A closed-loop on-site wastewater treatment system (OWT) was studied comprising steps of septic tank to remove organics (Biological Oxygen Demand (BOD)), biofiltration clarifier for biological removal of nitrogen (N), phosphorus (P) and BOD, reactive Polonite® filter for chemical adsorption and precipitation removal of dissolved P, and tidal flow constructed wetland (TFCW) sand filter for polishing the effluent to low P and N effluent Swedish standards. The field experimental data that have been used to optimize TFCW design in the numerical modelling using HYDRUS-2D coupled with and without PHREEQC indicated that the adsorption efficiency of the reactive Polonite® adsorbent was nearly double to that obtained in TFCW sand filters for PO4-P (95 %) and Total-P (85 %) removal in summer at a high temperature range (15.4–18.8 °C) and pH range (9.9–10.8). The weaker PO4-P (53 %) and Total-P (25 %) removal efficiency in winter was due to a low temperature (1.5–8.1 °C) and low pH (7.2–7.9). This decrease in pH was attributed to salinity in the domestic wastewater and dilution of rainwater. Modelling results revealed that the transport mechanisms and rate of P adsorption kinetics in the TFCW sand filters enhanced with calcium and iron flow from chemical dissolution in the preceding Polonite® adsorbent was increased with the increase in temperature. However, the P adsorption was less sensitive at high ferrihydrite (Fe(OH)3) dose, suggesting limited effects of cations dissolution and abundance of metal oxides and hydroxide ions at the mineral surface for anions exchange with phosphate for surface complexation. The strategy of combining field data and modelling provided valuable insights for assessing adaptability and optimizing TFCW design under variable fluxes and scenario effects of insulated/uninsulated and dilution by rainwater in cold-climate regions.

Stream restoration has been advocated as a key strategy to restore the ecosystem functioning of degraded stream systems, mitigate excess nutrient concentrations and reduce export to downstream recipients. Specifically small agricultural streams are suitable for such restoration efforts due their disproportionate contribution to downstream nutrient loading and their large proportion of the total stream network length. However, large-scale assessments of both the current removal efficiencies in and the enhancement potential of these streams are generally lacking. Here, we used a physically based model framework supported by an extensive dataset to simulate the transport and removal of nitrogen (N) in all local agricultural streams in Sweden, equivalent to 70,000 km or 33.5% of the total mapped stream network. The framework assumed that N removal occurred predominantly by denitrification in the hyporheic zone and was utilized to quantify the current and potential removal as well as to assess the limiting conditions in terms of Damköhler numbers. The removal rates were shown to be highly variable with location, but the aggregated results indicated that the current removal for all assessed reaches was 13.1, 0.1 and 37.6% of their load during mean (MQ), mean high (MHQ) and mean low discharge (MLQ) conditions, respectively. The theoretical potential of N removal, i.e., the aggregated removal under the assumption of optimal hyporheic conditions was estimated to be 15.4, 1.9 and 62.1% during MQ, MHQ and MLQ, respectively. The study also comprised a detailed investigation along one single stream reach that displayed the ability of engineered restoration actions to alter the hyporheic residence times and, in the end, the hyporheic N-removal during different discharge conditions. Overall, our results indicated that in-stream restoration efforts aiming to enhance hyporheic functioning in small agricultural streams had a high potential to reduce terrestrial N export, especially if implemented over larger areas.

Weather climate fluctuations cause large variations in renewable electricity production, which requires substantial amounts of energy storage to overcome energy drought periods. Based on daily hydroclimatic data and information about renewable power systems covering Europe, here we quantify the complementarity in the solar-wind-hydro energy components of the continental climate system. We show that the spatiotemporal management of renewable electricity production over Europe can induce a virtual energy storage gain that is several times larger than the available energy storage capacity in hydropower reservoirs. The potential electricity production matches the consumption by spatiotemporal management of suitable shares of solar and wind power complemented with the present hydropower. While the mixed renewable energy potential varies less than anticipated at the continental scale, utilization of the complementarity requires new continental electrical transmission lines and stable international trade. We highlight that management models need to consider incentives beyond national boundaries to appropriately benefit from continental climate conditions.

This study evaluates the impact of hydroclimate-driven periodic runoff on hydropower operations and production, with a focus on how the forecasted biennial periodicity of runoff time series could affect the efficiency of hydropower generation. Hydrologic stochastic processes are utilized to forecast long-term runoff, and seven hydroclimate scenarios are developed to be input into a production management model, allowing for an analysis of how periodic hydroclimate variations influence hydropower management and output. The results reveal that the biennial alternation between wet and dry years is a key factor affecting hydropower operations in the Dalälven River Basin. Notable differences between wet- and dry-year scenarios were observed in terms of power efficiency, production output, and forecasting accuracy. Operating hydropower systems based on dry-year runoff forecasts in wet years results in a 1.63% decrease in production efficiency and a reduction of 9,104 MWh in power generation. Conversely, applying wet-year forecasts in dry years slightly boosts production efficiency by 0.31% and increases power generation by 7,832 MWh. Scenarios that adhere to biennial periodicity offer the highest forecasting accuracy, particularly when applying dry-year forecasts in dry years in winter and spring, which produce the most precise predictions. In contrast, using dry-year forecasts in wet years results in the lowest forecasting accuracy.

Natural dams are formed most often in narrow, steep valleys in high mountains. The outburst floods triggered by natural dam failures result in the topography and landforms successively being altered. Boulder bars are common natural structures that are selected here to quantitatively evaluate the impact of outburst floods on the topographical and landform variations in downstream channels. In this study, we selected the Sedongpu natural dam on the Yarlung Tsangpo River formed as a result of a landslide in 2018 as an example, and studied the geomorphological changes in a river reach located 173 km downstream of the Sedongpu natural dam. The sizes and shapes of the boulder bars in this area were statistically analyzed. The results show that there are three shape types of boulder bars in this area, i.e., sickle, bamboo leaf and oval. Furthermore, it found that the relationship between the lengths and widths of boulder bars is similar before and after outburst floods, as is the relationship between perimeters and lengths of boulder bars, which means these relationships are not affected by outburst floods. And the perimeters of boulder bars are almost twice their lengths. In addition, the relationship between the areas and lengths of boulder bars follows a power function. The most important finding is that the riverine morphological features conserved self-similarity due the influence of the outburst flood erosion triggered by a natural dam failure. This finding adds to the previous observations since dam failures introduce sudden and dominating impacts on river systems.

This study examined the adsorption capacity and treatment efficiency of sand filters in on-site treatment systems for cold climate regions. The effects of different operating conditions, porosity and kinetics parameters were investigated in column experiments and COMSOL Multiphysics® modelling, to comprehensively reveal the mechanisms and optimize treatment efficiency of nitrogen (N) and phosphorus (P) removal in a field tidal flow constructed wetland (TFCW), treating effluent from a package treatment plant with P filter material. The results from column experiments with sand showed that Total-P adsorption rate was dependent on feed water quality (Septic tank >0.77 ± 0.06 g kg−1; Biotreatment >0.41 ± 0.07 g kg−1; Reactive material Polonite® <0.18 ± 0.07 g kg−1). In the field TFCW trial, Total-P adsorption in the top layer (>1.42 ± 0.55 g kg−1) and middle layer (>1.06 ± 0.51 g kg−1) was twice that in laboratory columns, due to strong interaction with the air-water interface and use of fluctuated domestic wastewater solutions. The breakthrough curve (BTCs) of the coarse sand matched the physical behaviour of tracer electrical conductivity (EC) in effluent from the sand column experiments. The modelling results demonstrated that high filter porosity and low hydraulic load were significant factors for optimal removal of NH4–N, Total-N, PO4–P, Total- P in the top layer (>99.95 ± 0.03 %, 44.37 ± 28.75%, 70.89 ± 28.30%, 76.18 ± 20.3%), middle layer (>98.94 ± 1.77%, 18.23 ± 23.04%, 76.62 ± 28.73%, 65.40 ± 31.85%) and deep layer (>99.99 ± 0.02%, 65.50 ± 20.64%, 75.53 ± 23.16%, 41.54 ± 28.81%) in the TFCW system, respectively. The results show that on-site wastewater treatment in cold climate TFCW can be applied as a technology to polish effluent from a three-step pretreatment system. However, hydraulic optimization is an important factor for the design of the TFCW to receive a successful long-term operating system.

Hydropower is the largest source of renewable energy in the world and currently dominates flexible electricity production capacity. However, climate variations remain major challenges for efficient production planning, especially the annual forecasting of periodically variable inflows and their effects on electricity generation. This study presents a model that assesses the impact of forecast quality on the efficiency of hydropower operations. The model uses ensemble forecasting and stepwise linear optimisation combined with receding horizon control to simulate runoff and the operation of a cascading hydropower system. In the first application, the model framework is applied to the Dalalven River basin in Sweden. The efficiency of hydropower operations is found to depend significantly on the linkage between the representative biannual hydrologic regime and the regime actually realised in a future scenario. The forecasting error decreases when considering periodic hydroclimate fluctuations, such as the dry-wet year variability evident in the runoff in the Dalalven River, which ultimately increases production efficiency by approximately 2% (at its largest), as is shown in scenarios 1 and 2. The corresponding potential hydropower production is found to vary by 80 GWh/year. The reduction in forecasting error when considering biennial periodicity corresponds to a production efficiency improvement of about 0.33% (or 13.2 GWh/year).

Significant attention has been given to hyporheic water fluxes induced by hydromorphologic processes in streambeds and the effects they have on stream ecology. However, the impact of hyporheic fluxes on regional groundwater flow discharge zones as well as the interaction of these flows are much less investigated. The groundwater-hyporheic interactive flow not only governs solute mass and heat transport in streams but also controls the retention of solute and contamination following the discharge of deep groundwater, such as naturally occurring solutes and leakage from geological waste disposal facilities. Here, we applied a physically based modeling approach combined with extensive hydrologic, geologic and geographical data to investigate the effect of hyporheic flow on groundwater discharge in the Krycklan catchment, located in a boreal landscape in Sweden. Regional groundwater modeling was conducted using COMSOL Multiphysics by considering geologic heterogeneity and infiltration constraint of the groundwater circulation intensity. Moreover, the hyporheic flow was analyzed using an exact spectral solution accounting for the fluctuating streambed topography and superimposed with the regional groundwater flow. By comparing the discharge flow fields with and without consideration of hyporheic flows, we found that the divergence of the discharge was substantially enhanced and the distribution of the travel times of groundwater was significantly shifted toward shorter times due to the presence of hyporheic flow. Particularly important is that the groundwater flow paths contract near the streambed interface due to the hyporheic flow, which leads to a phenomenon that we name “fragmentation” of coherent areas of groundwater upwelling in pinhole-shaped stream tubes.

As a part of the overall safety assessment for a geological disposal of radioactive waste, models for different ecosystems are used to evaluate doses to humans and biota from possible radionuclide discharges to the biosphere. In previous safety assessments, transport modelling of radionuclides in running waters such as streams has been much simplified to the extent that only dilution of the inflow of radionuclides has been considered with no regard of any other interactions.Hyporheic exchange flow (HEF) is the flow of surface water in streams that enters the subsurface zone and, after some time, returns to the surface. HEF has been studied for decades. Hyporheic exchange and the residence time in the hyporheic zone are key parameters controlling the transport of radionuclides in a stream. Further-more, recent studies have shown that HEF can reduce the groundwater upwelling area and increase the up -welling velocity in areas closest to the streambed water interface.In this paper, the development of an assessment model describing radionuclide transport with consideration of HEF and deep groundwater upwelling along streams is presented. An approach to parameterising the hyporheic exchange processes into an assessment model is based on a comprehensive study that has been performed in five different Swedish catchments. Sensitivity analyses are performed to explore the effect with consideration of the inflow of radionuclides with regard to HEF and deep groundwater upwelling in a safety assessment perspective. Finally, we include some suggestions for the application of the assessment model to long-term radiological safety assessments.

Hyporheic exchange flow (HEF) at the streambed–water interface (SWI) has been shown to impact the pattern and rate of discharging groundwater flow (GWF) and the consequential transport of heat, solutes and contaminants from the subsurface into streams. However, the control of geographic and hydromorphological catchment characteristics on GWF–HEF interactions is still not fully understood. Here, the spatial variability in flow characteristics in discharge zones was investigated and averaged over three spatial scales in five geographically different catchments in Sweden. Specifically, the deep GWF discharge velocity at the SWI was estimated using steady-state numerical models, accounting for the real multiscale topography and heterogeneous geology, while an analytical model, based on power spectral analysis of the streambed topography and statistical assessments of the stream hydraulics, was used to estimate the HEF. The modeling resulted in large variability in deep GWF and HEF velocities, both within and between catchments, and a regression analysis was performed to explain this observed variability by using a set of independent variables representing catchment topography and geology as well as local stream hydromorphology. Moreover, the HEF velocity was approximately two orders of magnitude larger than the deep GWF velocity in most of the investigated stream reaches, indicating significant potential to accelerate the deep GWF velocity and reduce the discharge areas. The greatest impact occurred in catchments with low average slope and in reaches close to the catchment outlet, where the deep GWF discharge velocity was generally low.