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 This includes identifying the amount of water that can be withdrawn from rivers at the river basin-scale without adjusting the flow regime based on the river basins hydrological characteristics which would ensure an adequate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. 

Freshwater Quality and Pollution

The quantity of global freshwater resources has been changing over time and human activity has been one of the largest drivers in this change. The sustainable use of freshwater resources has been one of the main themes that I have been exploring throughout my blogs in terms of ‘quantity’. However, another aspect of sustainable use can also include the ‘quality’ of freshwater resources, since polluted freshwater resources can limit the extent to which this resource can be consumed. In this blog, I will explore how agriculture, industry and urban waste impacts the quality of freshwater resources.

Freshwater resources and ecosystems are becoming more easily polluted, whereby waste from multiple sources (agricultural fertilisers, industrial chemicals, and urban/human untreated waste) are being deposited into freshwater systems, and are ultimately polluting these resources beyond natural levels (Zamparas & Zacharias, 2014). Eutrophication is a large concern in many freshwater ecosystems due to the increased deposition of phosphorous (P) and nitrogen (N) fertilisers and other polluting chemicals from both point and non-point sources.

Non-point sources include the global consumption of fertilisers in agriculture, and the poor management of fertiliser application increases the losses of these nutrients from the soil through surface runoff processes; 20% of nitrogen fertilisers is lost through surface runoff and leaching (Khan & Mohammad, 2014). The nutrient enrichment of freshwater bodies occurs where algal blooms develop over the surface of the water which results in a huge decline in the quality of the water, inducing a state of hypoxia – hypoxia occurs when dissolved oxygen levels fall below 2ml of O₂/litre, making it difficult for oxygen dependent plants and organisms to live in (Diaz & Rosenburg, 2008). An interesting study by Withers et al. (2014) explains that there is a delay during which the effects of applying inorganic fertilisers will appear. For example, in the UK the intense application of N and P fertilisers was encouraged during post-World War II period to produce more food for the nation. However, these nutrients were stored temporarily or permanently in the soil until runoff or leaching into groundwater resources occurred, and this left behind a ‘legacy’ of background leakage of nutrients in UK inland waters. Thus, nitrogen levels appear to be increasing across UK lowland aquifers, despite lower levels of nitrogen fertilisers being used today and the stores of fertilisers in the soil and groundwater provides sources of nutrients during periods have no runoff or leaching due to low rainfall (Howden et al., 2011). Hence, freshwater resources in the UK are expected to experience long-term declines in water quality from both historical and current uses of fertilisers.

Point sources include wastewater from industrial and treatment plants, whereby pathogenic organisms, inorganic and toxic chemicals contaminate local freshwater systems and environments. For example, Lake Geneva in Switzerland supplies water to 70,000 people, however Vida Bay in Lake Geneva is one of the most contaminated areas of the Lake due to wastewater contamination (Thevenon & Poté, 2012). In this study, Thevenon & Poté (2012) uses sediment cores to reconstruct the polluting elements over a decadal timescale from 1200 to present (Figure 1). The record shows that from 1900, trace metal elements have increased significantly, compared to the steady levels between 1200-1800 such as lead (Pb) from 20-30mg/g to 60mg/g from 1600 to 1960. The sediment record at Vida Bay (Figure 2) shows an increase in caesium (¹³⁷Cs) at ~45cm of the sediment record which coincides with the construction of the outlet pipe of the wastewater treatment plant in 1964 at Vida Bay (Thevenon & Poté, 2012). Hence, the discharge of treated industrial/domestic wastewater results in further contamination of the environment and aquatic ecosystems. Furthermore, the surface sediments which contains high organic matter contents (e.g. P and N) reflects faecal indicator bacteria, Escherichia coli – in 2007, high concentrations of E.coli of 10⁴-10⁶ CFU/g was located around the Vida Bay outlet pipe and 10⁵-10⁷ around Chamberone River compared to 1996 where levels were non-existent. Such increases in trace metals and faecal bacteria in Vida Bay will have adverse effects on human health due to contamination of drinking water; this case study highlights the significant role industries have on freshwater contamination in addition to the agricultural use of fertilisers.

Figure 1. Sedimentary trace elements from centre part of Lake Geneva (Thevenon & Poté, 2012)

Figure 2. Sediment record from Vida Bay in Lake Geneva (Thevenon & Poté, 2012)

Freshwater resources are also being polluted due to the poor management of wastewater in urban areas, and predominantly in rapidly urbanising cities that are unable to implement adequate sewerage and treatment facilities. For example, the per capita pollution load of urban discharge into the Bagmati River, Kathmandu Valley in Nepal is estimated at 31gBOD/capita/day (Kam & Harada, 2001). Biochemical Oxygen Demand (BOD) increased from 3.8 to 30mg/litre from 1995 to 1998, and faecal coliform increased from 1.0x10⁴ to 8.75x10³MPN/100ml in the same period. In Dhaka, BOD and faecal coliform levels are within 20-30mg/litre and 10⁴-10⁵MNP/100 range, but the environmental standards for safe drinking water are less than 3mg/litre for BOD and 5000MPN/100ml (Kam & Harada, 2001). Current levels in Dhaka exceeds the safe human consumption limit due to pollutants from various urban sources (domestic wastewater) which are being discharged unsafely into rivers and local water sources. 

Concluding Thoughts:

Freshwater resources and ecosystems are subjected to huge declines in water quality as a result of human activities, ranging from agriculture, industrial activity and urban living spaces. In addition to the over-consumption of freshwater resources, the degradation of freshwater quality can equally diminish the amount of freshwater available to us for human use. Overall, managing both the quality and quantity of freshwater resources are important in achieving water security, especially in a warming world where water resources are becoming more scarce. The application of fertilisers should be timed so as to avoid heavy rainfall periods, industrial wastewater should be better managed before discharging into the environment and urban wastewater and sewage systems should be at the forefront of urban policies. Freshwater quality can be easily maintained but will require human intervention and better management.

The role of dams and reservoirs in the consumption of freshwater

IPCC AR5 report suggests that under climate change, surface freshwater resources will likely decline due to the increased variability in rainfall, river flow and snow melt and ice storage (Cisneros et al., 2014). Areas near the Mediterranean, Southern Africa and East Asia will likely experience lower rainfall and higher temperatures and thus some areas will become more water-stressed, and population growth and urbanisation will only add to this water stress. One adaptive method to climate change impacts is the construction of dams and reservoirs. Watts et al. (2011) strongly argues that dam construction can reduce the effects of climate change while increasing the resilience of water systems and contributing to ecosystem restoration. In this blog, I am going to explore the role of dams in addressing the needs of water stress areas, the benefits and the downfalls of dam construction.

Dams have multiple uses ranging from hydropower, inland navigation, flood control, water supply for domestic and industrial use, and water supply for irrigation. These uses can be highly beneficial for those most vulnerable to changes in e.g. rainfall. 94% and 66% of agriculture is rain-fed in sub-Saharan Africa and Asia, respectively, thus these populations are highly vulnerable to climate change (McCartney and Smakhtin, 2010). In Africa, climate change is predicted to increase droughts and rainfall will become more intense and concentrated within the space of a few months which results in floods and large runoffs (IPCC, 2007). Ideally dams can bridge the gap between rainfall variability and supply, as well as reducing the impacts of intense runoffs and floods (ICID). However, the construction of dams in these vulnerable areas may not be able to derive any benefits from it. For example, the Kariba Dam in Zambia was declared to be in “dire” condition as a result of droughts, which are intensified by climate change (Leslie, the NewYorker).

Throughout my blogs, I have been arguing how human consumption of freshwater have been unsustainable to the extent where water resources are declining across the globe, however a study by Pekel et al. (2016) presents an alternative viewpoint, suggesting that surface freshwater resources are in fact increasing. Pekel et al. (2016) explains that globally, surface water resources have permanently declined in areas such as the Middle East and Central Asia as a result of human activities and overconsumption, however there has been an increase in surface water elsewhere which is mainly through dams and reservoir filling. They found that 90,000km² of surface water has been permanently lost, however 184,000km² of permanent water has formed in areas that previously had no water and Asia gained the largest amount of permanent water of 71,000km² since 1984. In terms of quantifying freshwater resources, this study has shown that we have not lost a significant amount of freshwater, but instead we have gained almost double of what we lost. However, this does not identify whether the use of dams is sustainable; what are the impacts of dams on a hydrological regime and the local environment?

The Three Gorges Dam (TGD) is located in the Yangtze river basin and is one of the world’s largest dam with a 600km long reservoir and 4000km² storage capacity (Gleick, 2013). This dam has large benefits such as having a large storage capacity and thus an extensive water supply during periods of long drought, and reducing major flooding downstream of the Yangtze basin (Sun et al., 2012). However, there are significant concerns to the environment, and people’s health and livelihoods. For example, sediment retention can significantly damage downstream environments. Jingjiang River and Dongting Lake usually received sediments before the TDG was built, but now it supplies the Yangtze river with sediments due to the retention of sediments at the TDG (Sun et al., 2012). The decreased sediment loads can cause suffocation and abrasions to biota and habitats in the surrounding environment. Moreover, the alteration of the hydrological regime has huge implications on wetlands and lakes whereby the storage of water during the autumn period and release during winter and spring resulted in the decline of water levels in the Lake Dongting wetland. Water levels decreased by 2.11m with an extreme value of 3.02m in 2009 since the dam’s construction (Sun et al., 2012). Other environmental problems include eutrophication, algal blooms, the decline of certain fish species, and the erosion of downstream riverbeds (Xu et al., 2013). The water quality in 38 small tributaries had declined since 2003 from 14-29% by 2008 depending on the portion of the river and the frequency of algal bloom events increased from 3 to 26 between 2003 to 2010 (Xu et al., 2013). By 2008, approximately 1.13 million people were displaced and resettled to areas near the reservoir or in urban cities, however the resettlement of these populations did not consider the environmental carrying capacity of these populations thus having wider implications on local resources (Xu et al., 2013).

Concluding Thoughts:

The overconsumption and pollution of freshwater resources, and climate change largely influences water availability in many areas of the world, and so the use of dams can prove to be a solution to addressing water security issues. Storing water can provide for a constant supply of water and especially during periods of drought, thereby reducing water scarcity. The study by Pekel et al. (2016) explains that surface water supplies has increased as a result of dams and reservoirs, however, case studies such as the TDG shows that dams and reservoirs have wider implications to the local environment, water quality and populations living near water resources, such as the water level decline of Lake Dongting in the Yangtze river basin, or the decline in water quality. Thus, this should only raise concerns regarding the sustainability of using dams in addressing water scarcity issues and if we should use dams as a permanent solution for guaranteeing a constant supply of freshwater.