Environmental-flow assessments for current and future run-off in a large river system regulated for hydropower production

Sammanfattning: In 2019, Sweden implemented legislative changes to renegotiate hydropower permits to both consider environmental rehabilitation and to ensure national supply of hydropower. This means that efforts for environmental rehabilitation of the 2,000 hydropower plants in Sweden need to be considered. Such rehabilitation measures include implementation of environmental flows, enhancing connectivity or morphological restoration. In order to enable prioritization among measures, it is necessary to assess the expected environmental benefits and consequences of implementation. We developed a new method to assess and prioritize among environmental-flow measures that aim to rehabilitate ecosystems in regulated rivers at the catchment level, with the Ume River in northern Sweden as an example. The Ume River is heavily regulated for hydropower production with 19 hydropower stations, with 13 run-of river impoundments in cascade and six storage reservoirs. Our strategy was to identify measures with minimal impact on hydropower production that also provide significant environmental benefits. Based on field studies of remaining natural values and potential for ecological rehabilitation, we quantified the estimated gain in the area of habitat for target organism groups, e.g. lotic fish species and riparian plants, if rehabilitation actions would be implemented along the entire Ume River. Regulated flows imply changes in the seasonal variation in flow, which often means that spring floods are lacking and that flows increase during the winter compared with natural unregulated flows. Hydropeaking, defined as rapid and frequent changes in flow and water levels to optimize hydropower production, is a common procedure that adversely affects habitats in river ecosystems. An important aspect of hydropeaking is zero-flow events, which occurs when hydropower stations are stopped due to low electricity demand or low electricity prices. We quantified the consequences for hydropower production of introducing environmental flows by identifying a set of rules of operation of the hydropower stations that reflect the limitations that ecological regulation of flows and water levels entail. In the work, consideration of technical limitations in the hydropower stations was a key to attain cost-effective measures. We then used hydropower production optimization programs to calculate changes in hydropower production and revenues. We also quantified the environmental benefits of environmental flows described as increases in the area of habitat for riverine species and improvements in ecosystem functions in the Ume River. We identified increasing the area of aquatic habitat with high flow velocity, providing suitable habitat for lotic species, enhancing the establishment of riparian and increasing longitudinal connectivity as the main aspects of the Ume River ecosystems in need of rehabilitation.This thesis focuses specifically on three aspects of environmental flows. (1) Analysis of hydropeaking and zero-flow events for all hydropower stations in a catchment and the introduction of a ban of zero-flow events as an environmental flow measure. The hydropower stations in Ume River system stand still without flow 9% to 55% of the time in a hydrologically normal year, transforming lotic habitat into stagnant water. (2) A comprehensive assessment of environmental flow measures which in addition to banning zero-flow events include improvements of connectivity, spill water to by-passed reached laid dry as well as more natural water-level variation, combined into a total of scenarios where the environmental benefits and impacts on electricity production were quantified. In addition, we modeled a spring-flood scenario and a scenario transforming the flow of the Ume River to its natural flow regime. (3) Predictions of the effects on hydropower production of introducing environmental flow scenarios were modeled using climate change projections for IPCC scenario A1B until the year 2040, where the efficiency of environmental flow measures in a future climate and detection of potential bottlenecks in flow linked to ecological extremes such as periods of drought and flood. Further, the thesis present the framework of collaborative management that facilitated the process to solve complicated societal challenges connected to mitigation measures and environmental flows in higly-regulated river basins. This framwork allowed both for finding the most cost effective environmental flow measures as well as detecting environmental rehabilitation measures that otherwise might go undetected, despite having little impact on hydropower production. Our results show that introducing a zero-flow ban with the aim of avoiding stagnant water would on average mean 0.5% electricity production loss per year and benefit existing and newly created 240 hectares of lotic habitat with a flow rate exceeding 0.1 m/s, suitable for lotic species such as grayling Thymallus thymallus. The small effect in electricity production is the result of an effort to route the flow through the turbines to generate electricity, which means that the main effect is to move electricity production from daytime to nighttime. Implementation of zero-flow restrictions in combination with allocating 1-12% of the average annual flow at all hydropower stations to side channels and reaches laid dry would result in a loss of 2.1% of the annual electricity production for the Ume River catchment. Adding flow to fish-ways would increase the loss to 3.1% per year. With the implementation of more natural water-level variation in the main channel, the loss increases to 3.8%. These measures would more than triple the habitat of lotic species such as grayling Thymallus thymallus, and increase the area of riparian vegetation by about 66%.Assessing hydropower production in the Ume River in a future climate shows that hydropower production is expected to increase by 2.6% compared to current conditions until 2040, which opens up for mitigating the effects of climate change by implementing flow measures that mimic conditions before climate change, which can help to avoid extreme hydrological events potentially harming the riverine ecosystem. The environmental flow scenarios developed in previous projects were tested in simulations with future flow conditions and the results show that all effects on electricity production were projected to be significantly smaller in the future compared with models without climate change. The operation of storage reservoirs is expected to become more important in a future climate. Our assessment is a way of predicting the effectiveness of environmental flow measures in the future with climate change.Our method forms the basis to guide future nationwide implementation of environmental rehabilitation of regulated rivers with the aim of maintaining and restoring riverine ecosystems.