Edited by: José Manuel Mirás-Avalos, Universidade de Santiago de Compostela, Spain
Reviewed by: Fei WANG, Institute of Soil and Water Conservation (CAS), China; Aitor García Tomillo, University of A Coruña, Spain
This article was submitted to Agroecology and Ecosystem Services, a section of the journal Frontiers in Environmental Science
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Global change is affecting agroforestry and its inherent ecosystems in Sweden. Here we examine the benefits of ecologically adjusted dam regulations to conserve biodiversity under climate change in floodplain habitats, including meadows and riparian mixed forests. The natural flood regime in snow-dominated regions has changed significantly during the last decades, in line with the projections for climate change. The ecosystems of temporary flooded forests show high biodiversity but are dependent on river high flows with long duration. These events are rare in the new climate scenario, but on the other hand, snow-fed rivers are also affected by hydropower dams and regulations. In this study we explored the potential of using reservoir regulation to artificially induce flood events; water management would then be a method to conserve biodiversity in forest habitats and adapt management to climate change. We made detailed calculations in lower Dalälven River, central Sweden, using observed time-series of river flow and dynamic scenario modeling for highly valuable Natura 2000 habitats. Here we show that long-term flooding is less frequent since extensive hydropower was introduced during the 1920s, and moreover, since the 1990's the spring floods are low due to low snow storage and short winter seasons. Sustainable management of 50% of the riparian forest requires flooding by 25 continuous days of 800 m3 s−1. We found that artificial floods using new ecological regulation regime of upstream hydropower reservoirs would help, but not be enough, to achieve this goal. The new regulation routines would correspond to a loss of 50-200 GWh in hydropower production for each artificial flood. Sustainable ecosystems in the study site do not request flooding every year, but some every fifth year. For practical implementation, the County Board is currently driving the process locally and we discuss the relevant social features, such as legal and funding aspects, of this adaptive management of water and forests. A smaller part of the forest could probably be rescued and costs could potentially be lowered by using only the most snow rich years and seasonal forecasting of river flow for optimal timing of water release from dams to induce flooding.
Currently, productive forests account for 57% of the Swedish land cover and they have been constantly expanding throughout the 20th century (KSLA,
The flood regime in snow-dominated regions has changed significantly during the last decades, which is in line with the projections for climate change impact but also an effect of extensive flow regulations (Arheimer et al.,
Arheimer et al. (
We here describe such a case study of central Sweden, where detailed calculations were made to: (i) analyse the reasons behind the reduced flooding that threatens a rare forest habitat, and (ii) explore the potential of using changed regulation strategy at upstream hydropower dams to induce river flooding for sustainable management. We found that reduction of snow storage was the main reason behind the loss of peak-flows during recent decades, and that changed regulation of hydropower dams could not save 50% of the habitats of the threatened temporarily flooded forest. We show that the regulations could help in climate adaptation, but there may be high costs for energy loss and melt-water must still be available in sufficient amounts from snow storage. Overall, the study highlights the importance of revising management protocols under non-stationary conditions due to global warming.
The floodplain forests of lower Dalälven River in central Sweden, receive water from a 29,000 km2 watershed starting in the mountains of Norway to the west, from which the river flows to the east and ends in the Baltic Sea (Figure
Map showing the catchment borders (red line) of Dalälven River (blue line) in Northern Europe, and the location of the hydropower plant “Näs” (black dot) upstream the regularly flooded residua forest of high biodiversity (checkered).
Several climate change impact studies have encompassed river flow in Dalälven River (e.g., Andréasson et al.,
The natural river regime with annual floods due to snowmelt during the spring has created a very unique zonation of ecosystems at various altitudes of the floodplains, depending on frequency and duration in flooding. The region is recognized as having the highest biodiversity in Sweden, with several valuable Natura 2000 habitats identified along the river floodplains. This is a unique service provided by the river from its original flow dynamics. The most vulnerable ecosystem is the regularly flooded riparian mixed forests, which requests flood duration of 25 continuous days to initiate the ecological processes and serve as habitat for the specific species living and growing there (Hedström-Ringvall et al.,
Differentiation of floodplain habitats was based on different elevation zones, using a topographic GIS analysis (Zinke,
Analysis of floodplain habitat elevation zones in relation to the observed and modeled stage-discharge rating curve for Färnebofjärden in Lower Dalälven. In order to flood riparian mixed forest habitats (Natura 2000 habitat code 91F0) a river discharge exceeding 800 m3s−1 is required.
Observed time-series of daily river flow were collected from the discharge stations at Långhag/Fäggeby (since 1851) and at Näs (since 1961). The stations are situated not far from each other in the main river channel close to the downstream floodplains. The observed data was used to analyse the long-term changes in river flow, which could be due to either climate variability/change or flow regulation in hydropower dams. Observed time-series from the hydropower stations Trängslet and Gråda (Lake Siljan) were used as a base-line when exploring effects from changed regulation routines to induce flooding for specific years (see below).
Dynamic modeling was performed using the numerical Hydrological Predictions for the Environment (HYPE) model (Lindström et al.,
The S-HYPE model was used to simulate the changes in snow storage between 1961 and 2015, to evaluate the impact from climate on observed changes in river regime. The S-HYPE model was also used to reconstruct the natural river flow, as it would have been without the influence of hydropower regulations, using the method described by Arheimer and Lindström (
To investigate future climate impact on river flow, the S-HYPE model was used for climate change projections. It was then fed with time-series from the Coupled Model Intercomparison Project Phase 5 (CMIP5) from the Intergovernmental Panel on Climate Change (IPCC), using projections for two different assumptions on societal development and emission scenarios (Representative Concentration Pathways (RCPs) 4.5 and 8.5, respectively). Data was extracted from the Regional Climate model RCA (Samuelsson et al.,
To explore the effects from inducing floods by changed regulation strategies, four alternative scenarios were constructed for the spring flood of recently wet years (1999, 2006, 2010, 2015):
- Business as usual, using daily corrected observations from monitoring stations by the Swedish Meteorological and Hydrological Institute (SMHI). - Scenario 1, reduced regulation from both Trängslet and Siljan hydropower dams. - Scenario 2, reduced regulation only from Lake Siljan hydropower dam (Gråda). - Natural flow calculated with the S-HYPE model as described by Arheimer and Lindström (
The calculations at both Trängslet and Siljan hydropower dams were made by the hydropower companies (Hedström-Ringvall et al.,
Long-term flooding is less frequent since extensive hydropower was introduced during the period 1920–1960 and the spring peak is less pronounced. Time-series of >50 years of observations both before and after building hydropower dams, show that the average in high water inflow causing flooding of the vulnerable forest habitat has dropped from 900 to 500 m3s−1 due to regulation (Figure
Average water discharge at Näs, using scaled observation from the nearby river gauge at Fäggeby for the period before constructing hydropower dams (purple) and observations at Näs after regulations (black).
Annual precipitation over the watershed upstream Näs hydropower station varies from 470 to 860 mm year−1, but there is a slight but significant increase in average annual precipitation over the last 55 years (Figure
Annual precipitation in Dalälven watershed upstream of Näs with fractions falling as rain and snow (
The annual maximum snow storage is a relatively good indicator of the spring flood volume, and therefore also flood duration (Figure
There is a temporal variability in calculated return periods but more so for floods with long duration (Figure
Temporal variability in the return period of daily discharge above 800 m3s−1 (blue) and daily discharge above 800 m3s−1 for at least 25 consecutive days in April-June (black).
Projections for Dalälven River suggest that climate change upstream the temporary flooded riparian mixed forest has less impact on the seasonal distribution of flow than current hydropower regulation (Figure
Impact on river flow at Näs by hydropower regulation and climate change, respectively. Regulated flow is observed river flow, while naturalized flow is modeled using the observed climate (green) and the climate model data of current climate (blue) and by the end of the century (red).
Although the changed strategies for regulating river flow helped to flood the riparian mixed forest of the floodplain, it would not have been possible to obtain 25 consecutive days with the environmental flow of > 800 m3 s−1 at Näs during the wet years studied using any of the alternative scenarios (Table
Number of days and production losses for different scenarios of artificially induced floods (modified from Hedström-Ringvall et al.,
B.a.U. |
0 | 0 | 7 | 4 | NA | NA | NA | NA |
Alt. No 1 | 14 | 6 | 15 | 12 | 190 | 174 | 200 | 215 |
Alt. No 2 | 14 | 5 | 11 | 7 | 54 | 47 | 35 | 54 |
Natural |
19 | 16 | 26 | 15 | NA | NA | NA | NA |
This detailed site-specific investigation for adapting the vulnerable riparian mixed forest at the floodplains of Dalälven River to climate change conditions, shows that induced floods by changed hydropower regulation will not help saving 50% of the habitats. The environmental goals must thus be revised to be realistic under climate change, as the snow storage will most likely be further reduced in the future. Sustainable management of the study site does not request flooding every year, but some every fifth year (Hedström-Ringvall et al.,
The County Board is currently driving the process locally and will proceed by establishing a working group for the next 5 years to further analyze effects and potential of changed regulations. The environmental flows will be reconsidered regarding area to be flooded, to also investigate the possibilities for sustainable management of smaller areas, which requests lower river flow. Besides from the flooded riparian mixed forest there are also flooded meadows of concern that request lower flow volumes than the forest to become sustainable under climate change. Both environmental goals will be negotiated and optimized against loss in energy production in new scenarios by the working group, to estimate the most cost effective climate adaptation for floodplains in Dalälven River. In addition, a committee for adaptive management will be established to elaborate operational decision-making of artificial flooding, taking fictive decisions from various sources of support material during spring each of the 5 years.
Finally, it should be mentioned that the hydropower companies and the engineering consultants that have been involved in the calculations for each hydropower dam are more reluctant to changes in regulation strategies. The calculations for each dam were based on statistics and observations, but in reality it would be very difficult to forecast exactly when the flow peaks will reach the riparian mixed forest. There is a high risk that the gates are opened too early or too late, which significantly would affect the result. It is thus difficult to match the flood peak from artificial flooding with the natural flooding from unregulated areas, while the joint effect is needed. The dam operators also see the difficulties in spring-flood forecasting and claim that the methods available are still too poor to be used for decision-making. They also see security risks, as when the discharge from Lake Siljan once has started it will be difficult to stop, due to the naturally inherent slowness of the system, and intense rains may challenge the upper limit of the reservoir. The new regulation strategies must thus also be analyzed from a security perspective as the dam was never designed for this purpose and the legal agreements on volume fluctuations must be further validated.
Apart from the concerns about ecology, actual costs and security mentioned here, there are also other policy concerns with changing regulation strategy from hydropower dams. Hydropower is referred to as a clean and renewable energy source, which is favored over fossil fuels. Reservoir storage is often used to balance out fluctuations in other renewable power sources, such as wind and solar, which may become more important in future energy production. Hence, climate mitigation may request more hydropower in the future and more flexible regulation schemes also taking this aspect in concern. Water governance always require collaborations among multiple actors to ensure sustainability in various sectors (Falkenmark and Molden,
Our analysis show that annual maximum snow storage in Dalälven River decreases despite an overall slight increase in annual precipitation during the last 55 years, and that these changes can be attributed to climate change. During the same period, hydropower regulations have reduced the flow peaks from snow melting, which naturally should overflow the floodplains. Both changes will affect forest habitats.
Searching for sustainable agroforestry requires an analysis where hydrologists and ecologists work in close collaboration. In lower Dalälven River, riparian biodiversity relies on occasional spring floods with relatively long duration to “reset” habitats. Artificially induced flooding is one possible adaptation measure, although it implies significant costs in lost energy production and changes in both regulation strategies and river basin management plans.
Managing floodplain ecosystems under climate change is facilitated by hydrological modeling tools. In this study we demonstrate that reference conditions are not stationary under climate change, which prevents the use of historic measurements to define reference conditions and targets of river basin management. Rather, reference conditions must be dynamically modeled to be comparable to the present-day situation and for separating the anthropogenic pressures from natural variability.
BA outlined the manuscript, analyzed the results, and wrote the text. NH collaborated with local stakeholders and made statistical analysis. GL contributed with hydrological modeling, plots, and graphs.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We would like to thank all participants in the collaborative project Sustainable hydropower in Dalälven River initiated by the Swedish Authority of Marine and Water Management; especially Per-Erik Sandberg (County board of Dalarna), Joel Berglund (County board of Uppsala), Anna Hedström-Ringvall (Regulation company of Daläven River), Claes Kjörk and Kent Pettersson (Fortum), Magnus Engström and Dag Wisaeus (ÅF/Vattenfall).