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Lakes are key components of biogeochemical and ecological processes, thus knowledge about their distribution, volume and residence time is crucial in understanding their properties and interactions within the Earth system. However, global information is scarce and inconsistent across spatial scales and regions. Here we develop a geo-statistical model to estimate the volume of global lakes with a surface area of at least 10 ha based on the surrounding terrain information. Our spatially resolved database shows 1.42 million individual polygons of natural lakes with a total surface area of 2.67 × 10 6  km 2 (1.8% of global land area), a total shoreline length of 7.2 × 10 6  km (about four times longer than the world’s ocean coastline) and a total volume of 181.9 × 10 3  km 3 (0.8% of total global non-frozen terrestrial water stocks). We also compute mean and median hydraulic residence times for all lakes to be 1,834 days and 456 days, respectively. Lakes play a key role in our ecosystems and thus it is vital to understand their distribution and volume. Here, the authors present a new global lake database (HydroLAKES) and develop a new geo-statistical model to show global lake area, shoreline length, water volume and hydraulic residence times.

Abstract Knowing where and when rivers cease to flow provides an important basis for evaluating riverine biodiversity, biogeochemistry and ecosystem services. We present a novel modeling approach to estimate monthly time series of streamflow intermittence at high spatial resolution at the continental scale. Streamflow intermittence is quantified at more than 1.5 million river reaches in Europe as the number of no‐flow days grouped into five classes (0, 1–5, 6–15, 16–29, 30–31 no‐flow days) for each month from 1981 to 2019. Daily time series of observed streamflow at 3706 gauging stations were used to train and validate a two‐step random forest modeling approach. Important predictors were derived from time series of monthly streamflow at 73 million 15 arc‐sec (∼500 m) grid cells that were computed by downscaling the 0.5 arc‐deg (∼55 km) output of the global hydrological model WaterGAP, which accounts for human water use. Of the observed perennial and non‐perennial station‐months, 97.8% and 86.4%, respectively, were correctly predicted. Interannual variations of the number of non‐perennial months at non‐perennial reaches were satisfactorily simulated, with a median Pearson correlation of 0.5. While the spatial prevalence of non‐perennial reaches is underestimated, the number of non‐perennial months is overestimated in dry regions of Europe where artificial storage abounds. Our model estimates that 3.8% of all European reach‐months and 17.2% of all reaches were non‐perennial during 1981–2019, predominantly with 30–31 no‐flow days. Although estimation uncertainty is high, our study provides, for the first time, information on the continent‐wide dynamics of non‐perennial rivers and streams.

Freshwater biodiversity and ecosystem services are under stress as climate change alters streamflow intermittence. We present the first continental‐scale quantification of future climate change impacts on streamflow intermittence, achieved for Europe at a high spatial resolution that captures headwater streams. A hybrid modeling approach combines physics‐based and data‐based modeling, with a random forest model, trained on historical streamflow observations, using predictors representing the impact of climate change on high‐resolution (500 m) streamflow. These predictors were derived from a low‐resolution (50 km) global hydrological model, WaterGAP, which was driven by the outputs of five global climate models. The generated monthly time series of intermittence status for over 1.5 million reaches were used to calculate five ecologically relevant indicators of streamflow intermittence change. In Europe, the number of non‐perennial reach‐months is projected to increase in the future, for both low (SSP1‐RCP2.6) and high (SSP5‐RCP8.5) greenhouse gas emissions scenarios, in almost all climate zones, particularly in August and September. Under SSP1‐RCP2.6, 3.8% of all reach‐months may experience no‐flow conditions in 2071–2100, only a small increase from 3.5% in 1985–2014. Under SSP5‐8.5, however, a larger increase to 4.8% of all reach‐months is expected; 2.8% of European reaches are projected to shift from being perennial to non‐perennial, even where annual precipitation increases, while 0.7% are projected to shift from non‐perennial to perennial. These shifts represent a fundamental change in ecological habitat and connectivity i.e. bound to erode aquatic species diversity and alter ecosystem functions across more than 87.000 km of river segments. Plain Language Summary Many streams and rivers in Europe experience dry periods without water flow, which may become more frequent in the future due to climate change. Until now, however, we lacked quantitative estimates of these shifts in drying periods at the scale and resolution necessary for water resource management. In this study, we quantified, for the first time, the impacts of climate change on streamflow intermittence at the continental scale, considering a low and a high greenhouse gas emissions scenario. We generated monthly time series of intermittence status for over 1.5 million European reaches to calculate the streamflow intermittence changes for each reach and month (i.e., reach‐month) using five indicators. We found that there is an increase in the number of non‐perennial reach‐months (with at least one day without streamflow) under both emissions scenarios, especially in August and September. By the end of the century, many more reaches will become non‐perennial than the reverse in all of Europe, except in the sub‐arctic and polar/alpine climate zones. Increases in streamflow intermittence are projected to be most widespread in the Mediterranean/semi‐arid and humid subtropical climate zones of Europe. Mitigating climate change by achieving a low emissions scenario would strongly reduce the projected changes. Key Points Hybrid modeling enables continental‐scale high‐resolution (500 m) projections of streamflow intermittence under climate change Under both low‐ and high‐emissions scenarios, the number of non‐perennial river reach‐months is expected to increase over Europe Streamflow intermittence will increase in almost all European climate zones, and most strongly in August and September

Flowing waters have a unique role in supporting global biodiversity, biogeochemical cycles and human societies(1-5). Although the importance of permanent watercourses is well recognized, the prevalence, value and fate of non-perennial rivers and streams that periodically cease to flow tend to be overlooked, if not ignored(6-8). This oversight contributes to the degradation of the main source of water and livelihood for millions of people(5). Here we predict that water ceases to flow for at least one day per year along 51-60 per cent of the world's rivers by length, demonstrating that non-perennial rivers and streams are the rule rather than the exception on Earth. Leveraging global information on the hydrology, climate, geology and surrounding land cover of the Earth's river network, we show that non-perennial rivers occur within all climates and biomes, and on every continent. Our findings challenge the assumptions underpinning foundational river concepts across scientific disciplines(9). To understand and adequately manage the world's flowing waters, their biodiversity and functional integrity, a paradigm shift is needed towards a new conceptual model of rivers that includes flow intermittence. By mapping the distribution of non-perennial rivers and streams, we provide a stepping-stone towards addressing this grand challenge in freshwater science.

Virtually every river network on Earth includes non-perennial rivers and streams (NPRs) that periodically cease to flow or dry. The recurrence of flowing, non-flowing and dry phases that characterize NPRs uniquely supports high biodiversity and biogeochemical cycles in entire river networks. Consequently, changing these hydrological cycles can threaten the integrity of riverine ecosystems and the people that depend on them for their livelihood and culture. Despite their prevalence and importance, NPRs are largely excluded from management practices, conservation laws, and scientific research that have been tailored to perennial rivers. This bias, which stems from a historical lack of consideration for the value and distinctiveness of NPRs, is resulting in their rapid degradation. The aim of this thesis is to advance our understanding of the global prevalence and diversity of NPRs, and to improve their integration in river policy and sustainable management. Leveraging an interdisciplinary perspective integrating hydrology, ecology, geography, and data science, this thesis addresses three main objectives through four articles (Chapters 2 to 5).i)Chapter 2 and 3 provide the first robust quantitative estimate of the prevalence, distribution, and diversity of NPRs worldwide. Using a machine learning model informed by global data on hydrology, climate, geology, and land cover, Chapter 2 reveals that water ceases to flow for at least one day per year along 51%–60% of the world’s rivers by length. This finding demonstrates that non-perennial rivers and streams are the rule rather than the exception on Earth, and that they occur within all climates and biomes, and on every continent. Chapter 3 identifies nine hydrological types of NPRs globally which differ in how often, how long, when and why they stop to flow.ii)Chapter 4 highlights the inadequate protection of NPRs by environmental protection laws. Through a case study of regulatory maps defining which watercourses are protected under the Water Law in France, this chapter sheds light on the socio-political factors influencing regulatory cartography, exposes the disproportionate exclusion of NPRs from regulatory frameworks, and discusses the implications of this exclusion for river network integrity.iii)Chapter 5 introduces a novel conceptual and operational framework to enhance the effectiveness of flow management programs to sustain freshwater ecosystems (i.e., environmental flows) in river networks with a high prevalence of NPRs. It proposes to broaden the set of ecological processes integrated into the design, implementation, and monitoring of environmental flows with the end goal of better protecting the distinct ecological structure and dynamics of NPRs.This thesis challenges the prevailing conceptual models of river ecosystems by demonstrating the global prevalence and diversity of NPRs, and by supporting their integration into science, policy, and management frameworks. In doing so, it contributes to an ongoing paradigm shift towards an integrated view of river networks. This integrated view involves studying and managing all reaches, their floodplain, and contributing catchment as a dynamically interconnected meta-ecosystem whose components span the aquatic-terrestrial continuum.

Virtually every river network on Earth includes non-perennial rivers and streams (NPRs) that periodically cease to flow or dry. The recurrence of flowing, non-flowing and dry phases that characterize NPRs uniquely supports high biodiversity and biogeochemical cycles in entire river networks. Consequently, changing these hydrological cycles can threaten the integrity of riverine ecosystems and the people that depend on them for their livelihood and culture. Despite their prevalence and importance, NPRs are largely excluded from management practices, conservation laws, and scientific research that have been tailored to perennial rivers. This bias, which stems from a historical lack of consideration for the value and distinctiveness of NPRs, is resulting in their rapid degradation.The aim of this thesis is to advance our understanding of the global prevalence and diversity of NPRs, and to improve their integration in river policy and sustainable management. Leveraging an interdisciplinary perspective integrating hydrology, ecology, geography, and data science, this thesis addresses three main objectives through four articles (Chapters 2 to 5).i) Chapters 2 and 3 provide the first robust quantitative estimate of the prevalence, distribution, and diversity of NPRs worldwide. Using a machine learning model informed by global data on hydrology, climate, geology, and land cover, Chapter 2 reveals that water ceases to flow for at least one day per year along 51%–60% of the world’s rivers by length. This finding demonstrates that non-perennial rivers and streams are the rule rather than the exception on Earth, and that they occur within all climates and biomes, and on every continent. Chapter 3 identifies nine hydrological types of NPRs globally which differ in how often, how long, when and why they stop to flow.ii) Chapter 4 highlights the inadequate protection of NPRs by environmental protection laws. Through a case study of regulatory maps defining which watercourses are protected under the Water Law in France, this chapter sheds light on the socio-political factors influencing regulatory cartography, exposes the disproportionate exclusion of NPRs from regulatory frameworks, and discusses the implications of this exclusion for river network integrity.iii) Chapter 5 introduces a novel conceptual and operational framework to enhance the effectiveness of flow management programs to sustain freshwater ecosystems (i.e., environmental flows) in river networks with a high prevalence of NPRs. It proposes to broaden the set of ecological processes integrated into the design, implementation, and monitoring of environmental flows with the end goal of better protecting the distinct ecological structure and dynamics of NPRs.This thesis challenges the prevailing conceptual models of river ecosystems by demonstrating the global prevalence and diversity of NPRs, and by supporting their integration into science, policy, and management frameworks. In doing so, it contributes to an ongoing paradigm shift towards an integrated view of river networks. This integrated view involves studying and managing all reaches, their floodplain, and contributing catchment as a dynamically interconnected meta-ecosystem whose components span the aquatic-terrestrial continuum.

Continued ecological impacts of invasive species on freshwater ecosystems is one of the main challenges confronting ecologists and decision makers in conserving biodiversity and ecosystem function today. Efforts to prohibit the initial introduction of nonnative species are widely recognized to be the most cost-effective management and policy strategy. However, when aquatic invasive species become established and start spreading through the landscape, efforts to slow their proliferation remain severely limited by a lack of adequate forecasting tools and understanding of their secondary spread. My thesis aims to address these challenges by improving our understanding of and predicting the leading edge dynamics of the invasive rusty crayfish Orconectes rusticus (now Faxonius rusticus) in the John Day River (JDR) basin, a major tributary of the Columbia River in northeastern Oregon. In Chapter 1, I demonstrate the use of a spatially explicit individual-based model to recreate the invasion history of rusty crayfish in the JDR and forecast its future distribution. This study shows that controlling the spread of invasive species is possible even after their establishment, when control efforts can be effectively allocated, and that spatially explicit individual-based models can provide unique insight into the secondary spread of aquatic invasive species and concretely support decision makers in choosing optimal control strategies. Chapter 2 investigates whether phenotypic differences exist between rusty crayfish individuals at the boundary of their invasion range compared to their conspecifics closer to their initial location of introduction. I show that rusty crayfish in the JDR have developed less competitive morphology and better physiological condition as they spread towards the edge of their current invasion range and feed lower in the food web in invasion front populations than in core areas. By accounting for variations in temperature, primary productivity, and macroinvertebrate biomass throughout the invasion gradient of rusty crayfish, my research suggests that low conspecific densities and natural selection by spatial sorting are the primary drivers of these phenotypic changes, which suggests that these trends are likely to grow stronger over time as rusty crayfish keep spreading. Together, these chapters not only improve our understanding of the leading edge dynamics of aquatic invasive species such as rusty crayfish but also improve our ability control their spread and reduce their impact on invaded ecosystems.