Investigating the Monthly Mean Stream Temperature Dynamics

Aurélien Gallice, Wolf Hendrik Huwald, Michael Lehning, Andrea Rinaldo, Bettina Schaefli
2014
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Résumé

Affecting the habitat suitability of many fish species, water temperature is a hydrological factor of great concern in the actual context of climate change. Despite more than 40 years of research on this topic, the impact of landscape on the dynamics of stream temperature is still not entirely understood. In the present study, we analyzed the monthly mean stream temperature measurements collected in 26 medium-sized catchments (3–300 km2) in Switzerland. While selecting the catchments, particular attention was given to cover a large range of different geomorphological conditions, especially regarding altitude, slope and aspect. Despite these differences, it was surprisingly found that the thermal regimes of almost all the investigated streams followed a same annual trend. Only the amplitude and the minimum value of this trend were observed to differ between the individual catchments. These two factors could be successfully related to geomorphological characteristics of the catchments using multi-linear regression. The shape of the annual trend was found to vary from one year to the other. This inter-annual variability was attributed to climate, based on the significant correlation between the annual trend and air temperature. As a result of the present study, we obtained a regression model to estimate the monthly mean stream temperature in ungauged catchments based on country-wide available geomorphological variables and the average of the monthly mean air temperature over Switzerland.

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https://infoscience.epfl.ch/record/206424?ln=fr

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Aurélien Gallice, Wolf Hendrik Huwald, Michael Lehning, Andrea Rinaldo, Bettina Schaefli
2014
Discussion par affiche

Résumé

Official source

https://infoscience.epfl.ch/record/206424?ln=fr

À propos de ce résultat

Concepts associés (16)

Concepts associés

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Publications associées (18)

Publications associées

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This report presents past observations and projects the future development of water temperature in Swiss lakes and rivers. Projections are made until the end of the 21st century using the CH2018 climate scenarios. Besides climate change effects on temperature, we also discuss effects on discharge for rivers, and effects on the thermal structure, and specifically the seasonal mixing regime and ice cover of lakes. Observations over the past 40 years show a clear increase in river temperatures, with a mean trend of 0.33 ± 0.03 °C per decade, corresponding to ~80% of the observed air temperature trend. This warming has been continuous over the last four decades and impacts the health of stream ecosystems (e.g. by favouring the spread of fish diseases) and their services (e.g., the water usage for industrial cooling). The temperature rise is more pronounced in the Swiss Plateau than in the Alps, where snow and glacier melt partially mitigates (for now) the effects of increasing air temperature. Conversely, annual average discharge shows no significant trend. Similar trends have also been reported for Swiss lakes with mean summer lake surface temperature increasing by 0.40 ± 0.08 °C per decade since the 1950s. This warming trend affects lake stratification. Warm periods may for instance increase the occurrence of deep-water anoxic conditions, as observed during the 2003 heat wave in Lake Zurich. In mild winters, ice cover duration is reduced in alpine lakes, and winter deep mixing is less intense in large-peri-alpine lakes. The mild winter 2006/7 limited, for instance, the seasonal mixing of Lake Constance to about 60 m depths. Effects of warming on lake thermal structure vary within and between regions, due to both lake and watershed characteristics as well as regional climate change patterns. We simulated the future evolution of stream temperature for 10 catchments in Switzerland for a historical reference period (1990–2000) and two future periods: 2055–2065 (mid-century) and 2080–2090 (end of the century). Results show that the temperature will stabilize by the end of the century for the RCP2.6 scenario (strong CO2 emission reduction), whereas the warming will accelerate with time for the RCP8.5 scenario (business as usual scenario). This expected warming will have significant impacts on the stream ecosystems. Alpine and lowland catchments will experience a similar annual mean temperature increase but display different seasonal effects. While Swiss Plateau rivers will become warmer both in winter and summer (but more in summer), alpine rivers will experience almost no warming in winter but a strong warming exceeding that of air temperature in summer. This is explained by an abrupt decrease in discharge, and by the soil warming resulting from the absence of snow and thus a lower albedo. Lake temperature projections are based on one-dimensional, vertically resolved, hydrodynamic simulations for 29 lakes. The simulated lakes cover a wide range of sizes, depths and water quality, and an altitude range from 200 to 1800 m a.s.l. Simulations indicate substantial changes in lake thermal structure for RCP8.5 with surface temperatures increasing on average by 3.3 °C at the end of the 21st century. This increase is limited to 0.9 °C in the mitigation scenario RCP2.6. We identified an altitude-dependent evolution of the durations of summer and winter stratification as well the ice-covered period. Larger changes in stratification duration are expected to occur at higher altitude lakes. Yet, these lakes will still maintain winter stratification and a shortened icecovered period while lower altitude lakes (below ~1500 m a.s.l.) risk drastic changes in the mixing regime. e.g., a complete loss of the ice cover and winter stratification under the RCP8.5 scenarios. Such changes in the mixing regime may strongly impact lake ecosystems. These low to mid altitude lakes may therefore be considered as the most vulnerable to climate change.

2021

Chargement

Hydrologic Drivers and Controls of Stream Ecological Processes

Serena Ceola

The temporal variability of streamflows is a key feature structuring and controlling ecological communities and ecosystem processes. The magnitude, frequency and predictability of streamflows, and thus of velocity and near-bed shear stress fields, control ecological structure and function, particularly of benthic organisms. River ecosystem dynamics is indeed deeply connected to streamflow variability, which chiefly relies on rainfall, climate, land use, and geomorphologic properties. Although alterations of streamflow regime due to climate change, habitat fragmentation or other anthropogenic factors are ubiquitous, their ecological implications remain poorly understood. The present Thesis therefore addresses a quantitative analysis of the implications of hydrological fluctuations on river ecosystems positing that they have major ecological importance. From a food-web perspective, the response of benthic biota to hydrologic change is analyzed through experimental and theoretical approaches. Starting from the theoretical characterization of the probability distribution function of streamflows, flow velocities, water depths and near-bed shear stresses at a site, where a comparison between measured and analytical streamflow statistics across several catchments is outlined, a flume experimental campaign has been carried out in order to analyze how flow variability affects biofilm growth and invertebrate grazing activity. Here, two contrasting flow regimes (a constant and a time-varying discharge sequence) and four different light conditions (from 90% to 27% of incoming light radiation), as a key control on biofilm algal productivity and grazer activity, have been performed. Average grazing rates were significantly enhanced under time-varying flow conditions and highest at intermediate light availability. This result suggests that the stochastic flow regime offers increased opportunities for grazing under more favorable shear stress conditions, with implications for stream ecology and trophic carbon transfer in stream food webs. A spatial generalization of the above results, upscaling them to whole river basins, is made possible by employing scaling relationships that are known to characterize the geometry and topology of natural landforms and riverine patterns. The proposed generalization accounts for hydrological characterizations of the driving streamflow variability where the explicit dependence of invertebrate suitability on geomorphic controls (e.g. river depth, bed shear stress and flow velocity) and on hydrologic conditions (e.g. embedded in the probability of streamflow, and the ensuing temporal correlations that define the persistence of the hydrologic signal in time) has been accounted for. Different invertebrate grazing species have been compared. Suitable dynamic models, describing benthic biota dynamics, in particular stream biofilm growth under both ungrazed and grazed conditions (that is, via the absence and presence of invertebrates, respectively), have been explored in order to assess the relevant processes that controlled biofilm-invertebrate temporal pattern. The present Thesis therefore aims to further our understanding of the linkages between hydrology and ecology, possibly leading to a comprehensive theory of hydrologic drivers and controls of biotic processes in stream benthic environments.

EPFL2012

Chargement

Modeling the monthly mean stream temperature dynamics

Aurélien Gallice, Wolf Hendrik Huwald, Michael Lehning, Bettina Schaefli

Water temperature is a hydrological factor which affects the habitat suitability of many aquatic (fish) species, and is therefore of great concern in the actual context of climate change. Two types of models are currently used to simulate stream temperature: physically-based models, which are typically applied only over limited areas, and regression models, which usually lack the ability to make predictions in ungauged areas. As an attempt to bridge the gap between these two types, we propose a hybrid model based on the energy-balance equation, in which the terms are related to catchment physiographic variables via empirical relationships. The physiographic variables are chosen so as to be available over the entire country (Switzerland), enabling the model to be used in ungauged catchments. This approach is on the one hand more physically-based than the usual regression models – hereby limiting the degree of empiricism associated with its derivation – but on the other hand seeks simplicity and applicability over large areas – making it more practical than the usual physically-based models. In order to test this model, we use it to predict the monthly mean stream temperature over 23 selected medium-sized catchments (3–300 km2) in Switzerland. While selecting the catchments, particular attention is given to cover a large range of different geomorphological conditions, especially regarding altitude, slope and aspect. It is shown that the model compares favorably with standard empirical models such as multi-linear regression.

2014

Chargement

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2021

Hydrologic Drivers and Controls of Stream Ecological Processes

Serena Ceola

EPFL2012