The hydrological significance of mountains: from regional to global scale

International audienceMountain regions supply a large share of the world's population with fresh water. Quantification of the hydrological significance of mountains, however, is subject to great uncertainty. Instead of focusing on global averages in advance, the present analysis follows a catchment-based approach using discharge data provided by the Global Runoff Data Centre (GRDC). The River Rhine originating in the European Alps is chosen as a first study area, revealing the hydrological relationship between mountainous and lowland regions in a well-documented area. Following the findings from this analysis, different aspects of runoff characteristics for a total of 22 case-study river basins world-wide have been investigated and compared, for a global view. The view has been extended through aspects of climate and human use of mountain runoff. The particular hydrological characteristics of mountain areas are characterised by disproportionately large discharges. In humid areas, mountains supply up to 20?50% of total discharge while in arid areas, mountains contribute from 50?90% of total discharge, with extremes of over 95%. The overall assessment of the hydrological significance of mountain areas reveals that the world's major "water towers" are found in arid or semi-arid zones where they provide essential fresh water for a significant proportion of a quickly growing global population. Keywords: mountain hydrology, global comparative assessment, runoff, water resources, sustainability, Rhine River, European Alp

Wasserschloss in einer durstigen Welt: Bedeutung der Gebirge für den Wasserhaushalt

Die Gebirgsregionen umfassen rund einen Viertel der Landoberfläche der Erde. Da sie aber weit mehr zum gesamten auf der Erde erzeugten Abfluss beitragen, als aufgrund dieses Flächenanteils zu erwarten wäre, werden sie zu Recht als 'Wasserschlösser' bezeichnet

Hochwasserereignisse aus kontinuierlicher Langzeitsimulation zur Überprüfung der Sicherheit der Stauanlagen. Schlussbericht vom 17.03.2021

Die in diesem Projekt entwickelte Methodik erlaubt es, auf der Basis von kontinuierlichen Langzeitsimulationen verschiedene Abflussverläufe von Hochwassern mit gegebenen Wiederkehrperioden durch realistische Ganglinien wiederzugeben. Der vorliegende Bericht beschreibt zum einen die Entwicklung dieser Methodik und zum anderen erste Auswertungen der Resultate aus dem Projekt „Extremhochwasser an der Aare“ (EXAR) für 19 Stauanlagen unter Bundesaufsicht im Einzugsgebiet der Aare. Der Vorteil der entwickelten Methodik ist, dass sich realitätsnahe repräsentative Ganglinien für eine Sicherheitsabschätzung zu definierten Jährlichkeitsbereichen ergeben. Dies kann zu realistischeren Abschätzungen führen als die sonst häufig verwendeten synthetischen Ganglinien, welche typischerweise nur durch einen oder zwei Parameter definiert werden. In einem ersten Schritt wurden aus den vorliegenden EXAR-Daten bivariate Jährlichkeiten bezüglich Abflussspitze und Hochwasservolumen berechnet und die entsprechenden Hochwasserganglinien bestimmten Jährlichkeitsbereichen (z.B. HQ100, HQ1’000, HQ5’000) zugeordnet. Innerhalb jedes Jährlichkeitsbereiches wurden dann die Ganglinien über funktionelles Clustering gruppiert. Dieses Clustering basiert auf einer Beschreibung der Ganglinien durch Funktionen, was bedeutet, dass die Ganglinien nicht nur nach bestimmten Charakteristika wie Abflussspitze oder Hochwasservolumen gruppiert werden, sondern die gesamte Form der Ganglinien in den Clustering-Prozess miteinbezogen wird. Aus jedem Cluster wurde anschliessend ein funktioneller Boxplot konstruiert, welcher wiederum die Form der Ganglinien im Cluster statistisch aggregiert darstellt. Die sich daraus ergebenden repräsentativen Ganglinien sollen den jeweils gewählten Jährlichkeitsbereich gut abdecken. Die Mittellinie des funktionellen Boxplots (was in etwa einem Median eines klassischen Boxplots entspricht) dient dann als repräsentative Ganglinie und entspricht einer tatsächlichen Ganglinie des Ausgangsdatensatzes. Um die Methode hinsichtlich ihrer Eignung als Grundlage für die Beurteilung der Hochwassersicherheit von Stauanlagen zu evaluieren, wurden zwei unterschiedliche Fälle betrachtet: 1) Stauanlagen mit beweglichen Organen zur Hochwasserentlastung und 2) Stauanlagen mit einem freien Überfall ohne zusätzliche bewegliche Organe zur Hochwasserentlastung. Für beide Fälle wurde jeweils der maximale Pegelanstieg im Stauraum berechnet und mit dem Volumen und der Abflussspitze der eingehenden Ereignisganglinien verglichen. In der Evaluation zeigte sich, dass die Mittellinie der funktionellen Ganglinien nicht immer am besten für eine Beurteilung der Hochwassersicherheit der Stauanlage geeignet ist. Deshalb wurden aus den funktionellen Boxplots jeweils weitere Ganglinien extrahiert. Zum einen waren dies repräsentative Ganglinien für Ereignisse mit sehr grossem Volumen innerhalb des Clusters, zum anderen repräsentative Ganglinien für Ereignisse mit grosser Abflussspitze innerhalb des Clusters. Diese zusätzlich ausgewählten Ganglinien decken den Bereich ungünstiger Pegelanstiege für die untersuchten Stauanlagen gut ab. Zusätzlich wurde die Methode univariat auf Anlagen angewendet, welche als Wehre betrachtet werden können. Der Fokus lag dabei auf der Abflussspitze. Für alle Anlagen ergaben sich mit der univariaten Methode enge funktionelle Boxplots, bei welchen die Mittellinie repräsentativ für die Kurvenschar der Jährlichkeitsbereiche war. Das 2020 angelaufene Projekt „Extremhochwasser Schweiz“ wird weiterentwickelte Langzeitsimulationen für grosse Einzugsgebiete (≥ 1‘000 km²) in der gesamten Schweiz bereitstellen und auch kleine (ca. 10–1‘000 km²) Einzugsgebiete abdecken können. Mit der hier entwickelten Methode und ersten Tests für hypothetische Anlagen mit freiem Überfall wurde eine gute Grundlage geschaffen, mit welcher diese Simulationen ebenfalls im Hinblick auf die Stauanlagensicherheit ausgewertet werden können

Human populations in the world's mountains: Spatio-temporal patterns and potential controls.

Changing climate and human demographics in the world's mountains will have increasingly profound environmental and societal consequences across all elevations. Quantifying current human populations in and near mountains is crucial to ensure that any interventions in these complex social-ecological systems are appropriately resourced, and that valuable ecosystems are effectively protected. However, comprehensive and reproducible analyses on this subject are lacking. Here, we develop and implement an open workflow to quantify the sensitivity of mountain population estimates over recent decades, both globally and for several sets of relevant reporting regions, to alternative input dataset combinations. Relationships between mean population density and several potential environmental covariates are also explored across elevational bands within individual mountain regions (i.e. "sub-mountain range scale"). Globally, mountain population estimates vary greatly-from 0.344 billion (31%) in 2015. A more detailed analysis using one of the population datasets (GHS-POP) revealed that in ∼35% of mountain sub-regions, population increased at least twofold over the 40-year period 1975-2015. The urban proportion of the total mountain population in 2015 ranged from 6% to 39%, depending on the combination of population and urban extent datasets used. At sub-mountain range scale, population density was found to be more strongly associated with climatic than with topographic and protected-area variables, and these relationships appear to have strengthened slightly over time. Such insights may contribute to improved predictions of future mountain population distributions under scenarios of future climatic and demographic change. Overall, our work emphasizes that irrespective of data choices, substantial human populations are likely to be directly affected by-and themselves affect-mountainous environmental and ecological change. It thereby further underlines the urgency with which the multitudinous challenges concerning the interactions between mountain climate and human societies under change must be tackled

Comprehensive space-time hydrometeorological simulations for estimating very rare floods at multiple sites in a large river basin

Estimates for rare to very rare floods are limited by the relatively short streamflow records available. Often, pragmatic conversion factors are used to quantify such events based on extrapolated observations, or simplifying assumptions are made about extreme precipitation and resulting flood peaks. Continuous simulation (CS) is an alternative approach that better links flood estimation with physical processes and avoids assumptions about antecedent conditions. However, long-term CS has hardly been implemented to estimate rare floods (i.e. return periods considerably larger than 100 years) at multiple sites in a large river basin to date. Here we explore the feasibility and reliability of the CS approach for 19 sites in the Aare River basin in Switzerland (area: 17 700 km2) with exceedingly long simulations in a hydrometeorological model chain. The chain starts with a multi-site stochastic weather generator used to generate 30 realizations of hourly precipitation and temperature scenarios of 10 000 years each. These realizations were then run through a bucket-type hydrological model for 80 sub-catchments and finally routed downstream with a simplified representation of main river channels, major lakes and relevant floodplains in a hydrologic routing system. Comprehensive evaluation over different temporal and spatial scales showed that the main features of the meteorological and hydrological observations are well represented and that meaningful information on low-probability floods can be inferred. Although uncertainties are still considerable, the explicit consideration of important processes of flood generation and routing (snow accumulation, snowmelt, soil moisture storage, bank overflow, lake and floodplain retention) is a substantial advantage. The approach allows for comprehensively exploring possible but unobserved spatial and temporal patterns of hydrometeorological behaviour. This is of particular value in a large river basin where the complex interaction of flows from individual tributaries and lake regulations are typically not well represented in the streamflow observations. The framework is also suitable for estimating more frequent floods, as often required in engineering and hazard mapping

Comprehensive space–time hydrometeorological simulations for estimating very rare floods at multiple sites in a large river basin

Estimates for rare to very rare floods are limited by the relatively short streamflow records available. Often, pragmatic conversion factors are used to quantify such events based on extrapolated observations, or simplifying assumptions are made about extreme precipitation and resulting flood peaks. Continuous simulation (CS) is an alternative approach that better links flood estimation with physical processes and avoids assumptions about antecedent conditions. However, long-term CS has hardly been implemented to estimate rare floods (i.e. return periods considerably larger than 100 years) at multiple sites in a large river basin to date. Here we explore the feasibility and reliability of the CS approach for 19 sites in the Aare River basin in Switzerland (area: 17 700 km2) with exceedingly long simulations in a hydrometeorological model chain. The chain starts with a multi-site stochastic weather generator used to generate 30 realizations of hourly precipitation and temperature scenarios of 10 000 years each. These realizations were then run through a bucket-type hydrological model for 80 sub-catchments and finally routed downstream with a simplified representation of main river channels, major lakes and relevant floodplains in a hydrologic routing system. Comprehensive evaluation over different temporal and spatial scales showed that the main features of the meteorological and hydrological observations are well represented and that meaningful information on low-probability floods can be inferred. Although uncertainties are still considerable, the explicit consideration of important processes of flood generation and routing (snow accumulation, snowmelt, soil moisture storage, bank overflow, lake and floodplain retention) is a substantial advantage. The approach allows for comprehensively exploring possible but unobserved spatial and temporal patterns of hydrometeorological behaviour. This is of particular value in a large river basin where the complex interaction of flows from individual tributaries and lake regulations are typically not well represented in the streamflow observations. The framework is also suitable for estimating more frequent floods, as often required in engineering and hazard mapping

Robust changes and sources of uncertainty in the projected hydrological regimes of Swiss catchments

Projections of discharge are key for future water resources management. These projections are subject to uncertainties, which are difficult to handle in the decision process on adaptation strategies. Uncertainties arise from different sources such as the emission scenarios, the climate models and their post-processing, the hydrological models and natural variability. Here we present a detailed and quantitative uncertainty assessment, based on recent climate scenarios for Switzerland (CH2011 data set) and covering catchments representative for mid-latitude alpine areas. This study relies on a particularly wide range of discharge projections resulting from the factorial combination of 3 emission scenarios, 10 to 20 regional climate models, 2 post-processing methods and 3 hydrological models of different complexity. This enabled us to decompose the uncertainty in the ensemble of projections using analyses of variance (ANOVA). We applied the same modeling setup to 6 catchments to assess the influence of catchment characteristics on the projected streamflow and focused on changes in the annual discharge cycle. The uncertainties captured by our setup originate mainly from the climate models and natural climate variability, but the choice of emission scenario plays a large role by the end of the century. The respective contribution of the different sources of uncertainty varied strongly among the catchments. The discharge changes were compared to the estimated natural decadal variability, which revealed that a climate change signal emerges even under the lowest emission scenario (RCP2.6) by the end of the century. Limiting emissions to RCP2.6 levels would nevertheless reduce the largest regime changes at the end of the 21st century by approximately a factor of two, in comparison to impacts projected for the high emission scenario SRES A2. We finally show that robust regime changes emerge despite the projection uncertainty. These changes are significant and are consistent across a wide range of scenarios and catchments. We propose their identification as a way to aid decision-making under uncertainty

Climate change and mountain water resources: overview and recommendations for research, management and policy

Mountains are essential sources of freshwater for our world, but their role in global water resources could well be significantly altered by climate change. How well do we understand these potential changes today, and what are implications for water resources management, climate change adaptation, and evolving water policy? To answer above questions, we have examined 11 case study regions with the goal of providing a global overview, identifying research gaps and formulating recommendations for research, management and policy. <br><br> After setting the scene regarding water stress, water management capacity and scientific capacity in our case study regions, we examine the state of knowledge in water resources from a highland-lowland viewpoint, focusing on mountain areas on the one hand and the adjacent lowland areas on the other hand. Based on this review, research priorities are identified, including precipitation, snow water equivalent, soil parameters, evapotranspiration and sublimation, groundwater as well as enhanced warming and feedback mechanisms. In addition, the importance of environmental monitoring at high altitudes is highlighted. We then make recommendations how advancements in the management of mountain water resources under climate change could be achieved in the fields of research, water resources management and policy as well as through better interaction between these fields. <br><br> We conclude that effective management of mountain water resources urgently requires more detailed regional studies and more reliable scenario projections, and that research on mountain water resources must become more integrative by linking relevant disciplines. In addition, the knowledge exchange between managers and researchers must be improved and oriented towards long-term continuous interaction