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Tropical Alpine Ecosystems are high-altitude grasslands located above 3000 m.a.s.l. along the tropical belt of three continents. Their unique vegetation and soil characteristics, in combination with low temperature and abundant precipitation, create the most advantageous conditions for regulating and storing surface and groundwater. However, increasing temperatures and changing patterns of precipitation due to greenhouse-gas-emission climate change are threatening these fragile environments, reducing their extent and modifying their altitudinal distribution range. Here, we investigate the impact of climate change on the distribution and extent of global Tropical Alpine Ecosystems. We use an ensemble of historical and projected climate data (SSP585) from seven General Circulation Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to estimate annual average values of temperature and annual accumulated values of precipitation for reference (1985-2014) and far future (2070-2100) 30-year periodos. We produced the 95% probability current and future hydroclimatic spaces for every ecosystem to determine the range at which Tropical Alpine Ecosystem currently thrives in the climatic space, and investigate a number of hydroclimatic variables. Then, we used the projected climate time-series data to assess the current Tropical Alpine Ecosystem areas that will be unable to keep up with the temperature and precipitation changes by exceeding their reference climatic boundaries in the far future. Overall, our results showed that the Tropical Alpine ecosystem would drastically reduce its extent. Approximately 45% of its current extent will experience hydroclimatic conditions beyond their reference climatic boundaries. For example, the Ethiopian montane moorlands in Africa will be the most impacted ecoregion with a reduction of approximately 95% of its current extent. For the case of páramos in the North of the South American continent, increasing temperatures and changing precipitation will render ~50% of the current extent unsuitable for these ecosystems during the dry season. Our results highlight the magnitude of the impacts of climate change on Tropical Alpine Ecosystem and the vulnerability of water security of millions of people who depend on its ecological functioning. These results also have implications for biodiversity conservation, as endemic species will be threatened by habitat reduction and shifts in their distribution ranges.

The loss of hydrological connectivity and fragmentation of natural wetlands have driven widespread wetland degradation worldwide. Monitoring techniques are needed to assess the degree of fragmentation and to aid with the restoration of affected wetlands. Hydrogeodetic tools such as wetland Interferometric Synthetic Aperture Radar (InSAR) can be used to monitor wetland hydrology as it provides information on three-dimensional flow dynamics at a high spatial resolution. While this technique has been utilized previously for the manual assessment of hydrological connectivity in wetlands, this study proposes the first deep learning-based approach for the automated detection of barriers to the natural water flow that cannot otherwise be identified by conventional space imagery. To this end, a deep convolutional network is trained by segmenting edge features in ALOS PALSAR-1 L-Band InSAR images captured between 2006 and 2011. The training dataset consists of manually labelled and delineated barriers showing abrupt changes in water surface elevation and 22 wrapped interferograms with high coherence across several sample sites in the Everglades and the wetlands of southern Louisiana, United States. The scenes were processed in the Interferometric synthetic aperture radar Scientific Computing Environment (ISCE). The network is set up using a UNet structure with alternating convolutional and pooling or upsampling layers along a contracting and expanding part. The validation of the resulting pixel-wise segmentation shows that the network can successfully detect hydrological barriers in wetlands. Apart from identifying the location of barriers, the CNN can be applied to identify the type and persistence of the fragmentation over the entire wetland. Utilizing the multitemporal data additionally helps detect seasonal changes in the presence or absence of hydrological barriers in the sample sites. This study demonstrates the potential of deep learning techniques for the automated detection of hydrological parameters in InSAR imagery and sets the groundwork for the automated monitor of wetland fragmentation across the world.

Water availability can be linked to a country's stability, internal security, and the occurrence of violence and governability, in the environmental change context. For instance, lack of access to water resources can trigger political conflicts, be used as a tool for political negotiation or attacks on water infrastructure can be used as a source of intimidation. The potential political risks associated with water availability take particular relevance at the scale of international and transboundary hydrological basins and under conditions of water-food scarcity or political instability. To date, although water risks occurring within the boundaries of the hydrological basin have been studied across several case studies in the literature, the issue of risks arising from water upwind-downwind dependency has been overlooked. For instance, precipitation in a hydrological basin or agricultural centre regions with a high dependency on terrestrial moisture recycling may originate in upwind terrestrial areas outside of the basin boundaries. Here we study geopolitical risk related to this water dependency by analizing terrestrial moisture recycling. Our analysis shows that some hydrological basins in Africa, Asia and South America present a high risk of experiencing geopolitical conflicts when there is a large extension of croplands, high moisture recycling dependency and their precipitationsheds extend over warmongering countries. Hence, our results indicate that addressing transboundary water security from a surface perspective can underlook potential geopolitical conflicts that may threaten regional water-food security and peace. These risks need special international attention to guarantee global peace and agricultural production.

Research on the protection and preservation of wetlands has traditionally focused on direct human drivers and impacts of climate change occurring in their upstream hydrological basin. However, since precipitation falling in the hydrological basin comprises both oceanic and terrestrial evaporation originating mostly outside of the basin boundaries, upwind land use and hydroclimatic changes affecting this supply of precipitation also need to be assessed. This study assesses the vulnerability of 40 wetlands of international importance to land use and hydroclimatic changes occurring upwind (i.e., in their precipitationsheds). We here use a dataset containing atmospheric moisture flows in combination with evaporation from natural and current vegetation to analyse the impact of extra-basin vegetation changes on the precipitation over the wetland basins. The analysis shows that historical land-use conversion has already caused reduced incoming precipitation into most wetland hydrological basins. The strongest effects are seen in (sub)tropical wetlands in South America, Africa and Asia and especially those located downwind of large agricultural areas. Based on our results and current wetland decline rates, we find that wetland sites in China, India, South America and Sub-Saharan Africa are especially threatened by hydroclimatic and vegetation changes outside of their basins. Additionally, larger basins appear to be more reliant on evaporation from within their basin boundaries than smaller hydrological basins. Using wetland ecosystems as an exemplary case, this study stresses the need to incorporate downwind effects to land-use changes in sustainable ecosystem management approaches. Since the transition from potential natural vegetation to agricultural land is often associated with changes in evaporation, land conversion may affect the resilience of wetland water availability. Following this analysis of the upwind moisture sources of wetland basins, future studies should investigate the potential effect of wetland loss on downwind precipitation patterns.

Eutrophication in the Baltic Sea has been one of the major environmental issues during the last century partly due to extensive land-use change, loss of natural retention systems, and insufficient management. European legislation such as the Water Framework Directive (WFD) attempts to guide the recovery of good ecological status from freshwater to the sea, and suggests wetlands as ecosystems that can potentially contribute to achieving this goal. Wetlands are considered remarkable Nature-based Solutions (NbS) for improving water quality by diminishing the nutrient loads. This study aims to set a background context of the WFD implementation in Sweden, determine the status of constructed wetlands, and evaluate the stakeholders’ perspectives to identify the main administrative hurdles of wetland implementation in Sweden. For this purpose, we conducted a narrative review, database analysis, and semi-structured interviews with members of the institutions involved in water management. Our results show that it is essential to find synergies among the WFD and other directives to expand cross-sectoral cooperation, implement adjustments on the funding scheme that includes restoration and maintenance of natural wetlands, and increase compensation periods and cost ceiling. Likewise, it is crucial to perform significant improvements in the monitoring system, including more frequent data collection, as well as exploring new strategies to capture landowners’ interest in the implementation of NbS, such as the Catchment Officers program. Finally, we suggest paludiculture as a promising farming practice to increase proprietors’ attention on novel market alternatives, and in turn, to provide benefits for climate, water, and biodiversity.Keywords Wetlands management · Water Framework Directive · Nature-based Solutions · Eutrophication · Semi-structured interviews · Sweden

Spatiotemporal characteristics of physical responses of lakes to external and environmental changes are still largely unknown due to the consistent lack of monitoring of water level and corresponding changes in water storage in lakes. Understanding these changes is a fundamental step in advancing regional management of natural and anthropogenic systems that depend on the water resources of lakes. As an illustrative example, we here report a case study involving lakes of the headwater topical Andes mountain range, which, despite guaranteeing water security to millions of downstream inhabitants, still remain significantly ungauged. We present a novel evaluation of the potential of Differential Interferometric Synthetic Aperture Radar DInSAR techniques for the spatiotemporal analysis of patterns of water level change in lakes such as the ones comprising these ungauged high-altitude lake systems. Time series of Sentinel-1B data for the years 2017 and 2018 were used to generate continuous interferograms representing water level changes in twenty-four lakes of the Cajas National Park, Ecuador. The relation of these water level changes with climatic and topographical factors were analyzed to validate the methodology, and determine any patterns of change and response to climatic drivers. We found relatively high Pearson correlation coefficients between regional precipitation and water level change as estimated from the interferograms. Furthermore, we found an important negative relationship between water level change, as obtained from the DInSAR phase, and lake surface area. The study revealed a spatial trend of this correlation in terms of the altitude of the lakes at the basin scale; that is, lower correlation values were found in the headers of the basins, whilst higher correlation values were found at lower basin altitudes. The results of the present study demonstrate the potential of DInSAR techniques based on Sentinel-1 data for the monitoring of hydrologic changes in open water surfaces, and the possible validation of the DInSAR results with precipitation when gauged water level data is missing. These results are a basis to propose monitoring strategies in ungauged high-altitude lake systems in regions with similar data gauging constraints. Future work will encompass the integration of ongoing water level gauging for further validation of the herein depicted lake water level estimation approach.

Lake Tarfala is a small (~0.5 km2) glacier-proximal lake in the Kebnekaise Mountains in Northern Sweden, located at an altitude of 1162 meters above sea level, and close to Tarfala Research Station run by Stockholm University. Only very limited direct monitoring of lake ice phenology using ground observations is available so far, and, long polar nights and often persistent cloud cover at such altitude limit the use of optical remote sensing. However, active microwave radar signals illuminate the target and penetrate through the cloud cover allowing to monitor the lake independent of weather or time of day. In this study, we opt for the Level-1 GRD (Ground Range Detected) and SLC (Single Look Complex) products from the twin Sentinel-1 satellites which provide a coverage of Lake Tarfala at a very high spatial and temporal resolution. We aim to make use of a total of 60 scenes (June 2020 - May 2021) to create the backscatter and coherence time series. Further, we aim to associate the variation in intensity seen in the backscatter time series to the backscattering potential of the medium. It has been shown [1] that an increase in intensity is observed when transitioning from ice-free waters to the initial freeze-up (ice-on) stage. Around ice-on, the intensity would, however, be comparatively low as the ice cover would be very thin and not yet fully developed. The availability of in-situ high-resolution time-lapse imagery and air temperature data from a pilot project carried out during the fall of 2020 [2] will be exploited to assist in the detection of the initial ice formation and freeze-up. Over the course of winter, ice will continue to thicken and a subsequent increase in backscatter intensity is expected until it reaches a saturation point where it stabilises, until the onset of melt in the subsequent spring/summer, when finally, the detection of ice-off (water free of ice) can be characterised by low backscatter values. Furthermore, loss of interferometric coherence upon the onset of melt will aid the backscatter time series when it fails to show a clear signal. We expect to track and provide a complete timeline of the different ice-phenology stages, namely the onset of freezing and the date of complete ice-on, the ice-thickening, the onset of surface melt and the date of complete ice-off. We expect that this study will provide a basis for Arctic lake ice monitoring for various applications such as management of winter water resources, understanding the seasonal and inter-annual land-atmosphere greenhouse gases and energy flux exchanges and biological productivity.References:1. Morris, K., Jeffries, M.O., Weeks, W.F. Ice processes and growth history on Arctic and sub-Arctic lakes using ERS-1 SAR data. Polar Rec. 1995, 31, 115-128.2. Weckström, J., Korhola, A. Kirchner, N., Virtanen, T., Schenk, F., Granebeck, A., Prakash, A. “Lake Thermal and Mixing Dynamics under Changing Climate” and “Towards a multi-approach detection and classification of ice phenology at Lake Tarfala”. Pilot projects funded by Arctic Avenue (a spearhead research project between the University of Helsinki and Stockholm University).

Hydrological connectivity is a critical determinant of wetland functions and ecosystems by controlling the movement of biogeochemical elements within wetlands and the flow of water between their hydrological units. Hydrological barriers exist when this connectivity is impaired, either by man-made infrastructure, agriculture developments, or naturally restricted by soil and ground composition. Determining hydrological barriers in wetlands is challenging due to the costs of high-resolution and large-scale monitoring, but radar observations can become a useful tool for such task. We here use an Interferometric Synthetic Aperture Radar (InSAR) to identify hydrological barriers in several iconic wetlands worldwide, with particular focus on the Baiyangdian wetland system in Northern China. For the first, we use Sentinel 1A and 1B data covering the period 2016-2019, while for the rest we rely on ALOS PALSAR data. We calculated profiles of water level change across hydrological transects showing high coherence and visualized them in maps. For instance, in the case of the Baiyangdian wetland, we find that of the 70 transects studied, 11% of all transects are permanently disconnected by hydrological barriers across all interferograms and 58% of the transects are conditionally disconnected. The occurrence of hydrological barriers varies between wetlands, with permanent barriers more related to ditches, infrastructure and the specific wetland landscape, and conditional barriers more to low water levels during dry seasons. This study highlights the potential of the application of wetland InSAR to determine hydrological barriers for wetland management and restoration.

Nature-based Solutions (NbS), inspired or supported by nature, aim to address societal challenges in a fast-changing environment via an integrated and sustainable approach. Effective implementation of such intervention certainly requires compliance with specific societal configurations in different geographies. Here two cases of NbS to hydrological disaster risks are used to demonstrate the relevance of social barriers and opportunities for the full function of NbS.Firstly, we introduce a novel large-scale NbS designed for reducing water scarcity in the Bolivian city of Santa Cruz de la Sierra. In this case, strategic reforestation was planned to bring rainfall to a downwind city taking advantage of atmospheric moisture pathways. In the process of co-designing reforestation sites, experiences from failed reforestation projects have improved the site selection originally based solely on the scientific evidence of the moisture pathways. Social barriers to implementation include underground economic activities and pressures for local food production. The latter factor also implies a trade-off between the fulfilments of different sustainable development goals.Secondly, a case of landscape-scale NbS that aims to mitigate flood risk from typhoons in Taiwan will be discussed. It consists of a flood diversion framework that directs excess runoff to local farmlands following Typhoon storms. The concept of payment for ecosystem services has been employed to increase the willingness of farmers and landowners to participate in this framework. Institution of compensation for agricultural loss established from previous meteorological disasters has paved the way for implementation. A combination of subsidies and agricultural loss compensation has offered an opportunity for the new intervention to take place in the rice-cropping landscape, while the effect of this ongoing framework will be further documented.These two cases show that the inertia from existing policy/institutional schemes and the lessons from past unsuccessful experiences provide an opportunity to identify and overcome social barriers to the implementation of innovative NbS.

Green water - soil moisture, evaporation, and precipitation over land - is fundamental to safeguard Earth system functioning. Nonlinear green-water driven changes in climate, ecosystems, biogeochemistry, and hydrology are becoming increasingly evident and widespread. Yet, considerations of continental to planetary scale green-water dynamics are yet to be assessed and incorporated in management and governance. Here, we propose a green water planetary boundary (PB) - as part of the planetary boundary framework that demarcates a global “safe-operating space” for humanity - for assessing green-water related changes that can affect the capacity of the Earth system to remain in Holocene-like conditions. We consider green-water related processes associated with all scales: spatially distributed units, regions or biomes, and the Earth system as a whole. The proposed green water PB variable is selected through expert elicitation based on a set of transparent evaluation criteria that consider both scientific and governability aspects. Finally, we clarify the appropriate use of a green water PB, outline remaining challenges, and propose a research agenda for future navigation and quantitative assessments of the biophysical Earth system scale boundaries of green water changes.

Compound anthropogenic pressures are driving critical mangrove degradation worldwide, threatening the wellbeing of coastal human populations historically associated with these systems. The Ramsar and Biosphere Reserve, Ciénaga Grande de Santa Marta (CGSM) is located in the northern of Colombia and is the largest coastal lagoon-delta in the Caribbean. It is inhabited by stilt communities that have developed intricate livelihood and cultural relationships with the mangroves. The CGSM has experienced sustained social and ecological degradation during the last six decades, triggered by land-use change and disruption of hydrological connections. This study integrates Social-ecological Memory from fishing communities and Paleoenvironmental frameworks to develop a historical perspective of the biophysical and social dimensions of environmental change in the CGSM. Integrating X-ray fluorescence (XRF) geochemical sediment analysis, C14 radiocarbon dating, and demographic inferences from archaeological evidence revealed three distinct periods over the last ~5000 years where sea-level rise and hydroclimatic variability shaped the transition between freshwater to prevailing marine conditions and modulated human occupation patterns in the area. Specifically, the period with the highest hydroclimatic variability and precipitation minima (4000 – 2500 yr BP) is consistent with the lowest human population estimates, whereas sea-level increase (~ 2000 yr BP) corresponds with a sustained increase in estimated population growth. In connection, participatory oral reconstructions conducted in the stilt-house communities of Buenavista and Nueva Venecia, offered nuanced descriptions about the spatial, temporal, and contextual aspects generating and reinforcing hypersalinization of the system, and their profound social-ecological consequences over the past several decades. The interdisciplinary approach of this study indicates that the CGSM is a highly dynamic socio-ecological system that has been changing and reconfiguring across different time scales in response to both natural and human-induced processes. Finally, it reveals the relative effects of biophysical and social drivers on driving social-ecological change on millennial to decadal time scales.

Sweden has approximately 100,000 lakes covering roughly nine percent of the country’s surface area. These lakes are one of the important sources of fresh water for urban, industrial, and agricultural use, further providing a wide range of ecosystem services. In order to conserve and protect the lakes from the impacts of climate change, hydrologic monitoring should ideally be conducted in all of these lakes. However, it is almost impossible to gauge all of these lakes on a regular basis, due to economical and logistic constraints. Radar altimetry has been successfully used to obtain water levels from specific lakes; however, the technology can only be used in large lakes that are located precisely under the orbit of the satellite, thus excluding most Swedish lakes. We here develop a new procedure based on the application of differential interferometric synthetic aperture radar (DInSAR) on sequential image pairs with short temporal baseline to measure the water level of 36 lakes. We processed Sentinel-1 twin satellite data with 6-day revisiting intervals, pair by pair, from March 2019 to November 2019. In total, we constructed 41 interferograms considering only the pixels with coherence values greater than 0.2 in all interferograms to ensure consistent scattering and good coherence in all images. We found that the pixels located near tree trunks in flat areas or near steep cliffs in mountainous areas showed a steady phase change in all interferograms that could be converted to water level change. In some of these lakes, the water level changes derived from this methodology correlated well with the in-situ water level of the gauge stations provided by the Swedish Meteorological and Hydrological Institute. We believe that this methodology has good potential for monitoring water level data in small lakes that cannot be monitored by radar altimetry, and serves as evidence of the unknown potential of DInSAR to track hydrological changes in open water surfaces.

In the recent years, southern Sweden has experienced drought conditions during the summer with potential risks of groundwater shortages. One of the main physical effects of groundwater depletion is land subsidence, a geohazard that potentially damages urban infrastructure, natural resources and can generate casualties. We here investigate land subsidence induced by groundwater depletion and/or seasonal variations in Gotland, an agricultural island in the Baltic Sea experiencing recent hydrological droughts in the summer. Taking advantage of the multiple monitoring groundwater wells active on the island, we explore the existence of a relationship between groundwater fluctuations and ground deformation, as obtained from Interferometric Synthetic Aperture Radar (InSAR). The aim in the long-term is to develop a high-accuracy map of land subsidence with an appropriate temporal and spatial resolution to understand groundwater changes in the area are recognize hydroclimatic and anthropogenic drivers of change.We processed Sentinel-1 (S1) data, covering the time span of 2016-2019, by using the Small BAseline Subset (SBAS) to process 119 S1-A/B data (descending mode). The groundwater level of Nineteen wells distributed over the Gotland island were used to assess the relationship between groundwater depletion and the detected InSAR displacement. In addition to that, the roles of other geological key factors such as soil depth, ground capacity in bed rock, karstification, structure of bedrock and soil type in occurring land subsidence also investigated. The findings showed that the groundwater level in thirteen wells with soil depths of less than 5 meters correlated well with InSAR displacements. The closeness of bedrock to ground surface (small soil depth) was responsible for high coherence values near the wells, and enabled the detection land subsidence. The results demonstrated that InSAR could use as an effective monitoring system for groundwater management and can assist in predicting or estimating low groundwater levels during summer conditions.

Man-made reservoirs and lakes are key elements in the terrestrial water system. The increased concern about the impact of anthropogenic interventions on and the dynamics of these water resources has given rise to various approaches for representing human-water interactions in land surface models. Synthetic aperture radar interferometry (InSAR) has become a powerful geodetic tool for this purpose, by evidencing changes of ground and water surfaces across time and space. In this research, the Lake Mead and associated Hoover Dam are studied using Small Baseline Subset (SBAS) technique. Lake Mead is the largest reservoir in the United States, in terms of water capacity, supplies water and hydropower for millions of people in Las Vegas, Los Angeles and southwestern part of the USA. In recent years, rising temperature, increasing evaporation and decreasing precipitation have decreased water levels substantially, and probably modified its surrounding groundwater and surface as well.This study aims to identify a hydrology-induced ground deformation around the lake Mead and a probable Hoover dam movement displacement. For the reservoir, we used the SBAS technique using 138 SAR data, including ERS1/2, Envisat, ALOS PALSAR and Sentinel-1, covering a time-spam between 1995 and 2019. For the analysis on the dam, we used the SBAS technique from 2014 to 2019 with descending and ascending modes of Sentinel-1A/B imageries. We found two main deformation patterns around the lake associated with the water level changes. Firstly, ERS and Sentinel-1 data evidenced a ground deformation that manifested itself as as a subsidence pattern in 1995 that has gradually changed into an uplift up to 2019. Secondly, the correlation trend between the deformation and water level changes has changed from negative to positive, with a transition point around March 2008. A possible interpretation for this is that the ground has initially reacted to the water fluctuations in the reservoir before March 2008 but after no longer plays a dominant role in the deformation occurring around the lake. The findings will help us to have a better understanding over the changes happened around the lake due to the water level changes and provide the valuable information for more effective management and maintenance of hydraulic structures and facilities near by the lake and water control in the future.

The planetary boundaries framework defines nine Earth system processes that together demarcate a safe operating space for humanity at the planetary scale. Freshwater - the bloodstream of the biosphere - is an obvious member of the planetary boundary framework. Water fluxes and stores play a key role for the stability of the Earth’s climate and the world’s aquatic and terrestrial ecosystems. Recent work has proposed to represent the water planetary boundary through six sub-boundaries based on the five primary water stores, i.e., atmospheric water, soil moisture, surface water, groundwater, and frozen water. In order to make it usable on all spatial scales we examine bottom-up and top-down approaches for quantification of the water planetary boundary. For the bottom-up approaches, we explore possible spatially distributed variables defining each of the proposed sub-boundaries, as well as possible weighting factors and keystone regions that can be used for aggregation of the distributed water sub-boundaries to the global scale. For the top-down approaches, we re-examine the stability of key biomes and tipping elements in the Earth System that may be crucially influenced by water cycle modifications. To identify the most appropriate variables for representing the water planetary boundary, we evaluate the range of explored variables with regard to scientific evidence and scientific representation using a hierarchy-based evaluation framework. Finally, we compare the highest ranked top-down and bottom-up approaches in terms of the scientific outcome and implications for governance. In sum, this comprehensive and systematic identification and evaluation of variables, weighting factors, and baseline conditions provides a detailed basis for the future operational quantification of the water planetary boundary.

Lake Baikal, located in eastern Russia, is the oldest (25 million years) and the deepest (~1800 meters) lake in the world. There are many rivers flowing into the Lake Baikal (~ 365 rivers), of which the Selenga River is the most important one being responsible for almost 55% of the runoff water into the system and also 60% of the transported sediments. As the hydrological changes of the river and its delta enormously alter the neighbouring area, it is of utmost importance to explore the dynamics of change in terms of flow magnitude, paths and fluvial geomorphology, and the related tipping points defining different states. The questions this study aims to answer are: What are the fluvial geomorphological and hydrological changes? What fluvial geomorphological tipping points can be identified during the last 34 years and what are the discharge and climatic conditions that induce them? In this study, we use the Global Surface Water Dataset (GSWD) to analyze the changes in the river’s stream network. With these products, we assess changes in several fluvial geomorphological proxies (e.g., sinuosity, fractal dimension, meandering characteristics, planform information) and identify possible tipping points. We relate these changes to different hydrological and climatic conditions such as precipitation, river discharge and Lake Baikal water level. We find evident changes in the meandering behaviour and flow path of the Selenga River tributaries in the Delta. The number of oxbow lakes based and corresponding size distribution has varied in time, and evident flow path changes occur that seem to be related to flooding periods, and there appears to be a consistent relationship between meandering and the river discharge variability. These results enable policymakers to understand different contributing factors altering the Selenga River Delta and ultimately leading to better decisions to manage the effects of these changes in the area.

The identification of tipping points in the water cycle has been recently ranked Nr. 1 in the list of the top 23 unresolved problems in Hydrology by the International Association of Hydrological Sciences (IAHS) and as a priority in the field of hydrology and water resources by several studies. Such daunting task is mainly attributed to the concerns that greenhouse gas emission climate change may tip the water cycle into an unfavorable new state. Up to date, tipping points occurring in complex dynamical systems have been identified across a large set of disciplines. In most proven tipping points, hydrologic variables are always taken as the control variables, as changes in water fluxes and stocks are known to act as stressors of socioecological systems, and the affected aquatic and terrestrial ecosystems as the response variables. The main objective of this study is to explore the existence of tipping points in catchment-scale freshwater availability, that is, the tipping points were the response variable is catchment water storage. We first review the existence of reported tipping points in the field of hydrology and water resources, to establish a coherent framework for the identification of hydrological tipping points. We explore their mathematical existence at the catchment scale by Linear Stability Analysis, illustrating cases with potential functions and bifurcation diagrams. We then explore any possible contribution to the existence of hydrological tipping points by adding complexity to the hydrological dynamic system through the inclusion of sociological feedbacks. We find that even with the inclusion of the moisture feedback of evapotranspiration to precipitation, constant socioecological conditions will most likely not present tipping points of water storage in the catchment. However, the inclusion of socioecological feedbacks does generate tipping points under certain assumptions, even without assuming a moisture feedback between evapotranspiration and precipitation. We hope that this study sheds some light on the existence, conditions, assumptions and characteristics of large-scale hydrological tipping points with long-term implications.

During the last six decades, forest biomass has expanded in the Northern basins, mainly due to forest management. This expansion should imply an increasing effect on evapotranspiration. However, increasing global CO2 emissions also trigger physiological plant water saving responses that induce an opposite effect on evapotranspiration. The dominant long-term and large-scale effect on evapotranspiration is still a matter of debate. In this study, we determined the dominant effect on evapotranspiration in Northern forests during the period 1961–2012 by studying change-effects on the ratio of actual evapotranspiration to precipitation, known as the evaporative ratio. We used the Budyko framework of water and energy availability at the basin scale to study the hydroclimatic movements in Budyko space of 65 Swedish basins. We found that changes in the evaporative ratio in 60 % of these basins could not be explained by climatic changes in precipitation and potential evapotranspiration. In both the temperate and boreal basin groups studied, a positive residual effect on the evaporative ratio counteracted the negative climatic effect. Furthermore, temporal change of this residual effect during the period 1961–2012 agreed with that of the standing forest biomass in both the temperate and boreal basin groups as well as with that of the forest cover area in the temperate group. Hence, our long-term and regional-scale results indicate that a positive effect on evapotranspiration from the increasing forest biomass overrode any possible negative stomatal water saving response from increasing atmospheric carbon dioxide concentration. Thus, we suggest that forest expansion is the dominant driver of long-term and large-scale evapotranspiration changes in Northern forests.