organic matter (OM)-rich riparian zones (RZs). Because these transition zones are key features controlling catchment biogeochemistry, appropriate RZ conceptualizations are needed to sustainably manage surface water quality in the face of a changing climate and increased demands for forest biomass. Here we present a simple, yet comprehensive, conceptualization of RZ function based on hydrological connectivity, biogeochemical processes, and spatial heterogeneity. We consider four dimensions of hydrological connectivity: (1) laterally along hillslopes, (2) longitudinally along the stream, (3) vertically down the riparian profile, and (4) temporally through event-based and seasonal changes in hydrology. Of particular importance is the vertical dimension, characterized by a 'Dominant Source Layer' that has the highest contribution to solute and water fluxes to streams. In addition to serving as the primary source of OM to boreal streams, RZs shape water chemistry through two sets of OM-dependent biogeochemical processes: (1) transport and retention of OM-associated material and (2) redox-mediated transformations controlled by RZ water residence time and availability of labile OM. These processes can lead to both retention and release of pollutants. Variations in width, hydrological connectivity, and OM storage drive spatial heterogeneity in RZ biogeochemical function. This conceptualization provides a useful theoretical framework for environmental scientists and ecologically sustainable and economically effective forest management in the boreal region and elsewhere, where forest headwaters are dominated by low-gradient, OM-rich RZs.

Cation release rates to catchment runoff from chemical weathering were assessed using an integrated catchment model that included the soil's unsaturated, saturated and riparian zones. In-situ mineral dissolution rates were calculated in these zones as a function of pH, aluminum and dissolved organic carbon (DOC) concentrations along a hillslope in Northern Sweden where soil water was monitored over nine years. Three independent sets of mineral dissolution equations of varying complexity were used: PROFILE, Transition-State Theory (TST), and the Palandri & Kharaka database. Normalization of the rate-coefficients was necessary to compare the equations, as published rate-coefficients gave base cation release rates differing by several orders of magnitude. After normalizing the TST- and Palandri & Kharaka-rate coefficients to match the base cation release rates calculated from the PROFILE-equations, calculated Ca2 + and Mg2 + release rates are consistent with mass balance calculations, whereas those of Na+ and K+ are overestimated. Our calculations further indicate that a significant proportion of base cations are released from the organic soils in the near-stream zone, in part due to its finer texture. Of the three sets of rate equations, the base cation release rates calculated from the normalized TST-equations were more variable than those calculated using the other two sets of equations, both spatially and temporally, due to its higher sensitivity to pH. In contrast, the normalized Palandri & Kharaka-equations were more sensitive to variations in soil temperature.

Background. Lake Tumba with a surface area of 830 km2 is very humic (pH = 3.6–4.6). The fishery is important for the riparian population but there are concerns about overfishing. The objective of this study was to assess changes in the fish species composition by comparing the presently reported survey (2005–2010) with earlier studies carried out in 1938–1939, 1955–1956, and 1959 Our hypothesis was that Lake Tumba had been overfished. Materials and methods. Fishing was done in the pelagial with gillnet (25 and 30 mm) and line with hooks at three sites within 166 days in 2005–2006 and 2009–2010. Results. A total of 40 species were identified. Five species were caught all years and at all sites, namely Chrysichthys cranchii (Leach, 1818), Chrysichthys ornatus Boulenger, 1902, Chrysichthys punctatus Boulenger, 1899, Mormyrops anguilloides (Linnaeus, 1758), and Tylochromis lateralis (Boulenger, 1898). Twelve species were only caught once. Weight Per Unit Effort (WPUE) differed with site and season: ranging from 0 to 4200 g · day–1 and the Number of fish Per Unit Effort (NPUE) varied from 0 to 27 fishes · day–1. Conclusion. Out of 49 pelagic species caught by Matthes in 1959, we considered five as diminished in numbers or even missing. This may be attributed to high fishing pressure. The protected site Mabali hosted the majority of previously reported species of which two were noted in Matthes’s list but not caught by him. Many parameters demonstrated a “positive” difference between Mabali (which is protected from fishing) and the other sites. This site should be a reference area for future monitoring if its status can be maintained. A trend in the fishery based on interviews with fishers indicated a change from larger species to smaller. This was deemed a result of fishing pressure and nine species were identified as diminishing. The presently reported survey introduced quantitative aspects while earlier studies were qualitative. This impairs the ability to draw strong conclusions about overfishing and other anthropogenic activities. The threat of overfishing calls for the need for consistent quantitative monitoring of the fish population, including monitoring of waters in the drainage area, to enable assessment of impact of anthropogenic pressure.

The study evaluates the biogeochemical model ForSAFE-2D, designed to simulate water and chemical transport from the forest to the stream, by simulating the hydrology and the transport of the chemical tracer chloride (Cl−) along a forest hillslope in Northern Sweden. The simulated Cl− exports were in balance with the simulated inputs but measurements suggested a net release of Cl− from the catchment. Underestimated deposition inputs (deposition peaks and possibly dry deposition) were probably and partially responsible for this mismatch. However, we could not exclude that other soil biogeochemical processes omitted in ForSAFE-2D could also contribute to Cl− exports from the catchment. The study showed that ForSAFE-2D is a promising tool to better understand the factors that regulate the chemical export from the forest to the stream. The results also confirmed that there are limitations in using Cl− as a tracer in forest ecosystems.

Improving the understanding of how stream flow dynamics are influenced by landscape characteristics, such as soils, vegetation and terrain, is a central endeavor of catchment hydrology. Here we inv ...

In this paper we combined a multi-model ensemble based on 15 regional climate models with a multi-catchment approach to explore the hydrologic sensitivity of 14 neighboring and rather similar catch ...

Using a 15 year stream record from a northern boreal catchment, we demonstrate that the inter-annual variation in dissolved organic carbon (DOC) concentrations during snowmelt was related to discharge, winter climate and previous DOC export. A short and intense snowmelt gave higher stream water DOC concentrations, as did long winters, while a high previous DOC export during the antecedent summer and autumn resulted in lower concentrations during the following spring. By removing the effect of discharge we could detect that the length of winter affected the modeled soil water DOC concentrations during the following snowmelt period, which in turn affected the concentrations in the stream. Winter climate explained more of the stream water DOC variations than previous DOC export during the antecedent summer and autumn.

The fate of anthropogenic emissions of mercury (Hg) to the atmosphere is influenced by the exchange of elemental Hg with the earth surface. This exchange holds the key to a better understanding of Hg cycling from local to global scales, which has been difficult to quantify. To advance research about land–atmosphere Hg interactions, we developed a dual-inlet, single detector relaxed eddy accumulation (REA) system. REA is an established technique for measuring turbulent fluxes of trace gases and aerosol particles in the atmospheric surface layer. Accurate determination of gaseous elemental mercury (GEM) fluxes has proven difficult due to technical challenges presented by extremely small concentration differences (typically

Headwater streams export CO2 as lateral downstream export and vertical evasion from the stream surface. CO2 in boreal headwater streams generally originates from adjacent terrestrial areas, so determining the sources and rate of CO2 transport along the hillslope–riparian–stream continuum could improve estimates of CO2 export via the aquatic pathway, especially by quantifying evasion at higher temporal resolutions. Continuous measurements of dissolved CO2 concentrations and water table were made along the hillslope–riparian–stream continuum in the Västrabäcken sub-catchment of the Krycklan catchment, Sweden. Daily water and CO2 export from the hillslope and riparian zone were estimated over one hydrological year (October 2012–September 2013) using a flow-concentration model and compared with measured lateral downstream CO2 export. Total water export over the hydrological year from the hillslope was 230 mm yr−1 compared with 270 mm yr−1 from the riparian zone. This corresponds well (proportional to the relative upslope contributing area) to the annual catchment runoff of 265 mm yr−1. Total CO2 export from the riparian zone to the stream was 3.0 g CO2-C m−2 yr−1. A hotspot for riparian CO2 export was observed at 30–50 cm depth (accounting for 71 % of total riparian export). Seasonal variability was high with export peaks during the spring flood and autumn storm events. Downstream lateral CO2 export (determined from stream water dissolved CO2 concentrations and discharge) was 1.2 g CO2-C m−2 yr−1. Subtracting downstream lateral export from riparian export (3.0 g CO2-C m−2 yr−1) gives 1.8 g CO2-C m−2 yr−1 which can be attributed to evasion losses (accounting for 60 % of export via the aquatic pathway). The results highlight the importance of terrestrial CO2 export, especially from the riparian zone, for determining catchment aquatic CO2 losses and the importance of the CO2 evasion component to carbon export via the aquatic conduit.

The current generation of catchment travel time distribution (TTD) research, integrating nearly three decades of work since publication of Water's Journey from Rain to Stream, seeks to represent the full distribution in catchment travel times and its temporal variability. Here, we compare conceptualizations of increasing complexity with regards to mixing of water storages and evaluate how these assumptions influence time-variable TTD estimates for two catchments with contrasting climates: the Gardsjon catchment in Sweden and the Marshall Gulch catchment in Arizona, USA. Our results highlight that, as long as catchment TTDs cannot be measured directly but need to be inferred from input-output signals of catchments, the inferred catchment TTDs depend strongly on the underlying assumptions of mixing within a catchment. Furthermore, we found that the conceptualization of the evapotranspiration flux strongly influences the inferred travel times of stream discharge. For the wet and forested Gardsjon catchment in Sweden, we inferred that evapotranspiration most likely resembles a completely mixed sample of the water stored in the catchment; however, for the drier Marshall Gulch catchment in Arizona, evapotranspiration predominantly contained the younger water stored in the catchment. For the Marshall Gulch catchment, this higher probability for young water in evapotranspiration resulted in older water in the stream compared to travel times inferred with assumptions of complete mixing. New observations that focus on the TTD of the evapotranspiration flux and the actual travel time of water through a catchment are necessary to improve identification of mixing and consequently travel times of stream water. Copyright © 2014 John Wiley & Sons, Ltd.

This study assessed variations in the concentrations of poly- and perfluoroalkyl substances (PFAS) in European perch ( Perca fluviatilis ) in Swedish lakes and the extent to which fish size, age and indicators of fish trophic ecology (δ 15 N and δ 13 C) correlate with the sum of individual PFAS concentrations ( Σ PFAS). Fish muscle tissue samples ( n = 80) were taken from six lakes across Sweden and analysed using solid-liquid extraction followed by liquid chromatography–electrospray ionization–tandem mass spectrometry (LC-ESI-MS/MS). PFAS levels in the lakes were affected by atmospheric deposition in relatively pristine areas with no direct point source of PFAS in the catchment. PFTeDA, PFUnDA, PFTriDA, PFDoDA, PFDA, PFOS and 6:2 FTSA were detected with a frequency between 68% and 99% and were included in the statistical evaluation. Σ PFAS differed between lakes (ANOVA: F = 50.6, p Σ PFAS of 3.4 ng g − 1 wet weight (ww) (mean ± SD: 0.99 ± 0.63 ng g − 1 ww), while the lowest levels were found in lake Bjorntjarn (0.31 ± 0.08 ng g − 1 ww) in northern Sweden (63°54′N, 18°51′E). PFOS was most abundant in perch from south-western Sweden, while other long-chain perfluorocarbons (> 10 carbon atoms) were relatively more abundant in lakes in northern Sweden. Pearson correlation coefficients indicated that concentrations of PFAS in perch did not show any relation to fish size or age and were negatively correlated with trophic position of the fish (δ 15 N). It was also found that Σ PFAS were negatively correlated with both latitude and altitude. The PFAS data in this study represent national background concentrations in freshwater fish across Swedish lakes.

Concentrations of nitrogen (N) in surface waters reflect the export of different organic and inorganic forms from terrestrial environments and the modification of these resources within aquatic hab ...

Forestry operations can increase the export of mercury (both total and methyl) to surface waters. However, little is known about the relative contribution of different forestry practices. We address this question using a paired-catchment study that distinguishes the effects of site preparation from the antecedent logging. Runoff water from three catchments, two harvested and one untreated control, was sampled biweekly during one year prior to logging, two years after logging, and three years after site preparation. The logging alone did not significantly increase the concentrations of either total or methyl-mercury in runoff, but export increased by 50-70% in one of the harvested catchments as a consequence of increased runoff volume. The combined effects of logging and site preparation increased total and methyl-mercury concentrations by 30-50% relative to preharvest conditions in both treated catchments. The more pronounced concentration effect after site preparation compared to logging could be related to site preparation being conducted during summer. This caused more soil disturbance than logging, which was done during winter with snow covering the ground. The results suggest that the cumulative impact of forest harvest on catchment mercury outputs depends on when and how forestry operations are implemented.

Community perception of forest–water relationship was gathered using participatory rural appraisal tools in four watersheds of the Blue Nile River Basin in Ethiopia. These were compared and contrasted with the observational records of forest cover and water flow. Upstream and downstream communities were assessed separately to check for differences in perception based on location within a watershed. The key result of the study was that people in the study watersheds had a range of perceptions about the forest–water relationship which were watershed specific. The perceptions were generally consistent with observational evidence from the same watersheds. This study highlighted the need for locale-specific approaches to land and water management in the Basin, as well as the potential value of using community perceptions to complement the observational records which can have spatial and temporal limitations.

Riparian zones (RZ) are a major factor controlling water chemistry in forest streams. Base cations' (BC) concentrations, fluxes, and cycling in the RZ merit attention because a changing climate and increased forest harvesting could have negative consequences, including re-acidification, for boreal surface waters. We present a two-year study of BC and silica (Si) flow-weighted concentrations from 13 RZ and 14 streams in different landscape elements of a boreal catchment in northern Sweden. The spatial variation in BC and Si dynamics in both RZ and streams was explained by differences in landscape element type, with highest concentrations in silty sediments and lowest concentrations in peat-dominated wetland areas. Temporal stability in BC and Si concentrations in riparian soil water, remarkably stable Mg/Ca ratios, and homogeneous mineralogy suggest that patterns found in the RZ are a result of a distinct mineralogical upslope signal in groundwater. Stream water Mg/Ca ratios indicate that the signal is subsequently maintained in the streams. Flow-weighted concentrations of Ca, Mg, and Na in headwater streams were represented by the corresponding concentrations in the RZ, which were estimated using the Riparian Flow-Concentration Integration Model (RIM) approach. Stream and RZ flow-weighted concentrations differed for K and Si, suggesting a stronger biogeochemical influence on these elements, including K recirculation by vegetation and retention of Si within the RZ. Potential increases in groundwater levels linked to forest harvesting or changes in precipitation regimes would tend to reduce BC concentrations from RZ to streams, potentially leading to episodic acidification.

The objective of this study was to quantify forest cover changes in three watersheds (Gilgel Abbay (1,646 km2), Birr (980 km2), and Upper-Didesa (1,980 km2) of the Blue Nile Basin between 1957 and 2001. Four land cover maps were produced for each watershed for 1957/1958, 1975, 1986, and 2000/2001. Nine different types of land cover were identified, five of which were forest cover classes. Between 1957 and 2001, the total forest cover increased in Gilgel Abbay (from 10 to 22 % cover) and decreased in Birr (from 29 to 22 % cover) as well as in Upper-Didesa (from 89 to 45 % cover). The increase in Gilgel Abbay was primarily due to the expansion of eucalyptus plantations. Natural forest cover decreased in all three watersheds. Wooded grassland decreased by two-thirds, dry/moist mixed forests decreased by half, and riverine forests had disappeared by 1975 in Gilgel Abbay and Birr. Major deforestation had already taken place in the northern watersheds, Gilgel Abbay and Birr, before the 1960s and 1970s, while in the southern watershed, Upper-Didesa, much of the deforestation occurred after 1975. The southern watershed still remained by far the most forested watershed in 2001 despite the strong ongoing deforestation. The changes in forest cover could affect natural resource management, greenhouse gas emissions, water resources, and agricultural production including coffee production. The patterns of change are different in the three watersheds. We therefore recommend further studies of the local conditions and drivers of change as the basis for designing effective policy to halt further loss of natural forest, which offers a wealth of ecosystem services.

[1] Aquatic CO2 concentrations are highly variable and strongly linked to discharge, but until recently, measurements have been largely restricted to low-frequency manual sampling. Using new in situ CO2 sensors, we present concurrent, high-frequency (

Groundwater flowing from hillslopes through riparian (near-stream) soils often undergoes chemical transformations that can substantially influence stream water chemistry. We used landscape analysis to predict total organic carbon (TOC) concentration profiles and groundwater levels measured in the riparian zone (RZ) of a 67 km2 catchment in Sweden. TOC exported laterally from 13 riparian soil profiles was then estimated based on the riparian flow–concentration integration model (RIM). Much of the observed spatial variability of riparian TOC concentrations in this system could be predicted from groundwater levels and the topographic wetness index (TWI). Organic riparian peat soils in forested areas emerged as hotspots exporting large amounts of TOC. These TOC fluxes were subject to considerable temporal variations caused by a combination of variable flow conditions and changing soil water TOC concentrations. Mineral riparian gley soils, on the other hand, were related to rather small TOC export rates and were characterized by relatively time-invariant TOC concentration profiles. Organic and mineral soils in RZs constitute a heterogeneous landscape mosaic that potentially controls much of the spatial variability of stream water TOC. We developed an empirical regression model based on the TWI to move beyond the plot scale and to predict spatially variable riparian TOC concentration profiles for RZs underlain by glacial till.

Forestry has been reported to cause elevated mercury (Hg) concentrations in runoff water. However, the degree to which forestry operations influence Hg in runoff varies among sites. A synoptic study, covering 54 catchments distributed all over Sweden, subjected to either stump harvest (SH), site preparation (SP) or no treatment (Ref), was undertaken to reveal the degree of forestry impact and causes of eventual variation. All streams were sampled twice, in autumn 2009 and summer 2010. There were no significant differences in total mercury (THg) and methylmercury (MeHg) concentrations between the three treatments in either 2009 or 2010. However, when pooling the treated catchments (that is, SH and SP) and taking catchment properties such as latitude into account, the treatment had a significant influence on the THg and MeHg concentrations. Although the treatment effect on THg and MeHg did not differ between SH and SP, the study did reveal significant forestry effects on potassium (K) and total nitrogen (TN) that were greater in the SH catchments and lower in the SP catchments. Partial least square (PLS) regressions indicated that organic matter was the most important variable influencing both the THg and MeHg concentrations. There were no significant differences between the treatment groups when comparing the ratios of THg/total organic carbon (TOC) and MeHg/TOC, suggesting that the high concentrations of THg and MeHg observed at some of the treated catchments are associated with increased concentrations of TOC rather than new methylation or increased mobilization caused by factors other than TOC.

This paper explores the flow paths and turnover times within a catchment characterized by the transmissivity feedback mechanism where there is a strong increase in the saturated hydraulic conductiv ...

Although water storage is an important variable to understand the hydrological functioning of a catchment, it is challenging to estimate the total water storage in a catchment. Catchment water stor ...

Lysevatten, a lake in southwest Sweden, has experienced both acidification and recent changes in the amount of lake-water organic carbon (TOC), both causing concern across Europe and North America. A range of paleolimnological tools – diatom-inferred pH, inferred lake-water TOC from visible-near-infrared spectroscopy (VNIRS), multi-element geochemistry and pollen analysis, combined with geochemical modeling were used to reconstruct the lake's chemistry and surroundings back to the most recent deglaciation 12 500 years ago. The results reveal that the recent anthropogenic impacts are similar in magnitude to the long-term variation driven by natural catchment changes and early agricultural land use occurring over centuries and millennia. The combined reconstruction of both lake-water TOC and lithogenic element delivery can explain the major changes in lake-water pH and modeled acid neutralizing capacity during the past 12 500 years. The results raise important questions regarding what precisely comprises "reference" conditions (i.e., free from human impacts) as defined in the European Water Framework Directive.

We bring together three decades of research from a boreal catchment to facilitate an improved mechanistic understanding of surface water dissolved organic carbon (DOC) regulation across multiple scales. The Krycklan Catchment Study encompasses 15 monitored nested research catchments, ranging from 3 to 6900 ha in size, as well as a set of monitored transects of forested and wetland soils. We show that in small homogenous catchments, hydrological functioning provides a first order control on the temporal variability of stream water DOC. In larger, more heterogeneous catchments, stream water DOC dynamics are regulated by the combined effect of hydrological mechanisms and the proportion of major landscape elements, such as wetland and forested areas. As a consequence, streams with heterogeneous catchments undergo a temporal switch in the DOC source. In a typical boreal catchment covered by 10-20% wetlands, DOC originates predominantly from wetland sources during low flow conditions. During high flow, the major source of DOC is from forested areas of the catchment. We demonstrate that by connecting knowledge about DOC sources in the landscape with detailed hydrological process understanding, an improved representation of stream water DOC regulation can be provided. The purpose of this study is to serve as a framework for appreciating the role of regulating mechanisms, connectivity and scaling for understanding the pattern and dynamics of surface water DOC across complex landscapes. The results from this study suggest that the sensitivity of stream water DOC in the boreal landscape ultimately depends on changes within individual landscape elements, the proportion and connectivity of these affected landscape elements, and how these changes are propagated downstream.

Short-term increases in soil solution nitrate (NO3-) concentration are often observed after forest harvest, even in N-limited systems. We model NO3- leaching below the rooting zone as a function of site productivity. Using national forest inventories and published estimates of N attenuation in rivers and the riparian zone, we estimate effects of stem-only harvesting on NO3- leaching to groundwater, surface waters and the marine environment. Stem-only harvesting is a minor contributor to NO3- pollution of Swedish waters. Effects in surface waters are rapidly diluted downstream, but can be locally important for shallow well-waters as well as for the total amount of N reaching the sea. Harvesting adds approximately 8 Gg NO3-N to soil waters in Sweden, with local concentrations up to 7 mg NO3-N 1(-1). Of that, similar to 3.3 Gg reaches the marine environment. This is similar to 3% of the overall Swedish N load to the Baltic. (C) 2010 Elsevier Ltd. All rights reserved.

Small streams with catchment areas

Acidification of soils and surface waters caused by acid deposition is still a major problem in southern Scandinavia, despite clear signs of recovery. Besides emission control, liming of lakes, streams, and wetlands is currently used to ameliorate acidification in Sweden. An alternative strategy is forest soil liming to restore the acidified upland soils from which much acidified runoff originates. This cost-benefit analysis compared these liming strategies with a special emphasis on the time perspective for expected benefits. Benefits transfer was used to estimate use values for sport ffishing and nonuse values in terms of existence values. The results show that large-scale forest soil liming is not socioeconomically profitable, while lake liming is, if it is done efficiently-in other words, if only acidified surface waters are treated. The beguiling logic of "solving" an environmental problem at its source (soils), rather than continuing to treat the symptoms (surface waters), is thus misleading.

To determine if forestry affects stream water dissolved organic carbon (DOC) concentrations, we conducted high frequency water sampling at a clear-cut catchment experiment in northern Sweden 1 year after harvesting. The overall finding was that harvesting significantly increased stream water DOC in these boreal forest catchments, at least during the growing season. The results indicate a DOC concentration increase of up to 50% during early summer on the two harvested catchments relative to the two control catchments. The analysis supports the hypothesis that a raised groundwater level following harvesting caused the increased DOC concentration during both hydrological episodes and low flow conditions. Harvesting resulted in a 70% increase in DOC export due to the combined effect of runoff and DOC concentration during the June–October study period. Given the extent of forestry activity in the boreal landscape, these results demonstrate that tree harvesting will affect the water quality of the region.

This paper defines landscape-scale patterns in the character of natural organic matter (NOM) and tests for relationships to catchment soil, vegetation and topography. The drainage network of a boreal catchment, subcatchment size 0.12–78 km2, in Northern Sweden was sampled in August 2002 during a period of stable low water flow. The NOM was characterized with UV/Vis spectroscopy, fluorescence, XAD-8 fractionation (%humic substances), gel permeation chromatography (apparent molecular weight), and elemental composition (C:N). The largest spatial variation was found for C:N, absorbance ratio, and specific visible absorptivity. The lowest variation was in fluorescence index, %humic substances and molecular retention time. But the variation in total organic carbon (TOC), iron and aluminium concentration was more than twice that of C:N. Between headwater and downstream sites no significant changes were distinguished in the NOM character. At stream reaches, junctions and lakes little change (

Summary Two different but complementary modelling approaches for reproducing the observed dynamics of total organic carbon (TOC) in a boreal stream are presented. One is based on a regression analysis, while the other is based on riparian soil conditions using a convolution of flow and concentration. Both approaches are relatively simple to establish and help to identify gaps in the process understanding of the TOC transport from soils to catchments runoff. The largest part of the temporal variation of stream TOC concentrations (4–46 mg L−1) in a forested headwater stream in the boreal zone in northern Sweden may be described using a four-parameter regression equation that has runoff and transformed air temperature as sole input variables. Runoff is assumed to be a proxy for soil wetness conditions and changing flow pathways which in turn caused most of the stream TOC variation. Temperature explained a significant part of the observed inter-annual variability. Long-term riparian hydrochemistry in soil solutions within 4 m of the stream also captures a surprisingly large part of the observed variation of stream TOC and highlights the importance of riparian soils. The riparian zone was used to reproduce stream TOC with the help of a convolution model based on flow and average riparian chemistry as input variables. There is a significant effect of wetting of the riparian soil that translates into a memory effect for subsequent episodes and thus contributes to controlling stream TOC concentrations. Situations with high flow introduce a large amount of variability into stream water TOC that may be related to memory effects, rapid groundwater fluctuations and other processes not identified so far. Two different climate scenarios for the region based on the IPCC scenarios were applied to the regression equation to test what effect the expected increase in precipitation and temperature and resulting changes in runoff would have on stream TOC concentrations assuming that the soil conditions remain unchanged. Both scenarios resulted in a mean increase of stream TOC concentrations of between 1.5 and 2.5 mg L−1 during the snow free season, which amounts to approximately 15% more TOC export compared to present conditions. Wetter and warmer conditions in the late autumn led to a difference of monthly average TOC of up to 5 mg L−1, suggesting that stream TOC may be particularly susceptible to climate variability during this season.

Running water comprises just over one millionth of the world’s water.The importance of those streams and rivers as a resource for humanwelfare and biodiversity, however, is far out of proportion to thatminuscule fraction. This explains why protecting running waters (theflow regimes, water quality and biota) is such a vital concern for society.Yet for all the focus and concern, how much do we actually know aboutthese running waters, and the lotic habitat they comprise?Consider what would happen if one asked any national environmentalauthority to assess the basic chemical and ecological status of runningwaters. At the river mouths, there would be enough information tomake a reasonable assessment of the status. But somewhere on theway upstream, available data would run dry, long before most streamchannels did (in non-arid regions).In Sweden, with an ambitious programme for monitoring and assess-ing surface waters, it came as a surprise several years ago to realizethat the length of all perennial streams on the country’s maps was notknown. When that was modelled in the form of a ‘virtual network’ froma50m

Inorganic aluminum (Al) concentrations are critical for defining the biological effects of acidification. The landscape's role in controlling the spatial variability of Al and its speciation has received only limited attention. We analyzed the speciation of stream Al at 14 sites within a 68 km2 boreal catchment during spring snowmelt, a period of episodic acidity. Three factors that influenced Al at these sites were landscape type (specifically the proportion of wetland areas), stream pH, and dissolved organic carbon (DOC). Forested catchment sites underlain by mineral soils had higher total Al concentrations and greater inorganic Al proportions than catchments with larger wetland areas, despite significantly higher pH. We suggest that this difference results from source limitation of Al in the peat wetlands. The control of Al solubilitywas dominated by organic complexes, with the organic carrying capacity exceeding Al in the majority of samples. When assessing the inorganic phase, only four percent of the samples were oversaturated with regards to commonly forming secondary Al minerals, with no samples showing supersaturation higher than 10 times with respect to any given solid phase. Inorganic Al rarely exceeded biological thresholds, except for short periods during peak flow in forested areas, despite two-thirds of the streams having minimum pH values below 4.9. Streams with a high percentage of wetland area were associated with lower Al:DOC ratios. The Al:DOC ratios were quite stable in each stream before, during, and after snowmelt, with the exception of isolated spikes in the Al:DOC ratio associated with particulate Al at a downstream site during high flow.

Spring flood is the period in boreal ecosystems that is most sensitive to acid deposition since as much as half of the year's precipitation melts and enters streams or the soil in the space of a few weeks. The ‘Episode Project‘ in northern Sweden found a consistent relationship between the SO42− concentration of snow and the anthropogenic component of acid neutralization capacity (ANC) decline during spring flood. This correlation creates the possibility for a regional prediction of the severity of anthropogenic episodic acidification during spring flood episodes using SO42− deposition data together with chemical data from 1240 lakes selected to be representative of northern Sweden. The regional assessment found that in 1998, ca 6% of the region was seriously affected by anthropogenic acidification during spring flood. The results from this study have important implications for both the national liming strategy and international negotiations to further reduce emissions of air pollutants in Europe.

There is an increasing demand for modelling the fluxes of chemical constituents at the catchment scale. Conceptual runoff models provide a basis for such modelling tasks provided that they capture the essential hydrological processes. However, most conceptual models do not fully address interactions between unsaturated and saturated storage. This can lead to unrealistic simulations for watersheds with shallow groundwater, where a large part of the soil volume can contribute to both the unsaturated and the saturated storage, depending on groundwater levels. Adding a small amount of water to the saturated storage will cause a significant amount of water stored in the unsaturated zone to change its status to ‘saturated’. The maximum volume of the unsaturated storage also decreases with rising groundwater levels, i.e. increasing saturated storage. In this study, a new model concept was proposed in which special emphasis was put on the interaction between saturated and unsaturated storage. The total storage was divided into two compartments, representing saturated and unsaturated storage, with a boundary moving up and down in response to the water budget of the compartments. Groundwater dynamics show a distinct pattern along the hillslope studied. Groundwater levels in an area close to the stream had dynamics similar to runoff, whereas levels further upslope responded to rainfall with a delay. To represent these differences in the model, the hillslope was subdivided into a riparian and an upslope reservoir. The performance of the new model was compared with that of simpler model variants without spatial differentiation and with or without the new formulation allowing for interactions between unsaturated and saturated storage. The new model approach provided the best results for simulating both runoff and groundwater dynamics. The subdivision of the hillslope accounted for most of the performance increase. To test the model structure further, 18O concentrations in the stream were simulated and compared with measured values. Copyright © 2003 John Wiley & Sons, Ltd.

Natural organic matter (NOM) from stream and soil water in a humic-rich headwater catchment in northern Sweden (initial total organic carbon (TOC) concentrations 10-40 mg C L-1) was rapidly degrade ...

The sources of episodic pH decline in four streams from northern Sweden during the autumn of 1996 were quantified. The events, in which pH dropped by between 1Ð 0a nd 2Ð4 units, were preceded by an extensive summer drought. Total organic carbon, which increased 100% to 160% during peak flow, was the most important driving mechanism of the episodic pH decline. Sulphate, however, was relatively more important during these autumn events than during spring flood. In the sites where past and present anthropogenic deposition were believed to be the main source of sulphate in stream water, sulphate contributed less than 0Ð3 pH units to the pH decline. In catchments where natural sources of sulphate are known to be important, sulphate contributed up to 0Ð6 units of pH decline. The export of sulphate during the episodes was two to nine times higher than what was expected from deposition only. The drought preceding the study episodes resulted in some of the lowest ground water levels during the 1990s in that region. The large export of sulphate was probably due to oxidation of natural sulphate bearing minerals in the soil and/or previously deposited sulphate driven by the low ground water level preceding the episodes. Copyright  2002 John Wiley & Sons, Ltd.

Soil frost effects on soil water and runoff dynamics along a boreal forest transect. 1. Field investigation

In northern regions, spring snowmelt generally results in an episodic decline of surface water pH. Natural organic acids may be an important factor in this type of pH change. We studied the variations in the acid/base character of aquatic organic acids during spring snowmelt in 1997 at Svartberget, a stream rich in total organic carbon (TOC) that is located in the boreal zone of northern Sweden. Snowmelt at Svartberget was accompanied by a drop in stream pH of up to 1.8 pH units, as well as the dilution of base cation and strong acid anion concentrations. Aluminum and TOC increased or decreased during snowmelt, depending on the sampling site within the 50-ha catchment. Although there were distinct differences in pH, TOC, and major inorganic ions in the runoff from three subcatchments, there was very little variation in the acid/base character of TOC. Thus organic acids in the Svartberget catchment share a common set of acid/base properties despite markedly different subcatchment drainage patterns, vegetation, and soils. The dissociation behavior of organic acids at Svartberget could be described with high precision (R2 = 0.91, p < 0.001, and n = 115) by a triprotic acid analog model (pKa1 = 2.5, pKa2 = 4.0, and pKa3 = 5.8), together with the measured site density of organic acids (8.6±0.8 μeq (mg TOC)−1). A Gaussian pKa distribution (μ = 4.20 and σ = 1.43) predicted organic acid dissociation with similar precision (R2 = 0.91, p < 0.001, and n = 94). Variations in site density among the tributary streams could largely be explained by aluminum complexation. Sites with lower measured site densities had greater concentrations of organically bound Al. Thus Al binding reduces the potential release or neutralization of H+ by organic acids.

The situation in northern Sweden did not figure prominently in the intense period of research during the 1980's that laid the basis for many acidification-related policies now in effect in Europe and Sweden. Northern Sweden has not only relatively low acid deposition levels and significant sources of natural acidity, but also intense episodes of pH decline during spring flood that are a major focus of liming activity. Controversy over that liming and natural acidity has led to scientific advances. These include discovery of a correlation between sulfur in snow and the anthropogenic contribution to the subsequent spring flood ANC decline, but also that natural organic acidity is responsible for most of the spring pH decline in the region. This paper compares the developments in liming policy with the scientific developments of relevance to the region during the last decade. Considerable discrepancies are noted which create opportunities for revising remediation policies to better reflect the state of knowledge in 2000.

Previous studies at the Svartberget catchment in northern Sweden have identified potential terrestrial sources of methylmercury (MeHg) and total mercury (THg) in runoff as well as processes controlling MeHg/Hg transfers from soil to runoff water. This paper considers whether the concepts based on a few seasons of observations are consistent with catchment budgets of MeHg/THg over half a decade. Inter-annual and seasonal variations in the input and output fluxes of THg/MeHg, in open field wet deposition (OF), litterfall (LF) and runoff water are evaluated together with more recently measured concentrations that include the throughfall water (TF) data. The input and output flux data of THg and MeHg from the Svartberget catchment are also compared to those from the Gårdsjön Catchment. The average annual MeHg input fluxes in OF, TF and LF are 0.08, 0.17 and 0.3 g km(-2) year(-1), respectively. The comparable inputs for THg are 7, 15 and 17 g km(-2) year(-1). Thus, LF is as important as TF for THg inputs, while LF is twice as important as TF for MeHg inputs. The annual output flux of MeHg varied between 0.05 and 0.14 g km(-2) year(-1). The annual output flux of THg varied between 1 and 3.4 g km(-2) year(-1). The large inter-annual variations in catchment output did not follow the smaller variations in atmospheric input. This suggests that changes in climate can effect terrestrial outputs of THg/MeHg to surface water more than atmospheric deposition. These data do not contradict the earlier findings that it is the hydrological and biogeochemical processes in the riparian zone that have a central role in determining the amount of MeHg reaching surface waters from forested catchments. The juxtaposition of major flow paths and organic-rich soils in the riparian zone may create the locations of most importance for net MeHg production, runoff export and a larger pool of MeHg.