Accounting for environmental flow requirements in global water assessments

With growing water needs for food production, it is necessary to improve the quantification of EFRs to secure enough water for the freshwater ecosystems. Three of the methods reviewed here already existed (Smakhtin, Tennant and Tessmann) and two were developed in this study (the Variable Monthly Flow method and the Q90_Q50 method).

The Variable Monthly Flow (VFM) method mimics for the first time the natural flow regimes while being “validated” at global and local scales. The VFM uses algorithms to classify flow regime into high, intermediate and low-flow months to take into account intra-annual variability by allocating EFRs with a percentage of mean monthly flow (MMF). The Q90_Q50 method allocates annual flow quantiles (Q50 and Q90) depending on the flow season.

The results showed that, over all methods, 37% of annual discharge was allocated to “Nature” with a higher pressure on low flow requirements (LFR = 46% to 71% of average low flows) than on high flow requirements (HFR = 17% to 45% of average high flows). Environmental flow methods using fixed annual thresholds such as Tennant, Q90_Q50 and Smakhtin seemed to overestimate EFRs of stable flow regimes and underestimate EFRs of variable flow regimes.

VFM and Tessmann methods showed the highest correlation with the locally-calculated EFRs (R2 = 0.91). The main difference between the Tessmann and VFM methods is that Tessmann method does not allow any water withdrawals during the low-flow season. Those five methods were tested within the global vegetation and hydrological model LPJml. The calculated global annual EFRs for “fair” ecological conditions represent between 25 to 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 (EFRs ranging from 30 to 67% of MAF).

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