97 research outputs found
Hyporheic Exchange and Water Chemistry of Two Arctic Tundra Streams of Contrasting Geomorphology
The North Slope of Alaskaâs Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the tundra surface and beneath streambeds during the summer, facilitating hyporheic exchange. Our goal was to understand how active layer extent and stream geomorphology influence hyporheic exchange and nutrient chemistry. We studied two arctic tundra streams of contrasting geomorphology: a high-gradient, alluvial stream with riffle-pool sequences and a low-gradient, peat-bottomed stream with large deep pools connected by deep runs. Hyporheic exchange occurred to ~50 cm beneath the alluvial streambed and to only ~15 cm beneath the peat streambed. The thaw bulb was deeper than the hyporheic exchange zone in both stream types. The hyporheic zone was a net source of ammonium and soluble reactive phosphorus in both stream types. The hyporheic zone was a net source of nitrate in the alluvial stream, but a net nitrate sink in the peat stream. The mass flux of nutrients regenerated from the hyporheic zones in these two streams was a small portion of the surface water mass flux. Although small, hyporheic sources of regenerated nutrients help maintain the in-stream nutrient balance. If future warming in the arctic increases the depth of the thaw bulb, it may not increase the vertical extent of hyporheic exchange. The greater impacts on annual contributions of hyporheic regeneration are likely to be due to longer thawed seasons, increased sediment temperatures or changes in geomorphology
Influence of Morphology and Permafrost Dynamics on Hyporheic Exchange in Arctic Headwater Streams under Warming Climate Conditions
We investigated surface-subsurface (hyporheic) exchange in two morphologically distinct arctic headwater streams experiencing warming (thawing) sub-channel conditions. Empirically parameterized and calibrated groundwater flow models were used to assess the influence of sub-channel thaw on hyporheic exchange. Average thaw depths were at least two-fold greater under the higher-energy, alluvial stream than under the lowenergy, peat-lined stream. Alluvial hyporheic exchange had shorter residence times and longer flowpaths that occurred across greater portions of the thawed sediments. For both reaches, the morphologic (longitudinal bed topography) and hydraulic conditions (surface and groundwater flow properties) set the potential for hyporheic flow. Simulations of deeper thaw, as predicted under a warming arctic climate, only influence hyporheic exchange until a threshold depth. This depth is primarily determined by the hydraulic head gradients imposed by the stream morphology. Therefore, arctic hyporheic exchange extent is likely to be independent of greater sub-stream thaw depths
Potential ecological impacts of climate intervention by reflecting sunlight to cool Earth
As the effects of anthropogenic climate change become more severe, several approaches for deliberate climate intervention to reduce or stabilize Earthâs surface temperature have been proposed. Solar radiation modification (SRM) is one potential approach to partially counteract anthropogenic warming by reflecting a small proportion of the incoming solar radiation to increase Earthâs albedo. While climate science research has focused on the predicted climate effects of SRM, almost no studies have investigated the impacts that SRM would have on ecological systems. The impacts and risks posed by SRM would vary by implementation scenario, anthropogenic climate effects, geographic region, and by ecosystem, community, population, and organism. Complex interactions among Earthâs climate system and living systems would further affect SRM impacts and risks. We focus here on stratospheric aerosol intervention (SAI), a well-studied and relatively feasible SRM scheme that is likely to have a large impact on Earthâs surface temperature. We outline current gaps in knowledge about both helpful and harmful predicted effects of SAI on ecological systems. Desired ecological outcomes might also inform development of future SAI implementation scenarios. In addition to filling these knowledge gaps, increased collaboration between ecologists and climate scientists would identify a common set of SAI research goals and improve the communication about potential SAI impacts and risks with the public. Without this collaboration, forecasts of SAI impacts will overlook potential effects on biodiversity and ecosystem services for humanity
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Electrical resistivity imaging of the architecture of substream sediments
The modeling of fluvial systems is constrained by a lack of spatial information about the continuity and structure of streambed sediments. There are few methods for noninvasive characterization of streambeds. Invasive methods using wells and cores fail to provide detailed spatial information on the prevailing architecture and its continuity. Geophysical techniques play a pivotal role in providing spatial information on subsurface properties and processes across many other environments, and we have applied the use of one of those techniques to streambeds. We demonstrate, through two examples, how electrical resistivity imaging can be utilized for characterization of subchannel architecture. In the first example, electrodes installed in riparian boreholes and on the streambed are used for imaging, under the river bed, the thickness and continuity of a highly permeable alluvial gravel layer overlying chalk. In the second example, electrical resistivity images, determined from data collected using electrodes installed on the river bed, provide a constrained estimate of the sediment volume behind a log jam, vital to modeling biogeochemical exchange, which had eluded measurement using conventional drilling methods owing to the boulder content of the stream. The two examples show that noninvasive electrical resistivity imaging is possible in complex stream environments and provides valuable information about the subsurface architecture beneath the stream channels
Toward catchment hydro-biogeochemical theories
Headwater catchments are the fundamental units that connect the land to the ocean. Hydrological flow and biogeochemical processes are intricately coupled, yet their respective sciences have progressed without much integration. Reaction kinetic theories that prescribe rate dependence on environmental variables (e.g., temperature and water content) have advanced substantially, mostly in well-mixed reactors, columns, and warming experiments without considering the characteristics of hydrological flow at the catchment scale. These theories have shown significant divergence from observations in natural systems. On the other hand, hydrological theories, including transit time theory, have progressed substantially yet have not been incorporated into understanding reactions at the catchment scale. Here we advocate for the development of integrated hydro-biogeochemical theories across gradients of climate, vegetation, and geology conditions. The lack of such theories presents barriers for understanding mechanisms and forecasting the future of the Critical Zone under human- and climate-induced perturbations. Although integration has started and co-located measurements are well under way, tremendous challenges remain. In particular, even in this era of "big data," we are still limited by data and will need to (1) intensify measurements beyond river channels and characterize the vertical connectivity and broadly the shallow and deep subsurface; (2) expand to older water dating beyond the time scales reflected in stable water isotopes; (3) combine the use of reactive solutes, nonreactive tracers, and isotopes; and (4) augment measurements in environments that are undergoing rapid changes. To develop integrated theories, it is essential to (1) engage models at all stages to develop model-informed data collection strategies and to maximize data usage; (2) adopt a "simple but not simplistic," or fit-for-purpose approach to include essential processes in process-based models; (3) blend the use of process-based and data-driven models in the framework of "theory-guided data science." Within the framework of hypothesis testing, model-data fusion can advance integrated theories that mechanistically link catchments' internal structures and external drivers to their functioning. It can not only advance the field of hydro-biogeochemistry, but also enable hind- and fore-casting and serve the society at large. Broadly, future education will need to cultivate thinkers at the intersections of traditional disciplines with hollistic approaches for understanding interacting processes in complex earth systems.This article is categorized under:Science of Water > Method
Low flow controls on stream thermal dynamics
Water level fluctuations in surface water bodies, and in particular low flow drought conditions, are expected to become more frequent and more severe in the future due to the impacts of global environmental change. Variations in water level, and therefore in-channel water volume, not only have the potential to directly impact stream temperature, but also aquatic vegetation coverage which, in turn, may affect stream temperature patterns and dynamics. Manipulation experiments provide a systematic approach to investigate the multiple environmental controls on stream temperature patterns. This study aims to use temperature data loggers and fibre optic distributed temperature sensing (FO-DTS) to investigate potential drought impacts on patterns in surface water and streambed temperature as a function of change in water column depth. To quantify the joint impacts of water level and associated vegetation coverage on stream temperatures, investigations were conducted in outdoor flumes using identical pool-riffle-pool features, but with spatially variable water levels representative of different drought severity conditions. Naturally evolved vegetation growth in the flumes ranged from sparse vegetation coverage in the shallow flumes to dense colonization in the deepest. Observed surface water and streambed temperature patterns differed significantly within the range of water levels and degrees of vegetation coverage studied. Streambed temperature patterns were more pronounced in the shallowest flume, with minimum and maximum temperature values and diurnal temperature variation being more intensively affected by variation in meteorological conditions than daily average temperatures. Spatial patterns in streambed temperature correlated strongly with morphologic features in all flumes, with riffles coinciding with the highest temperatures, and pools representing areas with the lowest temperatures. In particular, the shallowest flume (comprising multiple exposed features) exhibited a maximum upstream-downstream temperature warming of 3.3 °C (T in = 10.3 °C, T out = 13.5 °C), exceeding the warming observed in the deeper flumes by ⌠2 °C. Our study reveals significant streambed and water temperature variation caused by the combined impacts of water level and related vegetation coverage. These results highlight the importance of maintaining minimum water levels in lowland rivers during droughts for buffering the impacts of atmospheric forcing on both river and streambed water temperatures
Fine-Scale in Situ Measurement of Riverbed Nitrate Production and Consumption in an Armored Permeable Riverbed
Alteration of the global nitrogen cycle by man has increased nitrogen loading in waterways considerably, often with harmful consequences for aquatic ecosystems. Dynamic redox conditions within riverbeds support a variety of nitrogen transformations, some of which can attenuate this burden. In reality, however, assessing the importance of processes besides perhaps denitrification is difficult, due to a sparseness of data, especially in situ, where sediment structure and hydrologic pathways are intact. Here we show in situ within a permeable riverbed, through injections of 15N-labeled substrates, that nitrate can be either consumed through denitrification or produced through nitrification, at a previously unresolved fine (centimeter) scale. Nitrification and denitrification occupy different niches in the riverbed, with denitrification occurring across a broad chemical gradient while nitrification is restricted to more oxic sediments. The narrow niche width for nitrification is in effect a break point, with the switch from activity âonâ to activity âoffâ regulated by interactions between subsurface chemistry and hydrology. Although maxima for denitrification and nitrification occur at opposing ends of a chemical gradient, high potentials for both nitrate production and consumption can overlap when groundwater upwelling is strong
A water cycle for the Anthropocene
International audienceHumor us for a minute and do an online image search of the water cycle. How many diagrams do you have to scroll through before seeing any sign of humans? What about water pollution or climate changeâtwo of the main drivers of the global water crisis? In a recent analysis of more than 450 water cycle diagrams, we found that 85% showed no human interaction with the water cycle and 98% omitted any sign of climate change or waterpollution (Abbott et al., 2019). Additionally, 92% of diagrams depicted verdant, temperate ecosystems with abundant freshwater and 95% showed only a single river basin. It did not matter if the diagrams came from textbooks, scientific articles, or the internet, nor if they were old or new; most showed an undisturbed water cycle, free from human interference. These depictions contrast starkly with the state of the water cycle in the Anthropocene, when land conversion, human water use, and climate change affect nearly every water pool and flux (Wurtsbaugh et al., 2017; Falkenmark et al., 2019; Wine and Davison, 2019). The dimensions and scale of human interference with water are manifest in failing fossil aquifersin the worldâs great agricultural regions (Famiglietti, 2014), accelerating ice discharge from the Arctic (Box et al., 2018), and instability in atmospheric rivers that support continental rainfall (Paul et al., 2016).We believe that incorrect water cycle diagrams are a symptom of a much deeper and widespread problem about how humanity relates to water on Earth. Society does not understand how the water cycle works nor how humans fit into it (Attari, 2014; Linton, 2014; Abbott et al., 2019). In response to this crisis of understanding, we call on researchers, educators, journalists, lawyers, and policy makers to change how we conceptualize and present the global water cycle. Specifically, we must teach where water comes from, what determines its availability, and how many individuals and ecosystems are in crisis because of water mismanagement, climate change, and land conversion. Because the drivers of the global water crisis are truly global, ensuring adequate water for humans and ecosystems will require coordinated efforts that extend beyond geopolitical borders and outlast the tenure of individual administrations (Keys et al., 2017; Adler, 2019). This level of coordination and holistic thinking requires widespread understanding of the water cycle and the global water crisis. Making the causes and consequences of the water crisis visible in our diagrams is atractable and important step towards the goal of a sustainable relationship with water that includes ecosystems and society
Human domination of the global water cycle absent from depictions and perceptions
International audienceHuman water use, climate change and land conversion have created a water crisis for billions of individuals and many ecosystems worldwide. Global water stocks and fluxes are estimated empirically and with computer models, but this information is conveyed to policymakers and researchers through water cycle diagrams. Here we compiled a synthesis of the global water cycle, which we compared with 464 water cycle diagrams from around the world. Although human freshwater appropriation now equals half of global river discharge, only 15% of the water cycle diagrams depicted human interaction with water. Only 2% of the diagrams showed climate change or water pollutionâtwo of the central causes of the global water crisisâwhich effectively conveys a false sense of water security. A single catchment was depicted in 95% of the diagrams, which precludes the representation of teleconnections such as oceanâland interactions and continental moisture recycling. These inaccuracies correspond with specific dimensions of water mismanagement, which suggest that flaws in water diagrams reflect and reinforce the misunderstanding of global hydrology by policymakers, researchers and the public. Correct depictions of the water cycle will not solve the global water crisis, but reconceiving this symbol is an important step towards equitable water governance, sustainable development and planetary thinking in the Anthropocene
Advancing river corridor science beyond disciplinary boundaries with an inductive approach to catalyse hypothesis generation
A unified conceptual framework for river corridors requires synthesis of diverse site-, method- and discipline-specific findings. The river research community has developed a substantial body of observations and process-specific interpretations, but we are still lacking a comprehensive model to distill this knowledge into fundamental transferable concepts. We confront the challenge of how a discipline classically organized around the deductive model of systematically collecting of site-, scale-, and mechanism-specific observations begins the process of synthesis. Machine learning is particularly well-suited to inductive generation of hypotheses. In this study, we prototype an inductive approach to holistic synthesis of river corridor observations, using support vector machine regression to identify potential couplings or feedbacks that would not necessarily arise from classical approaches. This approach generated 672 relationships linking a suite of 157 variables each measured at 62 locations in a fifth order river network. Eighty four percent of these relationships have not been previously investigated, and representing potential (hypothetical) process connections. We document relationships consistent with current understanding including hydrologic exchange processes, microbial ecology, and the River Continuum Concept, supporting that the approach can identify meaningful relationships in the data. Moreover, we highlight examples of two novel research questions that stem from interpretation of inductively-generated relationships. This study demonstrates the implementation of machine learning to sieve complex data sets and identify a small set of candidate relationships that warrant further study, including data types not commonly measured together. This structured approach complements traditional modes of inquiry, which are often limited by disciplinary perspectives and favour the careful pursuit of parsimony. Finally, we emphasize that this approach should be viewed as a complement to, rather than in place of, more traditional, deductive approaches to scientific discovery
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