| 1. |
- Bin Ashraf, Faisal, et al.
(författare)
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A Method for Assessment of Sub-Daily Flow Alterations Using Wavelet Analysis for Regulated Rivers
- 2022
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 58:1
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Tidskriftsartikel (refereegranskat)abstract
- New tools are needed to evaluate the impacts of short-term hydropower regulation practices on downstream river systems and to progress towards sustainable river-flow management. As hydropower is increasingly being used to balance the energy load deficit caused by other less flexible sources, sub-daily flow conditions across many regulated river (RR) systems are changing. To address this, we used wavelet analyses to quantify the discharge variability in RRs and categorized the level of variability based on the conditions in natural free-flowing rivers. The presented framework used the definition of fluvial connectivity (Grill et al., 2019) to identify free-flowing rivers used in the study. We tested the developed framework in 12 different RRs in Finland and found higher overall averaged sub-daily variations, with up to 20 times larger variability than natural conditions. A large, highly regulated Finnish river system was found to have the highest sub-daily variations in winter, while smaller RRs with lower levels of regulation the highest variations in summer. The proposed framework offers a novel tool for sustainable river management and can be easily applied to various rivers and regions globally. It had flexibility to analyze sub-daily variations in desired seasonal or other ecologically sensitive periods.
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| 2. |
- Bin Ashraf, Faisal, et al.
(författare)
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Changes in short term river flow regulation and hydropeaking in Nordic rivers
- 2018
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Quantifying short-term changes in river flow is important in understanding the environmental impacts of hydropower generation. Energy markets can change rapidly and energy demand fluctuates at sub-daily scales, which may cause corresponding changes in regulated river flow (hydropeaking). Due to increasing use of renewable energy, in future hydropower will play a greater role as a load balancing power source. This may increase current hydropeaking levels in Nordic river systems, creating challenges in maintaining a healthy ecological status. This study examined driving forces for hydropeaking in Nordic rivers using extensive datasets from 150 sites with hourly time step river discharge data. It also investigated the influence of increased wind power production on hydropeaking. The data revealed that hydropeaking is at high levels in the Nordic rivers and have seen an increase over the last decade and especially over the past few years. These results indicate that increased building for renewable energy may increase hydropeaking in Nordic rivers.
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| 3. |
- Earon, Robert, et al.
(författare)
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Insight into the influence of local streambed heterogeneity on hyporheic-zone flow characteristics
- 2020
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Ingår i: Hydrogeology Journal. - : Springer Science and Business Media Deutschland GmbH. - 1431-2174 .- 1435-0157. ; 28:8, s. 2697-2712
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Tidskriftsartikel (refereegranskat)abstract
- Interaction between surface water and groundwater plays a fundamental role in influencing aquatic chemistry, where hyporheic exchange processes, distribution of flow paths and residence times within the hyporheic zone will influence the transport of mass and energy in the surface-water/groundwater system. Geomorphological conditions greatly influence hyporheic exchange, and heterogeneities such as rocks and clay lenses will be a key factor for delineating the hyporheic zone. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to investigate the streambed along a 6.3-m-long reach in order to characterise geological layering and distinct features which may influence parameters such as hydraulic conductivity. Time-lapse ERT measurements taken during a tracer injection demonstrated that geological features at the meter-scale played a determining role for the hyporheic flow field. The penetration depth of the tracer into the streambed sediment displayed a variable spatial pattern in areas where the presence of highly resistive anomalies was detected. In areas with more homogeneous sediments, the penetration depth was much more uniformly distributed than observed in more heterogeneous sections, demonstrating that ERT can play a vital role in identifying critical hydraulic features that may influence hyporheic exchange processes. Reciprocal ERT measurements linked variability and thus uncertainty in the modelled resistivity to the spatial locations, which also demonstrated larger variability in the tracer penetration depth, likely due to local heterogeneity in the hydraulic conductivity field.
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| 4. |
- Hao, Shuang, et al.
(författare)
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A Model for Assessing the Importance of Runoff Forecasts in Periodic Climate on Hydropower Production
- 2023
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Ingår i: Water. - : MDPI AG. - 2073-4441. ; 15:8
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Tidskriftsartikel (refereegranskat)abstract
- 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).
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| 5. |
- Jaeger, Anna, et al.
(författare)
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Time Series of Electrical Conductivity Fluctuations Give Insights Into Long-Term Solute Transport Dynamics of an Urban Stream
- 2023
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 59:7
-
Tidskriftsartikel (refereegranskat)abstract
- Artificial tracers are often used for quantitative estimates of solute transport properties in rivers. However, single-injection tracer tests give insights in transport characteristics limited to the ecohydrological conditions at the testing time. Series of time-consuming and laborious tracer tests would be required to properly capture seasonal changes. The present study uses intrinsic diurnal fluctuations of electrical conductivity (EC) caused by discharge of treated wastewater as a tracer to evaluate solute transport processes along a 4.7-km reach of the River Erpe, Germany. By reproducing the fluctuations recorded along the river using the solute transport model one-dimensional transport with inflow and storage, this study investigated the long-term dynamics in solute transport properties. Individual 48-hr curves of EC were used in the steady state configuration of the model to gain 48-hr-integrated estimates of selected transport parameters. Using a sliding window approach in 1-hr steps along the 2,270-hr time series of EC the temporal variability of solute transport between April and June 2016 was assessed. To test the identifiability of parameters using the proposed method, sensitivity analyses and a breakthrough curve analysis of selected 48-hr windows were implemented. With time advancing into the summer, a significant rising trend (Mann-Kendall test p-value < 0.05) of the cross sectional area of the channel was observed and attributed to the growth of macrophytes and a significant slightly decreasing trend for the storage rate was found. The presented method is of high value for river management, as promoting transient storage enhances biogeochemical cycling and benefits water quality.
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| 6. |
- Lewandowski, Jörg, et al.
(författare)
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Is the Hyporheic Zone Relevant beyond the Scientific Community?
- 2019
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Ingår i: Water. - : MDPI AG. - 2073-4441. ; 11:11
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Tidskriftsartikel (refereegranskat)abstract
- Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors.
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| 7. |
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| 8. |
- Mojarrad, Babak Brian
(författare)
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Multi-Scale Surface Water-Groundwater Interaction : Implications for GroundwaterDischarge Patterns
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the impact on individual scale intervals can be evaluated separately. Additionally, the impacts of streamflow discharge intensity on hyporheic exchange flow fields are investigated through field investigation. In this study, the hyporheic fluxes velocity at the streambed interface were generally at least one order of magnitude higher than groundwater flow velocity. This reflects the domination of hyporheic fluxes at the streambed interface, leading to significant impacts on the discharge of deeper groundwater through the hyporheic zone. Significant effects were found in flow travel time, direction and discharge areas at the streambed sediment. Thus, the upward groundwater flow contracted near the streambed surface and discharged in a fragmented pinhole pattern at the sediment–water interface. The results also indicated that the magnitude of groundwater flow and the heterogeneity of the subsurface sediment (i.e., the depth decaying hydraulic conductivity of streambed sediment) controlled the depth of hyporheic exchange flow in aquatic sediment. Furthermore, the increased stream flow intensity led to a wide range of hyporheic flow residence times in which temperature was used to evaluate stream segments with gaining and losing conditions.
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| 9. |
- Morén, Ida, et al.
(författare)
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Cross‐Validating Hydromechanical Models and Tracer Test Assessments of Hyporheic Exchange Flow in Streams With Different Hydromorphological Characteristics
- 2021
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 57:12
-
Tidskriftsartikel (refereegranskat)abstract
- Hyporheic exchange flow (HEF) can generally be quantified through two different approaches. The first approach, which is deductive, entails physically based models, supported with relevant observations. The second approach includes inductive assessments of stream tracer tests using solute transport models, which provide a useful mathematical framework that allows for upscaling of results, but included parameters often have a vague physical base, which limits the possibilities of generalizing results using independent hydromorphological observations. To better understand how the physical basis of HEF-quantifying parameters relates to stream hydromorphology at different spatial scales, we cross-validated the results from (a) tracer test assessments using a 1D solute transport model that accounts for HEF and (b) an independent hydromechanical model that represents HEF driven by multiscale pressure gradients along the streambed interface. To parameterize the models, topographical surveys, tracer tests, and streambed hydraulic conductivity measurements were performed in 10 stream reaches, differing in terms of geomorphology, slope, and discharge. The results show that the models were cross-validated in terms of the average exchange velocity, providing a plausible physical explanation for this parameter in small alluvial streams with low discharges, shallow depth, and moderate slopes. However, the hydromechanical model generally resulted in wider residence time distributions and occasionally higher average residence times compared to the tracer test assessments. From the cross-validated multiscale hydromechanical model, we learned that water surface profile variations were the main drivers of HEF in all investigated streams and that spatial scales between 20 cm and 5 m dominated the estimated HEF velocity.
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| 10. |
- Morén, Ida, et al.
(författare)
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Design of Remediation Actions for Nutrient Mitigation in the Hyporheic Zone
- 2017
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 53:11, s. 8872-8899
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Tidskriftsartikel (refereegranskat)abstract
- Although hyporheic exchange has been shown to be of great importance for the overall water quality of streams, it is rarely considered quantitatively in stream remediation projects. A main driver of hyporheic exchange is the hydraulic head fluctuation along the streambed, which can be enhanced by modifications of the streambed topography. Here we present an analytical 2-D spectral subsurface flow model to estimate the hyporheic exchange associated with streambed topographies over a wide range of spatial scales; a model that was validated using tracer-test-results and measurements of hydraulic conductivity. Specifically, engineered steps in the stream were shown to induce a larger hyporheic exchange velocity and shorter hyporheic residence times compared to the observed topography in Tullstorps Brook, Sweden. Hyporheic properties were used to parameterize a longitudinal transport model that accounted for reactions in terms of first-order decay and instantaneous adsorption. Theoretical analyses of the mitigation effect for nitrate due to denitrification in the hyporheic zone show that there is a Damkohler number of the hyporheic zone, associated with several different stream geomorphologies, that optimizes nitrate mass removal on stream reach scale. This optimum can be limited by the available hydraulic head gradient given by the slope of the stream and the geological constraints of the streambed. The model illustrates the complex interactions between design strategies for nutrient mitigation, hyporheic flow patterns, and stream biogeochemistry and highlights the importance to diagnose a stream prior remediation, specifically to evaluate if remediation targets are transport or reaction controlled.
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| 11. |
- Morén, Ida, et al.
(författare)
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Geographic and hydromorphologic controls on interactions between hyporheic flow and discharging deep groundwater
- 2023
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Ingår i: Hydrogeology Journal. - : Springer Nature. - 1431-2174 .- 1435-0157. ; 31:3, s. 537-555
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 12. |
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| 13. |
- Morén, Ida
(författare)
-
The influence of multiscale hyporheic flow on solute transport : Implications for stream restoration enhancing nitrogen removal
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stream water that flows into and out of streambeds is called hyporheic exchange flow (HEF).It continuously interacts with groundwater and thereby affect the water quality of local streamreaches as well as downstream recipients by providing an environment where solutes andenergy can be retained and degraded. Because of anthropogenic activities, many streams andrivers have been physically, chemically and biologically degraded during the last centuries andnatural functions, such as HEF, have to some extent been lost. The general aim of this thesiswas to advance the understanding of the physical controls of HEF in small streams and toinvestigate how HEF influences solute reactive transport in streambeds and surface waternetworks before and after stream restoration. To reach the aim, the consistency and deviationbetween HEF parameters evaluated with two common approaches were investigated in tendifferent alluvial streams with low discharge, shallow depth and moderate slope. The twoapproaches were: 1) developing and using a deductive hydro-mechanical model to assessed therelationship between the multiscale streambed geomorphology and the reach scale averageHEF parameters, and 2) evaluating HEF parameters from in-stream tracer tests using a 1Dlongitudinal transport model. The relatively high consistency between the approaches connectstheories that previously have been relatively fragmented and provides a tool for upscaling(parameterizing) of HEF in solute transport models over stream networks based onindependent observations of stream topography, streambed sediment properties and in-streamhydraulics. Applying the modelling framework at the network scale and supporting it withcomprehensive datasets provided information regarding physical mechanisms and spatialvariability of HEF as well as its influence on longitudinal solute transport. Specifically, thefractal properties of the water surface profile were shown to represent the average HEF velocitywell. Furthermore, hydraulic head variations over shorter wavelengths (0.1-5 m) were found todrive the main part of the HEF and the static hydraulic head variations dominated over dynamichydraulic head variations as drivers of HEF in all investigated streams. Moreover, this thesishighlights the importance of the hyporheic zone as a bio-chemical and mechanical filter forstream water. It shows that common engineered stream restorations can influence HEF andimprove the water quality in local stream reaches as well as downstream recipients. Specifically,the thesis presents exact solutions to the nitrogen transport, which shows that the mass removalof nitrogen in the hyporheic zone is either transport or reaction limited and that the maximalremoval rate corresponds to an optimal hyporheic residence time and a typical denitrificationDamköhler number. The results also show that potential exists to reduce the agriculturalnitrogen load to the Baltic Sea by stream restorations that optimize the hyporheic residencetimes. However, the large spatiotemporal variability in the potential between reaches stressesthe importance for further studies on which processes that are driving HEF under specifichydromorphologic conditions and careful design of stream restoration measures at each localstream reach.
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| 14. |
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| 15. |
- Riml, Joakim, 1979-, et al.
(författare)
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Potential of stream restorations to enhance the hyporheic removal of agricultural nitrogen in Sweden
- 2024
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Ingår i: Ecological Engineering. - : Elsevier B.V.. - 0925-8574 .- 1872-6992. ; 201
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 16. |
- Riml, Joakim, 1979-
(författare)
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Solute Transport Across Scales : Time Series Analyses of Water Quality Responses to Quantify Retention and Attenuation Mechanisms in Watersheds
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The intra-continental movement of waterborne contaminants is governed by the distribution of solute load in the landscape along with the characteristics and distribution of the hydrological pathways that transport the solutes. An understanding of the processes affecting the transport and fate of the contaminants is crucial for assessments of solute concentrations and their environmental effect on downstream recipients. Elevated concentration of nutrients and the presence of anthropogenic substances, such as pharmaceutical residues, are two examples of the current problems related to hydrological transport. The overall objective of this thesis is to increase the mechanistic understanding of the governing hydrological transport processes and their links to geomorphological and biogeochemical retention and attenuation processes. Specifically, this study aims to quantify the processes governing the transport and fate of waterborne contaminants on the point, stream reach, and watershed scales by evaluating time series obtained from stream tracer tests and water quality monitoring data. The process quantification was achieved by deriving formal expressions for the key transport characteristics, such as the central temporal moments of a unit solute response function and the spectral scaling function for time series of solute responses, which attributes the solute response in the Laplace and Fourier domains to the governing processes and spatial regions within the watershed. The results demonstrate that in addition to the hydrological and biogeochemical processes, the distribution of the load in the landscape and the geomorphological properties in terms of the distribution of transport pathway distances have defined effects on the solute response. Furthermore, the spatial variability between and along the transport pathways significantly affect the solute response. The results indicate that environments with high retention and attenuation intensity, such as stream-reaches with pronounced hyporheic zones, may often dominate the solute flux in the watershed effluent, especially for reactive solutes. The mechanistic-based framework along with the evaluation methodologies presented within this study describes how the results can be generalized in terms of model parameters that reflect the hydrology, geomorphology and biogeochemistry in the studied area. This procedure is demonstrated by the parameterization of a compartment-in-series model for phosphorous transport.
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| 17. |
- Riml, Joakim, 1979-, et al.
(författare)
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Spatiotemporal decomposition of solute dispersion in watersheds
- 2015
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 51:4, s. 2377-2392
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Tidskriftsartikel (refereegranskat)abstract
- Information about the effect of different dispersion mechanisms on the solute response in watersheds is crucial for understanding the temporal dynamics of many water quality problems. However, to quantify these processes from stream water quality time series may be difficult because the governing mechanisms responsible for the concentration fluctuations span a wide range of temporal and spatial scales. In an attempt to address the quantification problem, we propose a novel methodology that includes a spectral decomposition of the watershed solute response using a distributed solute transport model for the network of transport pathways in surface and subsurface water. Closed form solutions of the transport problem in both the Laplace and Fourier domains are used to derive formal expressions of (i) the central temporal moments of a solute pulse response and (ii) the power spectral response of a solute concentration time series. By evaluating high-frequency hydrochemical data from the Upper Hafren Watershed, Wales, we linked the watershed dispersion mechanisms to the damping of the concentration fluctuations in different frequency intervals reflecting various environments responsible for the damping. The evaluation of the frequency-dependent model parameters indicate that the contribution of the different environments to the concentration fluctuations at the watershed effluent varies with period. For the longest periods (predominantly groundwater transport pathways) we found that the frequency typical transport time of chloride was 100 times longer and that sodium had a 2.5 times greater retardation factor compared with the shortest periods (predominantly shallow groundwater and surface water transport pathways).
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| 18. |
- Riml, Joakim, 1979-, et al.
(författare)
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Spectral Decomposition Reveals New Perspectives on CO2 Concentration Patterns and Soil-Stream Linkages
- 2019
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Ingår i: Journal of Geophysical Research - Biogeosciences. - : American Geophysical Union (AGU). - 2169-8953 .- 2169-8961. ; 124:10, s. 3039-3056
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Tidskriftsartikel (refereegranskat)abstract
- The rapid development of novel technologies to obtain high-frequency observations has provided new possibilities to observe and understand carbon cycling in inland waters. This study investigates carbon dioxide (CO2) dynamics along a boreal soil-stream transect using a state-of-the-art data set in combination with a spectral methodology to identify controls on stream CO2. The spectral decomposition of hourly observations revealed intermediate (multiday) to long-term (monthly) patterns across the upslope-riparian-stream continuum, with similar power law increases in CO2 concentration fluctuations with increasing period. High-frequency CO2 variabilities, specifically diel CO2 concentration fluctuations, were also identified at all locations but were substantially amplified in the stream compared to in the riparian groundwater. Moreover, the spectral coherence between soil and stream CO2 fluctuations was inconsistent and restricted to episodic events. In contrast, we found a strong and consistent spectral coherence between the riparian groundwater level and stream CO2 concentration, indicating a hydrological control on stream CO2 dynamics. However, during some time spans even these patterns were obscured, suggesting that additional processes, such as CO2 evasion and in-stream metabolism, modulated the influence from riparian sources. The scales and patterns of temporal coherences (or lack thereof) between CO2 at different points in the catchment, as well as with other factors, for example, groundwater levels and Photosynthetically Active Radiation, provide new perspectives on the range of processes governing stream CO2 dynamics. Thus, this study highlights the potential of using spectral decomposition of high-resolution, spatially distributed data of different types to investigate biogeochemical transformations and pathways linking terrestrial and aquatic systems.
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| 19. |
- Torabi Haghighi, A., et al.
(författare)
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A power market-based operation support model for sub-daily hydropower regulation practices
- 2019
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Ingår i: Applied Energy. - : Elsevier Ltd. - 0306-2619 .- 1872-9118. ; 255
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Tidskriftsartikel (refereegranskat)abstract
- With increasing power production from renewable energy sources, sub-daily variations in energy demand need to be balanced. Today, hydropower is commonly used as balancing power. In this study, we quantified the impact of capacity constraints, in terms of reservoir volume and hydropower capacity, on the potential to comply with instant energy demand. To evaluate the impact, we developed two new metrics, power market impact and system efficiency ratio, which are based on two threshold flow regimes derived from natural flow as lower threshold release and regulated flow (based on hourly energy prices) as upper threshold release. The operation support model comprises 96 different regulation scenarios based on varying combinations of hydropower and reservoir capacities. For each scenario, an hourly water balance was simulated, to obtain the highest complying with upper threshold release based on actual energy demand. We tested the framework on the Kemijoki river with defined thresholds based on the natural flow regime (tributary river Ounasjoki) and the hourly energy price in Finland in 2017, and estimated the impact of regulation on hourly flow regime at the Taivalkoski hydropower station. The annual flow regime impact in 2013, 2014 and 2015 was estimated to be 74%, 84% and 61%, respectively, while the monthly impact varied from 27% to 100%. Our framework for evaluating interactions between the power market and sub-daily regulation practices is a useful novel tool for sustainable river management and can be easily applied to different rivers and regions and evaluated for different timescales.
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| 20. |
- Wörman, Anders, et al.
(författare)
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Convergence of groundwater discharge through the hyporheic zone of streams
- 2023
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Ingår i: Ground Water. - : Wiley. - 0017-467X .- 1745-6584. ; 61:1, s. 66-85
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 21. |
- Wörman, Anders, et al.
(författare)
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Fragmentation of the Hyporheic Zone Due to Regional Groundwater Circulation
- 2019
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 55:2, s. 1-21
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Tidskriftsartikel (refereegranskat)abstract
- By use of numerical modeling and field observations, this work quantified the effects of catchment-scale upwelling groundwater on the hyporheic (below stream) fluxes over a wide range of spatial scales. A groundwater flow model was developed that specifically accounted for the hydrostatic and dynamic head fluctuations induced by the streambed topography. Although the magnitudes and relative importance of these streambed-induced fluxes were found to be highly sensitive to site-specific hydromorphological properties, we showed that streambed topographic structures exert a predominant control on the magnitude of hyporheic exchange fluxes in a Swedish boreal catchment. The magnitude of the exchange intensity evaluated at the streambed interface was found to be dominated by the streambed-induced hydraulic head across stream order. However, the catchment-scale groundwater flow field substantially affected the distribution of groundwater discharge points and thus decreased the fragmentation of the hyporheic zone, specifically by shifting the cumulative density function toward larger areas of coherent upwelling at the streambed interface. This work highlights the spectrum of spatial scales affecting the surface water-groundwater exchange patterns and resolves the roles of key mechanisms in controlling the fragmentation of the hyporheic zone.
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| 22. |
- Wörman, Anders, et al.
(författare)
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Parameterizing water fluxes in the geosphere-biosphere interface zone : For use in biosphere modelling as part of the long-term safety assessment
- 2019
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Ingår i: International High-Level Radioactive Waste Management 2019, IHLRWM 2019. - : American Nuclear Society. ; , s. 554-558
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Konferensbidrag (refereegranskat)abstract
- The post-closure safety assessment of the disposal system for high-level nuclear waste should reflect a sufficient understanding of the natural environment surrounding the engineered barriers and future effects of releases of contaminants on human health and the environment. A critical part of radiological models for the biosphere is due to the transport of deep groundwater and subsequent mixing with surface waters when it emerges instreams and wetlands. These processes are highly influenced by the so-called hyporheic flow; the small-scale flow field in the sediments below the stream caused by local pressure gradients at the streambed interface, which contributes to a fragmentation of the upwelling groundwater. The present work addresses the effects of catchment-scale upwelling groundwater on the hyporheic fluxes over a wide range of spatial scales in boreal, glacial landscapes. In such landscapes, the groundwater surface generally follows the topography and soil layers are relatively thin. A model framework was developed to account for both the effects of the regional (groundwater) and locally (streambed) induced flow fields on the hyporheic exchange, specifically accounting for the hydrostatic and dynamic head fluctuations induced by the streambed topography. We show that the hyporheic flow field substantially affected the distribution of deep groundwater discharge points in streams and thus increased the fragmentation of the upwelling, here defined as the size of coherent up- or downwelling areas at the streambed interface. Due to the inverse relationship between the rate coefficient and the groundwater discharge areas, this fragmentation implies a considerable increase in both the flow velocities and rate coefficients through the upper part of the GBI, which are crucial components in dose assessments. Furthermore, the groundwater-surface water interactions were studied in several sub-watersheds to provide a basis for understanding the importance of different topographic and geographic factors and for statistical derivation of general relationships for mass transfer rate coefficients to be used to parameterize water fluxes in radiological dose models for the biosphere. It was found that site specific conditions were difficult to generalize in the form of proxy factors with high confidence, but the most essential factors were found to be stream-order, landscape slope, thickness of Quaternary deposits and indices representing the fractal nature of the landscape topography.
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| 23. |
- Wörman, Anders, et al.
(författare)
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Spectral decomposition of regulatory thresholds for climate-driven fluctuations in hydro- and wind power availability
- 2017
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Ingår i: Water resources research. - : AMER GEOPHYSICAL UNION. - 0043-1397 .- 1944-7973. ; 53:8, s. 7296-7315
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Tidskriftsartikel (refereegranskat)abstract
- Climate-driven fluctuations in the runoff and potential energy of surface water are generally large in comparison to the capacity of hydropower regulation, particularly when hydropower is used to balance the electricity production from covarying renewable energy sources such as wind power. To define the bounds of reservoir storage capacity, we introduce a dedicated reservoir volume that aggregates the storage capacity of several reservoirs to handle runoff from specific watersheds. We show how the storage bounds can be related to a spectrum of the climate-driven modes of variability in water availability and to the covariation between water and wind availability. A regional case study of the entire hydropower system in Sweden indicates that the longest regulation period possible to consider spans from a few days of individual subwatersheds up to several years, with an average limit of a couple of months. Watershed damping of the runoff substantially increases the longest considered regulation period and capacity. The high covariance found between the potential energy of the surface water and wind energy significantly reduces the longest considered regulation period when hydropower is used to balance the fluctuating wind power. Plain Language Summary The availability of renewable energy fluctuates significantly with climate and needs to be regulated to be sufficient at all times. This regulation can be achieved by storing hydropower in water reservoirs, but is complicated by the vast spatial distribution of storage locations, size variations in reservoirs, the covariation of renewable energy, and the range of frequencies that need to be considered in climate variations. This study provides a new method of analysis that can provide estimates of the most effective use of hydropower reservoirs and the limits of their use for regulating renewable energy. Based on data from entire Sweden we show how the storage bounds can be related to a spectrum of the climate-driven modes of variability in water availability and to the covariation between water and wind availability. The high covariance found between the potential energy of the surface water and wind energy significantly reduces the longest considered regulation period when hydropower is used to balance the fluctuating wind power.
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| 24. |
- Wörman, Anders, et al.
(författare)
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The power of runoff
- 2017
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Ingår i: Journal of Hydrology. - : ELSEVIER SCIENCE BV. - 0022-1694 .- 1879-2707. ; 548, s. 784-793
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Tidskriftsartikel (refereegranskat)abstract
- Although the potential energy of surface water is a small part of Earth's energy budget, this highly variable physical property is a key component in the terrestrial hydrologic cycle empowering geomorphological and hydrological processes throughout the hydrosphere. By downscaling of the daily hydrometeorological data acquired in Sweden over the last half-century this study quantifies the spatial and temporal distribution of the dominating energy components in terrestrial hydrology, including the frictional resistance in surface water and groundwater as well as hydropower. The energy consumed in groundwater circulation was found to be 34.6 TWh/fy or a heat production of approximately 13% of the geothermal heat flux. Significant climate driven, periodic fluctuations in the power of runoff, stream flows and groundwater circulation were revealed that have not previously been documented. We found that the runoff power ranged from 173 to 260 TWh/y even when averaged over the entire surface of Sweden in a five-year moving window. We separated short-term fluctuations in runoff due to precipitation filtered through the watershed from longer-term seasonal and climate driven modes. Strong climate driven correlations between the power of runoff and climate indices, wind and solar intensity were found over periods of 3.6 and 8 years. The high covariance that we found between the potential energy of surface water and wind energy implies significant challenges for the combination of these renewable energy sources.
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| 25. |
- Wörman, Anders, et al.
(författare)
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Virtual energy storage gain resulting from the spatio-temporal coordination of hydropower over Europe
- 2020
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Ingår i: Applied Energy. - : Elsevier. - 0306-2619 .- 1872-9118. ; 272
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Tidskriftsartikel (refereegranskat)abstract
- The viability of a renewable electricity system depends on a relatively small share of hydropower storage resources to regulate climate variations and the spatially uneven distribution of renewable energy. By spatio-temporal coordination of hydropower production over larger regions, the energy storage demand will be reduced and contribute to a "virtual" energy storage gain that in Europe was found to be almost twice the actual energy storage capacity of hydropower reservoirs. In an attempt to quantify this gain, hydropower availability was simulated for most parts of the European continent for a 35-year period based on historical hydrometeorological data. The most significant benefits from spatio-temporal management arise at distances between 1200 and 3000 km, i.e., on the continental scale, which can have implications for a future renewable energy system at large. Furthermore, we discuss a condition termed "energy-domain-specific drought", which is a risk that can be reduced by the spatio-temporal management of power production. Virtual energy storage gain is not explicitly considered in the management models of hydropower production systems but could in principle complement existing management incentives.
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| 26. |
- Xu, Shulan, et al.
(författare)
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Development of a model for radionuclide transport in streams for biosphere assessment purpose
- 2023
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Ingår i: Journal of Environmental Radioactivity. - : Elsevier BV. - 0265-931X .- 1879-1700. ; 264
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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