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A review of the literature on water governance reveals that most studies focus on blue water governance; while there is some literature on green and atmospheric water, explicit literature on how to govern green and atmospheric water is lacking. Hence, this paper addresses the question: What are the arguments for governing green and atmospheric water? In order to address this question, we have undertaken a scoping analysis of the literature on green and atmospheric water. We conclude that water governance must proactively address green and atmospheric water since: (a) blue water represents only a part of the available fresh water; (b) blue river basins represent only a subset of the wider systemic nature of water; (c) land use change has significant impacts on various water flows, which all may need to be governed; (d) climate variability and change influences blue, green, and atmospheric water availability; (e) an understanding of the socio‐ecological uses of the different colors of water is critical for a more optimal and legitimate governance of water; (f) new water technologies make it increasingly possible to modify the use of green and atmospheric water; and (g) global trade infrastructures pressurize local green water resources. Neglecting the need for explicit governance of green and atmospheric water could create new forms of “water grabbing” that would impact water availability beyond the basin scale. This article is categorized under: Human Water > Water Governance Science of Water > Hydrological Processes Water and Life > Stresses and Pressures on Ecosystems

A significant part of the global terrestrial freshwater is stored in the soil. Green water, or plant-available soil moisture, enables vegetation growth and determines vegetation form and functioning (Eagleson, 2002). In turn, vegetation cover governs many green water processes, such as infiltration capacity, evaporation, and percolation (Figure 1). Vegetation changes can affect green water dynamics that subsequently affect moisture recycling patterns by altering the magnitude and timing of evaporation and transpiration (Wang-Erlandsson et al., 2014). Terrestrial moisture recycling (TMR) is referred to as the "process of terrestrial evaporation entering the atmosphere, traveling with the prevailing winds, and eventually falling out as rain" (Keys et al., 2017: 15). Globally, 57% of the rainfall over land returns to the atmosphere via evaporation or transpiration (Eagleson, 2003;Tuinenburg et al., 2020), of which 70% rains back again over land (Tuinenburg et al., 2020). Subsequently, terrestrial evaporation and transpiration comprise 40% of the total rainfall falling over land globally (Van Der Ent et al., 2010). TMR thus represents a significant hydrological pathway for the global distribution of water.Anthropogenic land-use change (LUC) following increasing demand for food, fuel, fiber, and timber (Schyns et al., 2019) might affect TMR patterns. Some studies suggest that deforestation and vegetation reduction can disturb TMR and affect local to regional rainfall patterns (

Human actions compromise the many life-supporting functions of the global freshwater cycle. Yet, an encompassing analysis of humanity’s aggregate impact on the freshwater cycle is still missing. We compare the current state of the freshwater cycle against a stable reference state by estimating the global area experiencing streamflow and soil moisture deviations beyond pre-industrial variability range. We propose replacing the current freshwater use planetary boundary (PB) with our thus-defined freshwater change PB. Our analysis indicates unprecedented change: locally, the impacts of e.g. climate change, land use, and dams, are clearly visible. Globally, we find 70% and 44% increases in areas experiencing streamflow and soil moisture deviations. This suggests a transgression of the PB, calling for urgent actions to reduce human disturbance of the freshwater cycle.

Ecosystems around the world are coping with increasing water stress due to climate change and anthropogenic disturbance. Global warming intensifies the hydrological cycle through increasing evaporation rates and precipitation intensity (Ficklin et al., 2019;Westra et al., 2014). Furthermore, rising atmospheric carbon concentrations enhance plant productivity (leading to "global greening," see Piao et al., 2020), affecting hydrology at local to global scales (Lu et al., 2016;Zeng et al., 2018). Meanwhile, anthropogenic disturbance through direct extraction of surface-and groundwater leads to a global overexploitation of water resources (Wada et al., 2010), while land cover change has affected a third of the global land area, mostly in the direction of reducing vegetation cover (Winkler et al., 2021). While studies indicate that the effects of land cover change on the continental water cycle may be buffered by shallow groundwater (Zipper et al., 2019), land cover change has led to a general loss in the capacity of ecosystems to capture and retain water (Gerten et al., 2005), and a reduced flux of terrestrial evaporation (E, often referred to as "evapotranspiration") (Sterling et al., 2013). E is pivotal for the distribution of water on our planet, being a key contributor to precipitation (P) over land (Eltahir, 1998;Van Der Ent et al., 2010). It is estimated that globally, 70% of terrestrial E rains back over the continents , which amounts to 40% of the total terrestrial P (Van Der Ent et al., 2010). The importance of terrestrial moisture recycling varies both in time and space, with some regions being highly dependent on this "self-supplied" moisture (Holgate et al., 2020;Keys et al., 2014). In Amazonia, for instance, the contribution of local E is important to maintain forest stability and buffer against droughts.

<p>Redistribution of evapotranspiration from land via atmospheric circulation is an important Earth system process. Globally, evapotranspiration contributes significantly to terrestrial rainfall, on both regional and more remote scales. In wet, tropical regions (e.g. the Congo basin), transpiration and interception loss from the dense forest cover are the primary drivers of moisture recycling, which plays a crucial role in preserving regional ecosystem functioning. However, for semi-arid and arid regions, our knowledge on the extent and significance of evapotranspiration for moisture recycling is still very limited, despite the significance this may have for addressing challenges of desertification in times of rapid environmental change. Considering this, we are taking the Sahel region as a case study and investigate its contribution to precipitation in the African continent. In addition, we specifically study what fraction of the precipitation originates from vegetation in the Sahel through transpiration and interception loss. Our study is based on simulated atmospheric moisture trajectories derived from the Lagrangian model FLEXPART with a 1-degree resolution, driven by ECMWF reanalysis data over 1980–2016. Preliminary results show (1) the temporal variability in the contribution of the region to precipitation in African drylands, and (2) a significant contribution of local precipitation recycling. We conclude that consideration of the naturally and anthropogenically-driven greening of the Sahel, as well as land use and land cover changes in the region, may have both local and far-reaching impacts via the transport of moisture through the atmosphere.</p>

<p>Green water - i.e., land precipitation, evaporation and soil moisture - is fundamental for the functioning of the biosphere and the Earth System, but is increasingly perturbed by continental-to-planetary scale human pressures on land, water and climate systems. The planetary boundaries (PB) framework demarcates a global safe operating space for humanity, but does hitherto not explicitly account for green water. Here, we propose a green-water boundary within the existing PB framework, of which a control variable could be defined as "the percentage of ice-free land area on which root-zone soil moisture deviates from Holocene variability for any month of the year". We provide provisional estimates of baseline departures based on CMIP6 data, and review the literature on soil-moisture induced deterioration in Earth System functioning. The evidences taken together suggest that the green water PB is already transgressed, implying that human modifications of green water need to come to a halt and be reversed. Future research needs to advance our understanding of root-zone water dynamics, including associated large-scale and potentially non-linear interactions with ecohydrology, hydroclimate, biogeochemistry and societies.</p>

<p>The redistribution of biological (transpiration) and non-biological (interception loss, soil evaporation) fluxes of terrestrial evaporation via atmospheric circulation and precipitation is an important Earth system process. Overall, vegetation is the main contributor to terrestrial evaporation and subsequent precipitation over land. Yet, the specific contribution of different vegetation classes remains understudied. Here, we investigate how different vegetation classes (trees and non-tree vegetation) contribute to precipitation patterns through moisture recycling over African watersheds. Our study is based on simulated daily atmospheric moisture trajectories derived from the Lagrangian model FLEXPART, driven by 1° resolution reanalysis data over 1981–2019, aggregated at the monthly level. The data is constrained by evaporation and precipitation products, and unravels the annual and seasonal contribution from trees and non-tree vegetation to precipitation, employing fractional vegetation cover data. Our findings show that trees provide a higher water flux to precipitation over Africa compared to non-tree cover, with contributions of 777 mm year<sup>-1</sup> versus  342 mm year<sup>-1</sup>  respectively. However, the large extent of non-tree vegetation over the continent compensates for this difference and many watersheds depend even largely on non-tree vegetation for precipitation. As non-tree vegetation appears to be important for precipitation over Africa, its current contribution to water availability should not be overlooked and requires further research, particularly in relation to ongoing land use and land cover change that may affect hydrology. Providing an outlook on existing and projected land use and land cover change, we highlight the spatial heterogeneous impact on local and regional water availability over the continent.</p>