Encinas Fernández, Jorge
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Interdisciplinary Reservoir Management : a Tool for Sustainable Water Resources Management
Reservoirs are a common way to store and retain water serving for a multitude of purposes like storage of drinking and irrigation water, recreation, flood protection, navigation, and hydropower production, and have been built since centuries. Today, few reservoirs serve only one purpose, which requires management of present demands and interests. Since each reservoir project will cause negative impacts alongside desired advantages both on a local, regional and global scale, it is even more urgent to develop a common management framework in an attempt to mitigate negative impacts, incorporate different demands and make them visible within the discourse in order to avoid conflicts from early on. The scientific publications on reservoirs are manifold, yet a comprehensive and integrative holistic tool about management of this infrastructure is not available. Therefore, a comprehensive and integrated conceptual tool was developed and proposed by the authors of this paper that can contribute to the sustainable management of existing reservoirs. The tool presented herein is based on the results from the interdisciplinary CHARM (CHAllenges of Reservoir Management) project as well as the condensed outcome of relevant literature to aid and enhance knowledge of reservoir management. The incorporated results are based on field, laboratory and empirical social research. The project CHARM focused on five different aspects related to existing reservoirs in southern Germany (Schwarzenbachtalsperre, Franconian Lake District), namely: sedimentation of reservoirs, biostabilisation of fine sediments, toxic cyanobacteria(l) (blooms), greenhouse gas emissions from reservoirs and social contestation, respectively consent. These five research foci contributed to the topics and setup of a conceptual tool, put together by the research consortium via delphi questioning, which can be found alongside this publication to provide insights for experts and laymen. Conceptualising and analysing the management in combination with quantitative and qualitative data in one descriptive tool presents a novelty for the case studies and area of research. The distribution within the scientific community and interested public will possibly make a positive contribution to the goal of sustainable water resources management in the future.
Sediment fluxes rather than oxic methanogenesis explain diffusive CH4 emissions from lakes and reservoirs
Methane emissions from lakes and reservoirs are a major natural source in the global budget of atmospheric CH4. A large fraction of these emissions are due to diffusive transport of CH4 from surface waters to the atmosphere. It was suggested recently that CH4 production in the oxic surface waters is required to compensate for diffusive CH4 emissions from lakes. In contrast, we demonstrate here that typical diffusive CH4-fluxes from sediments in shallow water zones, Fsed,S, suffice to explain CH4 emissions to the atmosphere. Our analysis is based on the combination of an exceptional data set on surface concentrations of CH4 with a mass balance model of CH4 that is focused on the surface mixed layer and considers CH4-fluxes from sediments, lateral transport, gas exchange with the atmosphere, and includes temperature dependencies of sediment fluxes and gas exchange. Fsed,S not only explains observed surface CH4 concentrations but also concentration differences between shallow and open water zones, and the seasonal variability of emissions and lateral concentration distributions. Hence, our results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH4 in the surface waters and the CH4 emitted from lakes and reservoirs.
Lake Metabolism : Comparison of Lake Metabolic Rates Estimated from a Diel CO2- and the Common Diel O2-Technique
Lake metabolism is a key factor for the understanding of turnover of energy and of organic and inorganic matter in lake ecosystems. Long-term time series on metabolic rates are commonly estimated from diel changes in dissolved oxygen. Here we present long-term data on metabolic rates based on diel changes in total dissolved inorganic carbon (DIC) utilizing an open-water diel CO2-technique. Metabolic rates estimated with this technique and the traditional diel O2-technique agree well in alkaline Lake Illmensee (pH of ~8.5), although the diel changes in molar CO2 concentrations are much smaller than those of the molar O2 concentrations. The open-water diel CO2- and diel O2-techniques provide independent measures of lake metabolic rates that differ in their sensitivity to transport processes. Hence, the combination of both techniques can help to constrain uncertainties arising from assumptions on vertical fluxes due to gas exchange and turbulent diffusion. This is particularly important for estimates of lake respiration rates because these are much more sensitive to assumptions on gradients in vertical fluxes of O2 or DIC than estimates of lake gross primary production. Our data suggest that it can be advantageous to estimate respiration rates assuming negligible gradients in vertical fluxes rather than including gas exchange with the atmosphere but neglecting vertical mixing in the water column. During two months in summer the average lake net production was close to zero suggesting at most slightly autotrophic conditions. However, the lake emitted O2 and CO2 during the entire time period suggesting that O2 and CO2 emissions from lakes can be decoupled from the metabolism in the near surface layer.
Diurnal pumped-storage operation minimizes methane ebullition fluxes from hydropower reservoirs
Hydropower is considered green energy and promoted to reduce greenhouse warming. However, hydropower is typically generated using reservoirs and reservoirs are known to emit substantial amounts of the greenhouse gas methane (CH4) to the atmosphere. In many reservoirs ebullition is the dominant pathway of CH4 emission. We show that continuous diurnal pumped‐storage operation, which combines water pumping into the reservoir typically during the night and water drawdown during high demand of electricity, is beneficial for reducing CH4 ebullition associated with hydropower generation. This conclusion is based on ebullition fluxes and water levels measured over 3 months in Schwarzenbach reservoir located in Germany. The reservoir was managed using three modes of operation: (1) diurnal pumping and turbination, (2) no pumping and no turbination, and (3) diurnal turbination. Cross‐correlation analysis indicates that ebullition fluxes predominantly occur during diurnal water level decrease associated with turbination. Consistently, average ebullition fluxes of CH4 were negligible during Mode (2) and substantial during Modes (1) and (3). During Mode (3) the average CH4 ebullition flux was ~197 mg m−2day−1, ~12 times larger than during Mode (1) (16 mg m−2day−1). Our data indicate that overall CH4 ebullition is about 3 times larger during 51 days of operation consisting of 38 days of no turbination followed by 13 days of diurnal turbination than during 51 days of continuous diurnal pumped‐storage operation. This suggests that continuous diurnal pumped‐storage operation leads to reduced CH4 ebullition from reservoirs and is therefore advantageous compared to modes of operations involving long‐term, large‐amplitude turbination cycles.
Export of Dissolved Methane and Carbon Dioxide with Effluents from Municipal Wastewater Treatment Plants
Inland waters play an important role for regional and global scale carbon cycling and are significant sources of the atmospheric greenhouse gases methane (CH4) and carbon dioxide (CO2). Although most studies considered the input of terrestrially derived organic and inorganic carbon as the main sources for these emissions, anthropogenic sources have rarely been investigated. Municipal wastewater treatment plants (WWTPs) could be additional sources of carbon by discharging the treated wastewater into the surrounding aquatic ecosystems. Here we analyze seasonally resolved measurements of dissolved CH4 and CO2 concentrations in effluents and receiving streams at nine WWTPs in Germany. We found that effluent addition significantly altered the physicochemical properties of the streamwater. Downstream of the WWTPs, the concentrations of dissolved CH4 and CO2 were enhanced and the atmospheric fluxes of both gases increased by a factor of 1.2 and 8.6, respectively. The CH4 exported with discharged effluent, however, accounted for only a negligible fraction (0.02%) of the estimated total CH4 emissions during the treatment process. The CH4 concentration in the effluent water was linearly related to the organic load of the wastewater, which can provide an empirical basis for future attempts to add WWTPs inputs to regional-scale models for inland water-carbon fluxes.