Global Environmental Flows

The Global Environmental Flows for the Freshwater Quantity Target (FQT) Version 2 is derived from outputs of three Global Hydrological Models (GHMs) participating in ISIMIP Phase 3a:

For each model, monthly runoff, also referred to as Blue Water Runoff (BWR), was retrieved at a spatial resolution of approximately 30 × 30 arcminutes, or approximately 55 km, for the period 1990–2019. The modeled runoff was then aggregated to HydroBASINS Level 05 sub-catchments.

Environmental Flow Requirements (EFRs) were subsequently estimated from the modeled runoff using three environmental flow methodologies.

Smakhtin Method

The Smakhtin method estimates environmental flows using the runoff exceeded 90% of the time, \(Q_{90}\), and Mean Annual Runoff, (MAR).

\[ EFR = \begin{cases} Q_{90}, & \text{if } Q_{90} > 0.3 \times MAR \\[6pt] Q_{90} + 0.07 \times MAR, & \text{if } 0.3 \times MAR \geq Q_{90} > 0.2 \times MAR \\[6pt] Q_{90} + 0.15 \times MAR, & \text{if } 0.2 \times MAR \geq Q_{90} > 0.1 \times MAR \\[6pt] Q_{90} + 0.20 \times MAR, & \text{if } Q_{90} \leq 0.1 \times MAR \end{cases} \]

Richter Method

The Richter method defines environmental flows as 80% of Blue Water Runoff:

\[ EFR = 0.8 \times BWR \]

Pastor Method

The Pastor method defines environmental flows as a percentage of monthly flow depending on hydrological conditions. It allocates 60% during low-flow periods, 45% during intermediate-flow conditions, and 30% during high-flow periods.

\[ EFR = \begin{cases} 0.60 \times MAR, & \text{if } MAR \leq 0.4 \times MMF \\[6pt] 0.45 \times MAR, & \text{if } 0.4 \times MMF < MAR \leq 0.8 \times MMF \\[6pt] 0.30 \times MAR, & \text{if } MAR > 0.8 \times MMF \end{cases} \]

The variables used in the three environmental flow methods are:

  • BWR: Blue Water Runoff (m³/month)
  • MAR: Mean Annual Runoff (m³/month)
  • MMF: Mean Monthly Flow (m³/month)
  • Q₉₀: Runoff exceeded 90% of the time (m³/month)

Aggregation Across Hydrological Models

This process resulted in monthly EFR estimates for each hydrological model, environmental flow method, HydroBASINS Level 05 basin, month, and year.

To derive the environmental flow values presented in the dashboard, the monthly EFR estimates from the three hydrological models were combined by calculating the median value for each basin, environmental flow method, month, and year:

\[ EFR^{\mathrm{median}}_{b,e,m,y} = \operatorname{median} \left( EFR^{H08}_{b,e,m,y}, EFR^{MIROC}_{b,e,m,y}, EFR^{WaterGAP}_{b,e,m,y} \right) \]

These monthly median values were then averaged over the 1990–2019 period:

\[ \overline{EFR}_{b,e,m} = \frac{1}{30} \sum_{y=1990}^{2019} EFR^{\mathrm{median}}_{b,e,m,y} \]

In these equations:

  • b represents the HydroBASINS Level 05 basin
  • e represents the environmental flow method
  • m represents the calendar month
  • y represents the year

Global Coverage

Monthly environmental flow estimates were generated for 4,716 HydroBASINS Level 05 sub-catchments, representing approximately 86% of the 5,488 HydroBASINS Level 05 basins globally:

\[ \text{Global coverage} = \frac{4{,}716}{5{,}488} \times 100 \approx 86\% \]

References

  1. Richter, B. D., Davis, M. M., Apse, C., & Konrad, C. (2012). A presumptive standard for environmental flow protection. River Research and Applications, 28(8), 1312–1321. https://doi.org/10.1002/rra.1511

  2. Pastor, A. V., Ludwig, F., Biemans, H., Hoff, H., & Kabat, P. (2014). Accounting for environmental flow requirements in global water assessments. Hydrology and Earth System Sciences, 18, 5041–5059. https://doi.org/10.5194/hess-18-5041-2014

  3. Smakhtin, V., Revenga, C., & Döll, P. (2004). A Pilot Global Assessment of Environmental Water Requirements and Scarcity. Water International, 29(3), 307–317. https://doi.org/10.1080/02508060408691785

Interpretation Note

Important

These environmental flow estimates are derived from modeled runoff aggregated at the HydroBASINS Level 05 scale and are calculated separately for each HydroBASINS Level 05 basin.

They do not represent observed river discharge or streamflow measurements at a gaging station. They may therefore differ from environmental flow values estimated using local gage observations, locally calibrated hydrological models, or basin-specific ecological flow studies.