Accounting for environmental flow requirements in global water assessments

Abstract

Abstract. As the water requirement for food production and other human needs grows, quantification of environmental flow requirements (EFRs) is necessary to assess the amount of water needed to sustain freshwater ecosystems. EFRs are the result of the quantification of water necessary to sustain the riverine ecosystem, which is calculated from the mean of an environmental flow (EF) method. In this study, five EF methods for calculating EFRs were compared with 11 case studies of locally assessed EFRs. We used three existing methods (Smakhtin, Tennant, and Tessmann) and two newly developed methods (the variable monthly flow method (VMF) and the Q 90 _Q 50 method). All methods were compared globally and validated at local scales while mimicking the natural flow regime. The VMF and the Tessmann methods use algorithms to classify the flow regime into high, intermediate, and low-flow months and they take into account intra-annual variability by allocating EFRs with a percentage of mean monthly flow (MMF). The Q 90 _Q 50 method allocates annual flow quantiles (Q 50 and Q 90 ) depending on the flow season. The results showed that, on average, 37 % of annual discharge was required to sustain environmental flow requirement. More water is needed for environmental flows during low-flow periods (46-71 % of average low-flows) compared to high-flow periods (17-45 % of average high-flows). Environmental flow requirements estimates from the Tennant, Q 90 _Q 50 , and Smakhtin methods were higher than the locally calculated EFRs for river systems with relatively stable flows and were lower than the locally calculated EFRs for rivers with variable flows. The VMF and Tessmann methods showed the highest correlation with the locally calculated EFRs (R 2 = 0.91). The main difference between the Tessmann and VMF methods is that the Tessmann method allocates all water to EFRs in low-flow periods while the VMF method allocates 60 % of the flow in low-flow periods. Thus, other water sectors such as irrigation can withdraw up to 40 % of the flow during the low-flow season and freshwater ecosystems can still be kept in reasonable ecological condition. The global applicability of the five methods was tested using the global vegetation and the LundPotsdam-Jena managed land (LPJmL) hydrological model. The calculated global annual EFRs for fair ecological conditions represent between 25 and 46 % of mean annual flow (MAF). Variable flow regimes, such as the Nile, have lower EFRs (ranging from 12 to 48 % of MAF) than stable tropical regimes such as the Amazon (which has EFRs ranging from 30 to 67 % of MAF).

A good life for all within planetary boundaries

“…Our results suggest resource use could be reduced significantly in many wealthy countries without affecting social outcomes, while also achieving a more equitable distribution among countries. A focus on sufficiency would involve recognising that overconsumption burdens societies with a variety of social and environmental problems 42 , and moving beyond the pursuit of GDP growth to embrace new measures of progress 43 . It could also involve the pursuit of "degrowth" in wealthy nations 15 , and the shift towards alterative economic models such as a steady-state economy 24,44 .…”

“…The proposed basin-scale boundary draws on the concept of minimum "environmental flow requirements" to maintain healthy riparian/coastal ecosystems and also takes into account seasonal variation in freshwater availability by tracking monthly flows 42 . We believe it is important to take into account the spatial and temporal variation in freshwater availability, but we are unaware of any monthly, basin-scale data that also account for international trade of water-intensive products (i.e.…”

“…· Blue water and international trade. Although it is possible to measure water availability at the basin scale and in monthly intervals 42,43 , we were unable to identify a consumption-based footprint indicator (i.e. one that fully accounts for trade) that could be meaningfully compared to a monthly boundary (and hence used the blue water footprint in comparison to a per capita share of the global boundary).…”

Climate change imposed limitations on potential food production

“…These are assumed to amount to 30% of the long-term mean flow (30-year-mean) (Smakhtin et al, 2004;Gerten et al, 2013;Pastor et al, 2014), or rather the long-term streamflow that would have occurred if water hadn't been withdrawn. Note that, because the consumptive water use of other sectors is not explicitly represented, the fraction of water that may be withdrawn for irrigation was reduced to 65% instead of 70%, to not overestimate the water availability.…”

“…In the approach, the spatial extent of cultivated areas is modelled as a function of climatic 15 conditions as well as the agricultural water supply, and in regions where conditions allow for at least a minimum productivity the cultivated area is extended while in other regions the area under crops declines. To estimate the potential food production on a sustainable basis, future water withdrawals are limited to the fraction of renewable fresh water which exceeds environmental requirements (Pastor et al, 2014). Water for irrigation is removed from the river network and applied to the soil, from where it evaporates, is taken up by plants and transpired or returned to the river via subsurface runoff.…”