Too little, too much, too dirty water: Where, when, why? – Fellows’ seminar by Georgia Destouni

22 April 2024

“I’ve been fascinated by water all my life. The multidimensionality of water makes it simultaneously exciting and commonplace. H2O is the only chemical formula known by everyone,” said Georgia Destouni of the Department of Physical Geography, Stockholm University. “It’s a precondition for life as we know it and it’s the weirdest liquid on the planet, which doesn’t behave like other liquids. Because it’s everywhere, it’s also nowhere. Important realities are often the hardest to see.”

“Humans are mostly water. It fulfils many functions in the body,” she added. “We can’t be without it for more than a couple of days.”

“From a research perspective I look at it from a macroscopic scale – in nature, society, and ecosystems, and the links between them. Important links that we need to understand.”

“We are to some extent fooled by biases in data and models,” she added. “Water studies on and data for Africa in particular are major gaps.”

Following the water

Destouni’s work is about trying to understand where the water is, why there is occasionally too little (drought) or too much (flooding), and the where, why and when it may be too dirty (polluted). The aim is to understand trends and events that negatively affect the planet’s water, and how we can mitigate and protect ourselves and our ecosystems against their impacts going forward.

“We experience global change to a large degree through the variability of Earth’s water on land, where our human societies reside,” she explained. “As we move in space and time, water availability and quality may decrease or increase, while drought and flood extremes may become more or less frequent, intense, and/or temporally extended. Open research questions include: Do distinct water variability and change patterns emerge from past, through present, to projected future times around the global land area? Do the terrestrial water fluxes accelerate or decelerate? Do land conditions become wetter or drier? How do changes in water quantity and quality interplay with those in climate, human land use, and other societal and environmental developments? Like water itself, these questions are essential and we need to answer them to plan for and achieve sustainability for human societies, socio-economic sectors, ecosystems and global health.”

Destouni started by outlining the basics. She explained that most of the water on Earth is in the oceans (over 90%). Of all liquid freshwater on Earth, less than 1% is visible in lakes, streams and wetlands, with groundwater accounting for 99% and the source of one quarter of all the water used by humans. The flow rates and depths below the land surface of these large volumes of groundwater vary greatly across the globe, and not all groundwater is readily accessible or suitable for human consumption.

The water cycle consists of a combination of evapotransporation, precipitation and runoff in terms of main water fluxes, and collection in terms of water storages.

“Collection is via topographically determined hydrological catchments which are the basic spatial units to understand how water flows in nature. Any point in the landscape has an associated catchment.  We need to understand the water-flow distribution between the main water fluxes and changes in storage to understand change trends for Earth’s water on land,” she said.

Destouni explained that water cannot come from nowhere, so water fluxes and storage changes are balanced in every catchment – the weights of the different flux and storage changes in this balance can change due to climate change or changing utilisation of land and water by humans, and keeping track of this balance helps us constrain and calculate the flux and storage changes.

She also explained that we don’t know everything about the coupling of ground and surface water. “What you see in rivers is to large degree groundwater coming to the surface. Groundwater flow contributions to total runoff have been estimated to about 66% on global average, but other estimates are up to 98% – we don’t know for sure. Understanding and realistically estimating this coupling is important, for example if we want to understand how flood and drought extremes, and water quality are regulated.”

She also emphasised that we need to know where water comes from and how and where it flows, the different pathways and timeframes, to understand and be able to predict the evolution of water quality. This is the only way to link water quality to specific chemicals and pollutants, and choose effective ways and measures to mitigate water pollution and improve water quality now and for the future.

“There are also legacies from earlier pollutant inputs accumulated in the subsurface, in soil, slow-flowing groundwater and sediments, which continue to release water pollutants even after mitigation of active pollutant sources. For example, the Baltic Sea is one of the most polluted seas in the world and its water quality is not improving. We need to understand the different types of pollutant sources along all pathways of water flow that carries the pollutants. Legacy sources remain in the subsurface long after the active source inputs. We are now testing methods to distinguish the pollutant contributions from active and legacy sources in different parts of the world.”

Although we have a lot of data on all these water aspects, the data quality and the distribution of available data around the world are uneven. “We have, for example, continuous data time series fpr water flows from 1980 to 2010 for many hydrological catchments around the world,” explained Destouni. “However, these catchments still only cover 27% of the global land area (minus Antarctica) and only 14% of Africa, mostly in South Africa, Namibia, and the transboundary Congo River catchment. In contrast, nearly 80% of Europe is covered by catchments with such data, but there are important geographical gaps also there, for instance in the Eastern Mediterranean region.”

“Data from many places are needed to estimate and connect the different variables determining the balance of water fluxes and storage changes, and how this varies around the world and over time,” she added.

She outlined work considering four comparative data sources – ground observation, combined ground and satellite observation, and reanalysis with land-surface modelling and with earth-system modelling – for the following variables of water and climate over land: precipitation, evapotranspiration, runoff, soil moisture and temperature.

“These data show there is overall consistency on the warming, but large differences between datasets for water fluxes and whether they tend to be drying or wetting. This is not just a simple binary change – wetting or drying – when it comes to water on land. For example, there may be wetting in two main fluxes, like precipitation and evapotranspiration, but drying in the third, runoff, occurring at the same time. We need to understand this complexity. We use water-balance closure to check which dataset implications are realistic and which are not, for example implying huge water level decreases over time that are not observed or even physically possible, indicating a bias problem with that dataset.”

“Available data from Africa show particularly large dataset differences and associated uncertainties in the trend directions of ongoing water changes. Overall, a clear bias emerges in some data for Africa and the Southern Hemisphere, implying that use of just this data, without comparison with other datasets, can be hugely misleading.”

“We also don’t know enough about the interactions of water on land with other geospheres – the atmosphere, marine hydrosphere, cryosphere, and the anthroposphere. There are few studies of these links on large scales in the scientific literature.”

Water is the blue thread – SATORI

Destouni and colleagues research the coupling of natural-human systems, with a focus on water variability and change around the world, including in Africa and other neglected regions, at their SATORI (geoSpAtial daTa-mOdel-aRtificial Intelligence) Research Lab (SATORI Research Lab). SATORI brings together the multidisciplinary expertise of the Karolinska Institute, KTH Royal Institute of Technology, and Stockholm University as part of the ‘Stockholm Trio for Sustainable Action’ initiative. SATORI highlights water as a blue thread in its research and combines different types of geospatial data, physically based and AI/machine-learning modelling; as well as participatory approaches to address global change and sustainability challenges.

The idea is to investigate water variability and change, both as a science puzzle for the water system itself, and to look for solutions to societal water resource and water risk problems. The research areas include water availability and security; hazard and risk management; water quality; the water/energy/food nexus; cities and built environments; conflict and co-operation; and, planetary health. Specific projects include, for example, freshwater system variations; trends and drivers around the world; unravelling the legacy of historical, emerging and future groundwater pollution to the coastal ocean; understanding the balances and links between water on land and in the sea and the atmosphere; the impacts on and management of water by humans; and, what happens to water in different transition pathways to fossil-free energy systems.

“We need to understand hazards, risks and crisis management and develop early warning systems for droughts and floods. We also need to understand and model human-driven effects on such water extremes,” said Desouni. “We are trying to fill gaps in the data, as well as encourage consistency in how water is represented and understood through different disciplines and topics.”

While at STIAS, Destouni is developing a collaboration with CzASE – the Critical Zones Africa South & East Network based at the University of Cape Town. This is a large-scale, four-year project which includes aspects like critical zone appraisal; small-scale farming; African environmentalism; contaminant legacies and environmental justice; ecology economics; and, reducing precarity by amplifying habitability. It includes case studies in South Africa, Ethiopia, Mozambique, Tanzania, Malawi and Zimbabwe.

“The starting point is improving the data. Once data are available you can start answering questions about what to change to make things better,” concluded Destouni. “But, of course, data doesn’t tell us everything about how water conditions vary and change. You must look at the whole water system, including human activities using and both impacting and being impacted by these water conditions. You need to connect water in nature to the human water side which is context based. Solutions won’t happen unless people are prepared to do what it takes.”


Michelle Galloway: Part-time media officer at STIAS
Photograph: Ignus Dreyer

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