Elisa Calamita

Last Name

Calamita

First Name

Elisa

ORCID

0000-0002-2614-2942

Organisational unit

01859 - Lehre Geistes-, Sozial- und Staatswiss.

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Publications 1 - 9 of 9

On the use of averaged indicators to assess lakes' thermal response to changes in climatic conditions
Toffolon, Marco; Piccolroaz, Sebastiano; Calamita, Elisa (2020)
Environmental Research Letters
Studies on the impact of climate change in lakes have mainly focused on the average response of lake surface temperature during three summer months (July, August, September, usually termed JAS). Focusing on the Laurentian Great Lakes, we challenge this common assumption by showing that the thermal behaviour is diversified in time both among different lakes and within a single one. Deep regions experience a stronger warming concentrated in early summer, mainly due to anticipated stratification, while shallow parts respond more uniformly throughout the year. To perform such analysis, we use the difference between the five warmest and coldest years in a series of 20 years as a proxy of possible effects of climate alterations, and compare the warming of lake surface temperature with that of air temperature. In this way, based on past observations obtained from satellite images, we show how the warming is heterogeneously distributed in time and in space, and that the quantification of lakes' thermal response to climate change is chiefly influenced by the time window used in the analysis. Should we be more careful when considering averaged indicators of lake thermal response to climate change?
Modelling the effects of large dams on water quality in tropical rivers
Calamita, Elisa (2020)
River damming is a common way to use river systems to generate hydroelectric power, provide water for irrigation and supply drinking water, and it has been practised for millennia. The number of completed dam projects peaked in North America and Western Europe in the 1960s and 1970s. In the last decades, instead, the hydropower industry moved to build dams in the global south in order to serve growing industries and urban populations. The ongoing growth of the hydropower sector at low latitudes calls for an examination of the political, socio-economic and environmental effects of tropical dams. Despite the many services provided by dams, they affect the river ecosystem in many different ways. Dams disrupt the continuum of rivers by altering their natural hydrological regimes and also create new lentic systems by increasing water residence time. This has cascading effects on the morphology, biogeochemistry and ecology of downstream river environments. Concerning biogeochemistry, dams interrupt the flow of organic carbon, change the nutrient balance and alter oxygen and thermal conditions. Thus, they alter river water quality. Large reservoirs are also potential hotspots for mineralization processes. Thus, reservoirs, especially in the tropics, may be responsible for substantial amounts of greenhouse gas emissions. In recent years, the scientific community started analysing the environmental impacts of large dams in a more holistic manner to better inform stakeholders and decision-makers to find a balance between tapping hydropower potential and sustaining key natural resources. This project investigates the effects of damming on water quality at low latitudes with a specific focus on three water quality parameters: water temperature, dissolved oxygen and carbon dioxide (CO2). Water temperature and dissolved oxygen are key parameters for the survival and reproduction of aquatic species. Water temperature alterations can affect community composition and even trigger the local extinction of species. Low oxygen concentrations alter lifecycle performance, growth capacity, reproductive success and disease vulnerability of fish, whilst hypoxia leads to higher fish mortality. Carbon dioxide, instead, is a greenhouse gas and most of the carbon mineralized in inland waters is released as CO2 to the atmosphere. Major uncertainties remain regarding the consequences of anthropogenic hydrological alterations, especially those stemming from large dams, on carbon emissions. This thesis considers the Zambezi River Basin (southeastern Africa) as a specific case study. The Zambezi River Basin is one of the most dammed African river basins, and many additional dams are already planned or under construction. Among others, the Zambezi River Basin hosts Kariba Dam, which forms the largest artificial lake in the world by volume. Kariba Dam and its hydropower plant are transboundary structures, with management shared between Zambia and Zimbabwe. In general, the transboundary character of water infrastructures complicates the water resources management and thus, serious omissions in the discussion of downstream water quality effects often occur. This specific case study in the Zambezi Basin serves as a starting point to shed some light on the general effects of large dams on water quality at low latitudes. The first study of this thesis presents a global review and synthesis of the effects of river damming on water quality with a special focus on low latitudes. Two physical processes were identified as drivers of most water quality changes: the trapping of sediments and nutrients and the thermal stratification in reservoirs. Analysing the mixing behaviour of the 54 largest low-latitude reservoirs revealed that most, if not all, large low-latitude reservoirs stratify on at least a seasonal basis. Stratification creates density and temperature gradients within the lake water column, facilitating the development of low-oxygen conditions in the deep colder waters. By releasing such water, low-latitude large dams have the potential to impact downstream ecosystems by altering thermal regimes or causing hypoxic stress. The second study of this thesis shows how a detailed statistical analysis of vertical profiles in reservoirs allows generating an assessment tool for water quality alterations downstream of large dams. This finding suggests that designing and maintaining an efficient water quality monitoring of reservoirs is key for their sustainable management. Due to the spatial heterogeneity of water quality in large reservoirs, water quality monitoring should be designed for capturing the temporal dynamics close to outlets of dams in order to predict downstream water quality. In the third study, the alterations of the thermal and oxygen regimes of the Zambezi River downstream of Kariba Dam were quantified by means of a one-dimensional numerical lake model. Results suggest that these alterations depend on the stratification and the water level of the reservoir but also on the management of water withdrawal, thus on the transboundary policies of the dam. Scenarios show that cooperative management of the existing infrastructure of Kariba Dam has the potential to partially mitigate the actual downstream water quality alterations. These results reveal that transboundary dams may offer additional opportunities for optimized management. Moreover, outcomes show that biogeochemical lake models are effective tools to test the effectiveness of such transboundary management scenarios to mitigate downstream water quality alterations. Finally, the last part of the thesis addresses the effects of large dams on the carbon dioxide emission dynamics of inland waters. Monitoring the seasonal and sub-daily fluctuations of water quality properties downstream of Kariba Dam revealed that atmospheric CO2 emissions from the Zambezi River surface downstream of Kariba fluctuate strongly over different timescales. Seasonal changes were driven by reservoir stratification and the accumulation of carbon dioxide in hypolimnetic waters. Sub-daily variability of CO2 emissions, instead, was linked to the hydropeaking resulting from the daily variability in electricity production. Failing to account for these fluctuations in downstream CO2 emissions could lead to errors in the carbon budgeting of hydroelectric reservoirs. Thus, it is critically important to include both limnological seasonality and dam operation at sub-daily time steps in our assessment of carbon budgeting of reservoirs and carbon cycling along the aquatic continuum. This thesis underlines potential environmental drawbacks associated with hydropower, nevertheless recognizing the many benefits of such energy source to societies worldwide. Thus, it aims at inspiring innovative strategies for more sustainable design and management of dams.
Anthropogenic influences on Zambian water quality: hydropower and land-use change
Winton, R. Scott; Teodoru, Cristian R.; Calamita, Elisa; et al. (2021)
Environmental Science: Processes & Impacts
The Zambezi River Basin in Southern Africa is undergoing rapid development and population growth. Agricultural intensification, urbanization and future development of hydropower dams will likely lead to a degradation of surface water quality, but there have been few formal assessments of where, how and why these changes impact specific water quality parameters based on in situ data spanning a large region. We sampled a large suite of biogeochemical water quality parameters at 14 locations in four field campaigns in central and southern Zambia in 2018 and 2019 to characterize seasonal changes in water quality in response to large hydropower dams and human landscape transformations. We find that the major rivers (Zambezi and Kafue) are very clean with extremely low concentrations of solutes, but suffer from thermal changes, hypoxia and loss of suspended sediment below dams. Smaller tributaries with a relatively large anthropogenic landcover footprint in their catchments show signs of pollution in the form of higher concentrations of nutrients and dissolved ions. We find significant relationships between crop and urban land cover metrics and selected water quality metrics (i.e. conductivity, phosphorus and nitrogen) across our data set. These results reflect a very high-quality waterscape exhibiting some hotspots of degradation associated with specific human activities. We anticipate that as agricultural intensification, urbanization and future hydropower development continue to accelerate in the basin, the number and extent of these hotspots of water quality degradation will grow in response. There is an opportunity for governments, managers and industry to mitigate water quality degradation via investment in sustainable infrastructure and practice, such as wastewater treatment, environmental dam operations, or riparian protection zones.
Living with floating vegetation invasions
Kleinschroth, Fritz; Winton, R. Scott; Calamita, Elisa; et al. (2021)
Ambio
Invasions of water bodies by floating vegetation, including water hyacinth (Eichhornia crassipes), are a huge global problem for fisheries, hydropower generation, and transportation. We analyzed floating plant coverage on 20 reservoirs across the world's tropics and subtropics, using > 30 year time-series of LANDSAT remote-sensing imagery. Despite decades of costly weed control, floating invasion severity is increasing. Floating plant coverage correlates with expanding urban land cover in catchments, implicating urban nutrient sources as plausible drivers. Floating vegetation invasions have undeniable societal costs, but also provide benefits. Water hyacinths efficiently absorb nutrients from eutrophic waters, mitigating nutrient pollution problems. When washed up on shores, plants may become compost, increasing soil fertility. The biomass is increasingly used as a renewable biofuel. We propose a more nuanced perspective on these invasions moving away from futile eradication attempts towards an ecosystem management strategy that minimizes negative impacts while integrating potential social and environmental benefits.
On the role of local depth and latitude on surface warming heterogeneity in the Laurentian Great Lakes
Calamita, Elisa; Piccolroaz, Sebastiano; Majone, Bruno; et al. (2021)
Inland Waters
Lake surface water temperature (LSWT) responds rapidly to changes in climatic variables. This response is heterogeneous in space and its spatial distribution is primarily influenced by lake bathymetry and latitude. Such heterogeneity is not captured by one-dimensional water temperature models, which can accurately predict only the average LSWT. We performed a spatially distributed application of the hybrid physically based/data-driven model air2water to predict the LSWT variability in the 5 Laurentian Great Lakes and to deepen our understanding of the role of local depth and latitude in shaping this heterogeneous response. Daily remotely sensed LSWT data were used to calibrate and validate the model during 1995–2018, and additional simulations considering a synthetic warmer climate scenario in which air temperature was increased by 2 °C were run to assess the inter- and intra-lake differences in LSWT warming rates. The model reproduces the observed spatial distribution of LSWT with an average root mean squared error of 1.2 °C and suggests that, under the warmer scenario, the LSWT of the 5 lakes could increase heterogeneously, with the deepest zones showing the maximum warming rates. Summer stratification lengthening is expected to increase with higher local depth; this behaviour attenuates with increasing latitude, whereas the LSWT warming is essentially dependent on the local depth, irrespective of latitude. We highlight the importance of accounting for LSWT spatial heterogeneity to adequately assess the thermal response of the Great Lakes to a warming climate.
Sixty years since the creation of Lake Kariba: Thermal and oxygen dynamics in the riverine and lacustrine sub-basins
Calamita, Elisa; Schmid, Martin; Kunz, Manuel; et al. (2019)
PLoS ONE
The current boom of dam construction at low latitudes endangers the integrity and function of major tropical river systems. A deeper understanding of the physical and chemical func-tioning of tropical reservoirs is essential to mitigate dam-related impacts. However, the development of predictive tools is hampered by a lack of consistent data on physical mixing and biogeochemistry of tropical reservoirs. In this study, we focus on Lake Kariba (Southern Africa), the largest artificial lake in the world by volume. Kariba Dam forms a transboundary reservoir between Zambia and Zimbabwe, and therefore its management represents a socio-politically sensitive issue because the Kariba Dam operation completely changed the downstream hydrological regime. Although Lake Kariba represents a unique and scientifi-cally interesting case study, there is no consistent dataset documenting its physical and chemical behaviour over time. This limits the scope for quantitative studies of this reservoir and its downstream impacts. To address this research gap, we aggregated a consistent database of in situ measurements of temperature and oxygen depth profiles for the entire 60 years of Lake Kariba’s lifetime and performed a detailed statistical analysis of the thermal and oxygen regime of the artificial lake to classify the different behaviours of the lake’s sub-basins. We demonstrate that the seasonal stratification strongly depends on the depth of the water column and on the distance from the lake inflow. Satellite data confirm these spa-tiotemporal variations in surface temperature, and reveal a consistent longitudinal warming trend of the lake surface water temperature of about 1.5˚C from the inflow to the dam. Finally, our results suggest that the stratification dynamics of the lacustrine sub-basins have the potential to alter the downstream Zambezi water quality. Future research should focus on assessing such alterations and developing strategies to mitigate them.
Unaccounted CO2 leaks downstream of a large tropical hydroelectric reservoir
Calamita, Elisa; Siviglia, Annunziato; Gettel, Gretchen M.; et al. (2021)
Proceedings of the National Academy of Sciences of the United States of America
Recent studies show that tropical hydroelectric reservoirs may be responsible for substantial greenhouse gas emissions to the atmosphere, yet emissions from the surface of released water downstream of the dam are poorly characterized if not neglected entirely from most assessments. We found that carbon dioxide (CO2) emission downstream of Kariba Dam (southern Africa) varied widely over different timescales and that accounting for downstream emissions and their fluctuations is critically important to the reservoir carbon budget. Seasonal variation was driven by reservoir stratification and the accumulation of CO2 in hypolimnetic waters, while subdaily variation was driven by hydropeaking events caused by dam operation in response to daily electricity demand. This “carbopeaking” resulted in hourly variations of CO2 emission up to 200% during stratification. Failing to account for seasonal or subdaily variations in downstream carbon emissions could lead to errors of up to 90% when estimating the reservoir’s annual emissions. These results demonstrate the critical need to include both limnological seasonality and dam operation at subdaily time steps in the assessment of carbon budgeting of reservoirs and carbon cycling along the aquatic continuum.
Reviews and syntheses: Dams, water quality and tropical reservoir stratification
Winton, Robert S.; Calamita, Elisa; Wehrli, Bernhard (2019)
Biogeosciences
The impact of large dams is a popular topic in environmental science, but the importance of altered water quality as a driver of ecological impacts is often missing from such discussions. This is partly because information on the relationship between dams and water quality is relatively sparse and fragmentary, especially for low-latitude developing countries where dam building is now concentrated. In this paper, we review and synthesize information on the effects of damming on water quality with a special focus on low latitudes. We find that two ultimate physical processes drive most water quality changes: the trapping of sediments and nutrients, and thermal stratification in reservoirs. Since stratification emerges as an important driver and there is ambiguity in the literature regarding the stratification behavior of water bodies in the tropics, we synthesize data and literature on the 54 largest low-latitude reservoirs to assess their mixing behavior using three classification schemes. Direct observations from literature as well as classifications based on climate and/or morphometry suggest that most, if not all, low-latitude reservoirs will stratify on at least a seasonal basis. This finding suggests that low-latitude dams have the potential to discharge cooler, anoxic deep water, which can degrade downstream ecosystems by altering thermal regimes or causing hypoxic stress. Many of these reservoirs are also capable of efficient trapping of sediments and bed load, transforming or destroying downstream ecosystems, such as floodplains and deltas. Water quality impacts imposed by stratification and sediment trapping can be mitigated through a variety of approaches, but implementation often meets physical or financial constraints. The impending construction of thousands of planned low-latitude dams will alter water quality throughout tropical and subtropical rivers. These changes and associated environmental impacts need to be better understood by better baseline data and more sophisticated predictors of reservoir stratification behavior. Improved environmental impact assessments and dam designs have the potential to mitigate both existing and future potential impacts.
Lake Modeling Reveals Management Opportunities for Improving Water Quality Downstream of Transboundary Tropical Dams
Calamita, Elisa; Vanzo, Davide; Wehrli, Bernhard; et al. (2021)
Water Resources Research
Water quality in tropical rivers is changing rapidly. The ongoing boom of dam construction for hydropower is one of the drivers for this change. In particular, the stratification in tropical reservoirs induces oxygen deficits in their deep waters and warmer surface water temperatures, which often translate into altered thermal and oxygen regimes of downstream river systems, with cascading consequences for the entire aquatic ecosystem. Operation rules of reservoirs, involving water intakes at different levels, could mitigate the consequences for downstream water quality. However, optimized water management of deep reservoirs relies on predictive models for water quality, but such predictive capability is often lacking for tropical dams. Here we focus on the Zambezi River Basin (southern Africa) to address this gap. Using the one-dimensional General Lake Model, we reproduced the internal dynamics of the transboundary Lake Kariba, the world’s largest artificial lake by volume, created by damming the Zambezi River at the border between Zambia and Zimbabwe. Through this modeling approach, we assessed and quantified the thermal and oxygen alteration in the Zambezi River downstream of the reservoir. Results suggest that these alterations depend directly on Kariba’s stratification dynamics, its water level and the transboundary policies for water withdrawal from the reservoir. Scenario calculations indicate a large potential for mitigating downstream water quality alterations by implementing a hypothetical selective withdrawal technology. However, we show that a different and cooperative management of the existing infrastructure of Kariba Dam has the potential to mitigate most of the actual water quality alterations.

Publications 1 - 9 of 9

Elisa Calamita

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