Freshwater Planetary Boundary

Throughout my blog, I have been commenting on how the human use of freshwater consumption has been highly unsustainable, but now I want to address whether humans have passed the global freshwater planetary boundary (PB). This blog will explore how the PB of freshwater was determined, and its limitations. 

What is the global freshwater planetary boundary?

The PB concept was defined by Rockstrom et al. (2009) and then was updated by Steffan et al. (2015) as the global safe operating limit of certain environmental processes that humans should not move beyond. They explain that within environmental systems, there are thresholds that are intrinsic to that environmental process, however the boundaries that was defined in their study was subjective to a set of human-determined values in defining a safe limit from a dangerous limit that would have adverse and undesirable effects on the system and humans alike (Figure 1).

Figure 1. Diagram of the Planetary Boundaries concept, showing safe zone, and zone of uncertainty about the ‘threshold’ [Rockstrom et al., 2009]. 

Global freshwater use is identified as one of the nine PB, and it is categorised as a ‘slow’ planetary process which has no defined threshold but it is a system that contributes to the resilience of Earth System processes when the changes in this system at local and regional scales are aggregated (Rockstrom et al., 2009). The study estimated a boundary of 4,000km³yr^-1 (uncertainty range of 4,000–6,000km³yr^-1) of the consumptive use of blue water (rivers, lakes, reservoirs and renewable groundwater), and when freshwater resources are consumed beyond this limit, both blue and green water-induced thresholds will be met such that global moisture feedbacks, biomass production, ecosystem functioning and carbon uptake by terrestrial systems are adversely affected. 90% of global green water and 20–50% blue water is required to sustain ecosystem services and aquatic ecosystem functions, respectively (Rockstrom et al., 2009). Steffan et al. (2015) uses the same boundary values, however the study updated the control variable that was used to define this threshold by including the hydrological characteristics of a river basin i.e. environmental flow requirements. This includes identifying the amount of water that can be withdrawn from rivers at the river basin-scale without adjusting the flow regime based on the river basins hydrological characteristics which would ensure an adequate ecosystem state.

Have we passed the global freshwater planetary boundary?

Our current consumption of freshwater is ~2,600lm³yr^-1 and given that the boundary is estimated at 4,000km³yr^-1, so surely, we are still within this safe operating space. Right?

However, on the one hand, an interesting study by Jaramillo & Destouni (2015) compares Destouni et al. (2013) and Steffan et al. (2015) estimates of global freshwater use with, and argues that Steffan et al. (2015) may have underestimated total freshwater consumption whereby we may have already passed the freshwater boundary. For example, reservoir-related freshwater consumption affect humidity and evapotranspiration up to 100km from the reservoir borders, thus having a larger impact on surrounding water resources such as higher evapotranspiration rates from raised groundwater levels (Destouni et al., 2013), which contrasts to Steffan et al. (2015) study whereby the evapotranspiration losses from raised groundwater levels is negated by a corresponding decrease of evapotranspiration rates in surface water downstream. Furthermore, evaporation rates in water storage or hydropower reservoirs have increased on average. Based on the net increase in basin-related and hydropower evapotranspiration losses in Switzerland alone, Destouni et al. (2013) found that global freshwater consumption increased to 1257km³, which is ~1000km³ higher than the amount identified by Steffan et al. (2015) (Figure 2). Jaramillo & Destouni (2015) does explain that there are limitations to Destouni et al. (2013) study given that evapotranspiration losses do not include other non-hydropower sources, and that evapotranspiration rates are generally lower due to Switzerland’s cooler climate, hence the total freshwater consumption of 3,569km³yr^-1 is a conservative figure. However, Destouni et al. (2013) provides a global synthesis, accounting for Et rates in all different types of water use across the globe and find that total freshwater use amounts to 4,664km³yr^-1 (Figure 2C), showing that we already surpassed Steffan et al. (2015) 4,000km³yr^-1 limit. 

Figure 2. Comparison of freshwater consumption by (A) Steffan et al. (2015); (B) Destouni et al. (2013) and (C) Destouni et al. (2013) & Shikomanov (1997); [Jaramillo & Destouni (2015)]

On the other hand, another study by Gerten et al. (2013) argues that Steffen et al. (2015) while accounting for environmental flow regimes have overestimated the limits to which blue-water can be consumed from freshwater resource. Gerten et al. (2013) argues that while using a bottom-up approach to quantify environmental flow regimes of local freshwater resources, the limit that they proposed (~2,800mk³yr^-1) is much lower than that proposed by Steffan et al. (2015) of 4,000km³yr^-1. They argue that Rockstrom et al. (2009) and Steffan et al. (2015) used a top-down approach which was based on global estimates of water availability and other processes, rather than including the spatiotemporal patterns of regional water resources.

Concluding Thoughts:

These studies show that defining a threshold for freshwater consumption is very complicated because there are many complex processes in the water cycle that have yet to be covered and thus making it difficult to assess the certainty the limit of how much freshwater we can consume. However, setting a limit to how much freshwater that can be consumed can be useful in helping to govern how much water is being used. However, the overconsumption of freshwater in specific regions can lead to other impacts such as biodiversity loss or increased salt and chemical deposition (e.g. Aral Sea case study blog). Hence, it is important to address the unsustainable overconsumption of freshwater resources with other PB thresholds.