Introduction

"Water is the bloodstream of the biosphere" is a wise insight coined by Professor Malin Falkenmark (Falkenmark and Biswas 1995), a world-leading international hydrologist, who passed away on 3 December 2023, at the age of 98 years (Fig. 1). Falkenmark was a scientific visionary, calling for global water stewardship as a fundamental step towards human development, even before modern thinking on sustainable development was established through the 1987 Brundtland Commission and the 1992 Agenda 21 following the United Nations Conference on Environment and Development in Rio. Her lifelong passion was to eradicate water poverty in the world, and to do this with hydrological evidence and inter-disciplinary collaboration. She co-developed the most prestigious award in water science—the Stockholm Water Prize, and received multiple awards herself, including the prestigious Volvo Environment Prize in 1998 and the Blue Planet Award in 2018.

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Photo: Stockholm Resilience Centre

Malin Falkenmark (1925–2023).

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Academic contributions

Falkenmark’s contributions to science fundamentally challenged our perception of the water cycle. Her persistent influence on water science and practice cannot be overstated: she pointed to the modern disruption of the water-bloodstream at a global scale owing to human activities (e.g., Falkenmark and Rockström 2006; Falkenmark 2009; Falkenmark et al. 2018); she brought to light the fundamental importance of the invisible “green water” stored in soils and circulating as evapotranspiration (challenging and correcting the prevailing narrow focus on “blue” water in rivers, lakes, reservoirs, and aquifers, inherited by reductionist concepts of the water cycle of past decades) (e.g., Falkenmark 1995a; Falkenmark and Rockström 1996); she suggested to turn around our perspective in water management, no longer asking where to get ever more water from but how to make best use of available water (Falkenmark et al. 1989); and she promoted a fair balance of water for humans and nature in an ethical framework of hydro-solidarity and social-ecological resilience (Falkenmark and Folke 2002).

The Falkenmark water scarcity index (1970s and 1980s)

Falkenmark dedicated her life to the role of water for human development, and in particular to regional differences in hydro-climatic predicaments and what this means for food security and human well-being. She developed the empirically based, now so-called Falkenmark Index, presented in a ground-breaking paper in Ambio in the mid-1980s (Falkenmark 1986), which still today constitutes the standard water scarcity index in global assessments. In that analysis, Falkenmark combined national data on freshwater resources with socio-economic data and population numbers, finding that in countries with > 600 cap/Mm3/yr of accessible freshwater (blue), water management tends to get stressful, and difficult at ~ 1000 cap/Mm3/yr (see Falkenmark et al. 2013) (Fig. 2). With this calculation, Falkenmark introduced the notion of “Water Crowding” (which better highlights the population pressure on water resources than its inverse, the now mostly used Falkenmark Index; Ashton 2002). She showed that water-scarce countries are predominantly among the world’s most vulnerable and poor countries and that population growth without development pushes them further into absolute water scarcity as more people have to share each water unit (Falkenmark et al. 1989).

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Malin Falkenmark’s classic visual of water scarcity categories in the world. Each water cube was defined as a “flow unit” of one million m3/yr of available (blue) water in the terrestrial water system. Nations with > 1000 people sharing a flow unit face absolute water scarcity (Falkenmark 1989)

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Her concern over the world’s most water-poor emerged already in the 1970s. In 1974, she published research in Ambio (Falkenmark and Lindh 2020), analysing different water predicaments in different parts of the world. During the 1970s, she worked for the United Nations and wrote the report Water for a Starving World (Falkenmark and Lindh 1977), which was one of the underlying reports in the UN conference on water in Mar del Plata in 1977 where Falkenmark was General Rapporteur.

Falkenmark’s scientific breakthrough came in the mid-1980s with a series of papers on the role of freshwater for human development, starting with the seminal 1986 Ambio article "Freshwater—time for a modified approach" (Falkenmark 1986). This body of research was heavily inspired by her fellow global climatologists and hydrologists like Mark Isaakovich L’vovich, Michail Budyko, Peter Gleick, Charles Vörösmarty, and Sandra Postel. In particular, L’vovich’s World Resources and their Future (L’vovich 1979) strongly influenced Falkenmark's thinking. Here, for the first time, freshwater resources were mapped across the entire world, presenting hydroclimatic differences between different regions, in particular in terms of the partitioning between stable runoff and vapour fluxes of evapotranspiration.

Green and blue water and ecohydrology (1990s)

In the mid-1990’s, Falkenmark introduced the green and blue water concept (e.g., Falkenmark 1995a; Falkenmark and Rockström 1996), which has been widely adopted among water resource analysts and played a pivotal role for green water (soil moisture, evaporation, and transpiration) to be recognised as critical freshwater resources in urgent need of attention in both policy and science (Fig. 3). The mainstream focus on freshwater resources in rivers was in Falkenmark’s words ‘blind’ to the fact that the vast majority of the world’s cropland and food production (as well as the terrestrial ecosystems) in fact rely on green water. The concept of blue and green water has since its introduction helped clarify the importance of rainwater harvesting in agriculture and holistic integrated land–water management, and it has inspired further conceptual developments such as the distinction of blue and green water in assessments of water footprints and virtual water trade (e.g. Mekonnen and Hoekstra 2011), and more recently the planetary boundaries (Richardson et al. 2023).

Fig. 3
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The conceptual figure of the partitioning of precipitation into green and blue water resources and flows. Green water resource is the soil moisture in the unsaturated zone of the soil, and green water flow the water vapour flow of evaporation and transpiration. Water in lakes, dams, and groundwater is the blue water resource, and surface and sub-surface runoff flow is the blue water flow (Falkenmark and Rockström 2006)

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The green water concept was first introduced by Falkenmark at an internal seminar at the Food and Agriculture Organization of the United Nations (FAO 1995; Falkenmark 1995a), showing her genuine dedication to putting "science to work". Here she proved, in her typically quantitative and technical way combined with artistic drawings, iconic terms, and conceptual vision, that the conventional stable runoff focus (until then seen as the "only" accessible freshwater resource for development), was insufficient if the world is serious about lifting people out of poverty and meeting rapidly rising food needs. She claimed that such a view overlooks that rainfed agriculture, being practiced on 80% of the world’s cropland to produce 70% of the world's food, is independent of blue water but solely relies on green water. In sub-Saharan Africa, 95% of food production is dependent on green water, as always emphasised by Falkenmark. Managing green water resources (soil moisture) and green water flows (evaporation and transpiration), is therefore of critical importance, to lift water scarcity-prone regions out of food insecurity and vulnerability (Falkenmark 1989, 1995a, 1995b).

It was at the end of the 1990s that Falkenmark contributed to the integration of eco-hydrology with water resource research and management. A key step in this work was the publication in 1999 of the world's first attempt to quantify green water flows to sustain ecological functions in the biosphere on land (Rockström et al. 1999). This was followed by Falkenmark's efforts to develop an integrated conceptual framework, combining water resource development for socio-economic development with water for aquatic and terrestrial ecosystems (Falkenmark 2001, 2003; Falkenmark and Folke 2003; Falkenmark and Rockström 2004).

Eventually, a Green–Blue Water Initiative (GBI) was launched in 2005 (Falkenmark and Rockström 2005; Hoff et al. 2010a), engaging scholars and experts from the International Water Management Institute (IWMI), the International Union for Conservation of Nature (IUCN), the World Bank, the Potsdam Institute for Climate Impact Research (PIK), the Global Water Security & Sanitation Partnership (GWSP), UNESCO-IHE, the International Soil Reference and Information Centre (ISCRIC) and universities in the global South, like the Association for Strengthening Agricultural Research in Eastern and Central Africa (ASARECA) in East Africa, Mekelle University, in Ethiopia, and Anna University in India. This was an international and interdisciplinary green–blue water resource R&D effort, combining modeling, analyses with policy advice on the importance of taking an integrated approach to green and blue water for sustainable development. Results included first multi-model estimates of global green and blue water availability and use in agriculture (Hoff et al. 2010b), and since, green water is a standard variable analysed in intercomparison assessments such as by the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) (e.g. Liu et al. 2022).

In 2009, Falkenmark was one of the co-authors of the first paper on the Planetary Boundaries Framework dedicated to defining precautionary limits of human interference with key Earth system processes (Rockström et al. 2009), leading the work on the scientific evidence in support of a global freshwater boundary. This proposal has contributed to answering the question of how much modification of the global freshwater cycle can be tolerated so that Earth system stability would not be put at risk (e.g. Gerten et al. 2013; Gleeson et al. 2020a, b; Wang-Erlandsson et al. 2022).

The triple green revolution in Africa (1990–2020)

With the adoption of the UN Sustainable Development Goals (SDGs) in 2015, Falkenmark became increasingly concerned over the world's inability to take water scarcity, and increasingly climate change, and ecologically-induced human water extremes, seriously in policy and management. A particular passion of hers was to mobilize the world to collaboratively lift water-scarce regions in sub-Saharan Africa out of their water development limitations (Fig. 4). She often came back to the fact that Africa is the driest continent on Earth, with the highest share of its population (approx. 40%) living in semi-arid hot tropical regions (covering approx. 40% of the Earth’s surface)—yet with high unused potentials of green water (Rockström and Falkenmark 2015).

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The systemic hydro-climatic and food insecurity risks and challenges in sub-Saharan Africa (Falkenmark and Rockström 1993)

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Together with science and policy, this led her to launch the Triply Green Revolution in Africa initiative, sparked by a public statement at the World Water Week in 2016 (SIWI 2016), and a call for action in Nature journal (Rockström and Falkenmark 2015). The Triply Green Revolution idea was inspired by Conway's idea of a double-green revolution (green as sustainable and green in terms of the need for sustainable intensification, i.e., more food per unity input) (Conway 1999). The 3rd "green" was, obviously, Falkenmark’s green water, i.e., the need to invest in managing rainfall and local soil moisture to build more resilient rainfed agricultural systems in Africa—a precondition to meet the SDGs 1 and 2 on eradicating poverty and food insecurity.

Social-ecological resilience (2010–2023)

Falkenmark was central in advancing the role of water in building social-ecological resilience, both in terms of mitigation, adaptation, and social transformation (Rockström et al. 2012, 2014; Falkenmark and Wang-Erlandsson 2021). A key part of this work is the role of anthropogenic changes to the hydrological cycle with a particular view on triggering extreme events and potentially tipping points (Falkenmark et al. 2018). Her last work was concerned with the importance of the African rainforest as a precipitation shed for cropland across the continent (Nyasulu et al. 2024).

Throughout Falkenmark’s academic life, she has repeatedly returned to the notion of water as the ‘Bloodstream of the Biosphere’. The notion of the water cycle as the bloodstream echoes lines of thought developed in the seventeenth century, which suggest an analogy between the vital functions of cycling water on Earth and the blood circulation in the human body (Gerten 2024). For Falkenmark, water’s role as the bloodstream does not only convey the role of water for transporting solutes (Falkenmark and Biswas 1995) but encompasses the multiple functions of water for ecosystems and society—the entire ‘life-support system’ that is the premise for the human existence (Falkenmark 2009; Falkenmark et al. 2018) (Fig. 5).

Fig. 5
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Integrated approach to freshwater connecting the biosphere, society and the global hydrological cycle (Falkenmark 2020). Reproduced with permission from www.tandfonline.com

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An impressive professional journey as a path-breaking female scientist

Falkenmark embarked on her studies in hydrology in the 1950s in a male-dominated profession, to become Sweden's first Professor in International Hydrology. She gave birth to three children, lost her husband much too early, and published her most cited scientific paper at the age of 60 years (in 1986) when she introduced the water crowding index. A remarkable academic career spanning over 30 years followed.

28-year-old Falkenmark was working for the Swedish Meteorological and Hydrological Institute (SMHI) measuring environmental impacts on lakes and rivers arising from hydropower development. In parallel, she studied hydraulics at the Swedish Royal Institute of Technology (KTH) and hydrology at Uppsala University. After this, she joined the National Scientific Research Council to take responsibility for Swedish participation in the International Hydrological Decade (1965–1975). The appreciation for her commitment led to her subsequent appointment as the General Rapporteur of the UN Water Conference in Mar del Plata (1977; Fig. 6). Her leadership and expertise were broadly recognised, and she was asked to serve on several international platforms, such as a member of the UN Committee on Energy and Natural Resources for Development and the UN Millennium Project Task Force for Environmental Sustainability; vice-president of the International Water Resources Association, and Scientific Advisor to the Global Environment Facility and the Comprehensive Freshwater Assessment of the World.

Fig. 6
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Malin Falkenmark in her role as Rapporteur General at the UN global water summit in Mar del Plata in 1977. It was to this UN Summit that Malin led the first report on water scarcity and poverty in the world: UN Report "Water for a Starving World" based on Falkenmark and Lindh (1977)

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She led the development of the Department for Water and Environment Studies (“Theme Water”) at Linköping University, which she joined in 1980, and was then appointed Professor in International Hydrology at the Swedish Natural Science Research Council (later to become the Swedish Research Council) in 1986. In 1991, she joined the Natural Resource Management group at the Department of Systems Ecology at Stockholm University, to strategically advance her green–blue water thinking across human and ecological water demands and resilience building. It is along this interdisciplinary path that Falkenmark became a globally recognised scientific voice not only for water resource research, but also for water resilience thinking, and social-ecological systems thinking on freshwater (Falkenmark and Folke 2003; Rockström et al. 2012; Falkenmark and Wang-Erlandsson 2021).

Falkenmark has educated an entire generation of water professionals, and her legacy is also institutionalised through several initiatives. Among others, she led the establishment of the annual Stockholm World Water Week (initially named Stockholm Water Symposium), and she was the Chair of the Scientific Program Committee at the Stockholm International Water Institute (SIWI) (1991–2003). Falkenmark had a pragmatic and long-term perspective, which combined with a strong passion for mentoring future water professionals, made her an appreciated teacher educating students and supervisor of early career researchers. She actively mentored students and published scientific articles even in her 90s. And, those of us who had the privilege to work closely with her, felt that if anything, her tempo increased over the years.

A continued inspiration

Falkenmark was deeply concerned and felt an urgency over the water challenges facing the world, combined with a pragmatism that her time on Earth would one day come to an end. And she had (she felt) so much to accomplish, which she did, and her water legacy is so strongly codified in science, policy, and society at large, that there is no way her thinking gets lost. On the contrary, as she would have wished, there are many water professionals across the world, inspired by her work, and enriching even further evidence-based solutions of how to solve the global water crisis and build a more water-resilient world. Malin Falkenmark’s legacy will remain a continued inspiration for water and environmental science and practice in the decades to come. Fittingly, her lifelong dedication to science and society gained her the nickname ‘Madame Evergreen’.