Green water, blue water, silvergreen water, silverblue water

on planetary boundaries and the significance of different types of groundwater

In the 1980s, while studying water scarcity in sub-Saharan Africa, Swedish hydrologist Malin Falkenmark emerged as a leading voice among those who catalyzed a fundamental shift in how we understand the planet’s most vital resource, providing new language and focus for global policy. For centuries, water management had focused primarily on the visible, divertible supply - the water in rivers, lakes, and aquifers. Falkenmark recognized that this narrow scope overlooked a critical component of food security: the moisture locked in the soil that is essential for agriculture.

To make this concept graspable for governing bodies, she introduced the terminology at an FAO (Food and Agricultural Organization of the United Nations) conference: green water, which is the moisture in the soil used by plants, versus blue water, the visible, manageable water in lakes, rivers, and aquifers.

This distinction impacts water management. While traditional hydrology focuses on diverting and storing blue water, managing green water is fundamentally different. It relies on land-based practices like building up healthy soil, optimizing vegetation cover, and making sustainable land use choices.

In this essay I will further propose that there is an analogous distinction that could be useful for groundwater, classifying it into silvergreen and silverblue resources. I will explore the significance of this for the earth’s planetary boundaries.

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In 2009, Johan Rockström, a Swedish environmental science professor, gathered a group of scientists to lay out the concept of planetary boundaries for the earth. This framework has become hugely influential - it’s shaped how scientists, policymakers, and international organizations think about Earth system stability and safe operating spaces for humanity. The idea is that there are nine critical Earth system processes where we need to stay within certain boundaries to avoid catastrophic changes: climate change, change in biosphere integrity, stratospheric ozone depletion, ocean acidification, biogeochemical flows (phosphorus and nitrogen), land-system change, freshwater use, atmospheric aerosol loading, and chemical pollution. Each boundary represents a threshold beyond which Earth system functioning could be fundamentally altered in ways that undermine the conditions that have allowed human civilization to flourish.

The planetary boundaries framework has been adopted by the UN, cited in countless policy documents, and become a touchstone for thinking about sustainability. It gives us a way to think about multiple, interconnected environmental crises simultaneously rather than tackling climate or biodiversity or water in isolation.

Lan Wang-Erlandsson, a hydrologist working on Earth system science, then helped lead the work to integrate green water into the planetary boundaries framework. This was crucial because the original freshwater planetary boundary had focused primarily on blue water: consumptive use of rivers, lakes, and aquifers. But blue water is only part of the freshwater story. The other part is the water flowing through soil and vegetation back to the atmosphere.

[here’s a previous article on Wang-Erlandsson ]

She and other key people in the field co-authored a 2022 perspective piece in the pre-eminent science journal Nature. Perspective pieces are more about how we might strategize going forward, how we interpret findings, its a viewpoint. So this one brought together key researchers to articulate why the green water distinction matters for planetary boundaries.

In the piece they worked to create a planetary boundary specifically for green water. They defined it technically as the percentage of ice-free land area where root-zone soil moisture deviates from Holocene variability for any month of the year. The boundary they came up with was that at most we can have 11% of ice-free land having a deviation from normal soil moisture. The problem was we were already up to 15%.

So even though the blue water boundary was not yet transgressed, we had transgressed the green water boundary. This matters because green and blue water play fundamentally different roles in Earth system stability. You can’t manage them the same way, and you can’t substitute one for the other.

The perspective piece emphasize that green water is critical for circulation, for rain-fed agriculture which comprises a significant amount of earth, for forests and vegetation. It drives precipitation recycling, global circulation patterns, and maintains the carbon sink by helping sustain all the vegetation on earth. These are Earth system functions that blue water management - dams, diversions, irrigation infrastructure - simply cannot address. We were already disrupting the delivery of water to vegetation, global rain, and the carbon sink in a way that was transgressing planetary boundaries.

From a planetary boundaries perspective, the resilience of the Amazon and Congo rainforests is critical, as these forests are considered tipping elements of the Earth system. Radical changes in mean annual precipitation result in non-linear responses: changes in tropical tree cover, drought patterns, deciduousness in tropical forests, shifts in tropical tidal-wetland vegetation cover, reduction in evolutionary diversity, and interannual variability of net ecosystem carbon exchange. Below a critical threshold of around 2,000 mm per year, tropical rainforests cannot maintain year-round photosynthesis. Severe droughts in semi-arid regions reduce tree growth in ways that wet periods cannot compensate for, decreasing overall ecosystem production. Green water flows influence rainfall levels at the regional scale through moisture feedback and thereby the availability of blue water resources.

Other papers have estimated that 90% of global green water flows are required to sustain critical ecosystem services, whereas 20-50% of the mean annual blue water flows in river basins are required to sustain aquatic ecosystem functioning.

Groundwater

Groundwater, too, has been brought into focus by the planetary boundaries research. In a 2020 water planetary boundary paper led by hydrologist Tom Gleeson, researchers emphasized that groundwater is crucial for providing baseflow - the slow seepage of water that sustains river flows, particularly during dry seasons. This baseflow helps maintain aquatic biodiversity. Groundwater also supports wetlands and their associated ecosystems. Furthermore, the loss of groundwater contributes directly to global sea level rise, establishing it as a critical component of the Earth system.

The 2023 Planetary Boundary version 3.0 paper notes the boundary for this resource: the safe and just Earth System Boundary for groundwater is that natural and anthropogenic drawdown (total extraction and discharge) should be no greater than the average annual natural recharge.

Just as there is importance in distinguishing green water and blue water, it’s worth considering that it might be useful to distinguish two different ecological functions of groundwater. This distinction influences what we monitor and how we manage it. So here is a proposal for a distinction and color-coding scheme.

Silverblue Water - the groundwater that seeps up into rivers and wetlands.

Silvergreen Water - the groundwater that is accessed directly by vegetation roots

We could color code groundwater as silver water, as silver has that ancient vibe that groundwater has, since some of the water in aquifers comes from rain thousands of years ago.

The scientists Mooney, Richards and Caldwell figured out that that tree roots bring up water from the aquifers to hydrate the surrounding soil in a process called hydraulic redistribution. Later, Inez Fung and her team at UC Berkeley found that California trees depend heavily on this groundwater during summer droughts, using far more than previously thought. The extent of tree roots and their access to the water table directly influences the amount of silvergreen water available to the ecosystem and the atmosphere.

Michael Barlage at the National Center for Atmospheric Research found that including groundwater in climate models for the Central US increased evapotranspiration by six inches, reduced summer temperatures by about 3∘C, and increased rainfall in the region. In his models he simulated how tree roots reached down into the aquifers.

If silvergreen water contributes significantly to precipitation, it is critical part of green water for sustaining rain-fed agriculture and ecosystems. This aquifer rain can also influence large-scale atmospheric circulation and affect teleconnections (long-distance atmospheric linkages between climate patterns), making its management a global concern. During the dry season, it helps maintain crucial moisture levels needed to prevent widespread wildfires and biome phase changes. For instance, in California, groundwater withdrawal can lead to a shift in the biome due to the loss of this dry-season lifeline. The cooling effect from transpiration also makes it an essential local climate regulator.

[Here are some past articles in this newsletter about groundwater “The secret life of groundwater”, “The missing link: groundwater creates rain”, “Groundwater lessens wildfires”, “The unsung linchpin: groundwater helps stabilize the climate”,"Taming the hot winds that cause wildfires”]

To define the silvergreen planetary boundary, we consider three interconnected functions, whose failure could signal a tipping point:

1. Precipitation Recycling: Silvergreen water fuels plant transpiration, a crucial feedback that returns moisture to the atmosphere as rainfall. A boundary would require tracing how much of the water transpired by deep-rooted trees actually falls back as local or downwind rain.

2. Fire Mitigation: During dry seasons, ample silvergreen water resists ignition and slows the spread of wildfires. The boundary here would be a functional threshold: enough moisture to prevent catastrophic wildfire spread under typical climatic conditions.

3. Ecosystem Hydration: Silvergreen water sustains trees and soil health through droughts, preserving biodiversity and the carbon sink. This involves linking root depth and soil moisture availability to species survival and growth across landscapes.

The global water cycle has many key parts, each working in concert with others to form an integral whole. Blue water, green water, silvergreen, and silverblue are not separate resources to be managed in isolation, but interdependent currents in one living planetary circulation. When any link is broken, when soils dry, forests fall, or aquifers are drained, the rhythm of the whole system falters. But the same is true in reverse: when we restore vegetation, rebuild soil, and let slow water move again, the cycle begins to heal itself. Through local action that respects these hidden connections, the global water cycle can regain its balance, renewing both landscapes and climate from the ground up.

References

Barlage, M., Chen, F., Rasmussen, R., Zhang, Z., & Miguez‐Macho, G. (2021). The importance of scale‐dependent groundwater processes in land‐atmosphere interactions over the central United States. Geophysical Research Letters, 48(5), e2020GL092171

Gleeson, Tom, Lan Wang-Erlandsson, Samuel C. Zipper, Miina Porkka, Fernando Jaramillo, Dieter Gerten, Ingo Fetzer et al. “The water planetary boundary: interrogation and revision.” One Earth 2, no. 3 (2020): 223-234

Oyama, M. D., and C. A. Nobre. 2003. A new climate–vegetation equilibrium state for tropical South America. Geophysical Research Letters 30:2199. doi:10.1029/2003GL018600

Richardson, Katherine, Will Steffen, Wolfgang Lucht, Jørgen Bendtsen, Sarah E. Cornell, Jonathan F. Donges, Markus Drüke et al. “Earth beyond six of nine planetary boundaries.” Science advances 9, no. 37 (2023): eadh2458

Rockström, J., L. Gordon, M. Falkenmark, C.Folke, and M. Engvall. 1999. Linkages among water vapor flows, food production, and terrestrial ecosystem services. Conservation Ecology 3(2): 5.[online] URL: http://www.ecologyandsociety.org/vol3/iss2/art5

Rockström, Johan, Will Steffen, Kevin Noone, Åsa Persson, F. Stuart Chapin III, Eric Lambin, Timothy M. Lenton et al. “Planetary boundaries: exploring the safe operating space for humanity.” Ecology and society 14, no. 2 (2009)

Wang-Erlandsson, Lan, Arne Tobian, Ruud J. Van der Ent, Ingo Fetzer, Sofie te Wierik, Miina Porkka, Arie Staal et al. “A planetary boundary for green water.” Nature Reviews Earth & Environment 3, no. 6 (2022): 380-392

Smakhtin, V. 2008. Basin closure and environmental flow requirements. International Journal of Water Resources Development 24:227–233