Local Environmental Flows
Local environmental flows are provided as complementary information where basin-specific environmental flow assessments are available. These assessments may be derived from hydrological models, environmental flow studies, or other analyses developed for a particular basin or river system.
Unlike global environmental flow estimates, local assessments are generally more representative because they incorporate basin-specific hydrology, ecological requirements, water-management practices, and regional characteristics.
For the Rio Grande–Rio Bravo Basin, the Basin Threshold Tool provides seasonal and monthly environmental flow estimates derived from representative daily hydrographs constructed from naturalized and observed streamflow records. These estimates characterize natural, regulated, and resilient flow conditions under dry, moderate, and wet water-year types.
The dataset distinguishes three flow conditions:
- Natural Flow: Estimated streamflow under natural conditions with minimal human influence.
- Regulated Flow: Streamflow under current water-management conditions, including the effects of reservoirs, water diversions, and other human activities.
- Resilient Flow: The flow regime required to maintain ecological functions, biodiversity, and ecosystem services. In the Basin Threshold Tool, resilient flow is used as the Environmental Flow Requirement.
Functional Flow Metrics Approach
The local environmental flow dataset for the Rio Grande–Rio Bravo Basin is based on the Functional Flow Metrics framework developed by Saiz-Rodríguez et al. (2026).
The methodology uses historical daily streamflow records to characterize the annual flow regime and identify ecologically important seasonal flow components. Annual Functional Flow Metrics are calculated for each streamflow-gaging station.
Three Functional Flow Metrics are used to characterize the annual hydrograph:
- Winter Dry Season Flow: Represents winter baseflow conditions that sustain aquatic habitat and water quality during low-flow periods.
- Spring Flood Pulse: Represents snowmelt-driven high flows that maintain channel morphology, transport sediment, and reconnect floodplains.
- Monsoon Season Flow: Represents summer rainfall-driven flows that contribute to habitat diversity and groundwater recharge.
The start and end dates of these seasonal components are determined independently for each streamflow station. Therefore, the timing and duration of the Winter Dry Season, Spring Flood Pulse, and Monsoon Season vary across the basin according to local hydrologic conditions.
The 16 streamflow stations represent three hydrologic regimes.
| Hydrologic Regime | Characteristics |
|---|---|
| Snowmelt-dominated (8 stations) | Peak flows are generated primarily by spring snowmelt and are concentrated in the upper basin. |
| Monsoon-dominated (1 station) | Peak flows are driven predominantly by summer rainfall. |
| Bimodal (7 stations) | Two seasonal flow peaks occur each year: one associated with spring snowmelt and the other with the summer monsoon. |
To represent interannual variability, annual Functional Flow Metrics are summarized into three representative water-year types.
| Water-Year Type | Percentile | Description |
|---|---|---|
| Dry | 25th | Below-average flow conditions. |
| Moderate | 50th | Average or near-average flow conditions. |
| Wet | 75th | Above-average flow conditions. |
For each water-year type, representative daily hydrographs are constructed for the natural, regulated, and resilient flow conditions. These hydrographs preserve the characteristic timing and shape of the annual flow regime while representing typical dry, moderate, and wet conditions.
The representative daily hydrographs form the basis for both the seasonal and monthly environmental flow estimates provided in the Basin Threshold Tool.
Seasonal Environmental Flows
Seasonal environmental flow volumes are calculated directly from the representative daily hydrographs.
For each station, flow condition, and water-year type, the representative daily flow values are summed over the station-specific Winter Dry Season, Spring Flood Pulse, and Monsoon Season:
\[ V_s = \sum_{i \in s} V_i^{\mathrm{daily}} \]
where \(V_i^{\mathrm{daily}}\) is the representative daily flow volume on day \(i\), and \(V_s\) is the total flow volume for season \(s\).
Seasonal volumes are calculated separately for natural, regulated, and resilient flow conditions under dry, moderate, and wet water-year types.
The resulting seasonal flow volumes are reported in million cubic meters (MCM).
Using the natural, regulated, and resilient seasonal flow volumes, the Environmental Flow Deficit Ratio is calculated as:
\[ \mathrm{EFlow\ Deficit\ Ratio} = \frac{Q_e-Q_r}{Q_n-Q_r} \]
where \(Q_e\) is the resilient or environmental flow volume, \(Q_r\) is the regulated flow volume, and \(Q_n\) is the natural flow volume.
For example, when the deficit ratio is calculated for the Winter Dry Season, \(Q_e\), \(Q_r\), and \(Q_n\) represent the resilient, regulated, and natural flow volumes for that season and the selected water-year type.
The Environmental Flow Deficit Ratio provides a relative measure of the gap between current regulated flow conditions and the environmental flow requirement. Values close to zero indicate that regulated flows are already near the environmental flow requirement, while larger values indicate a greater departure from resilient flow conditions.
The corresponding Percentage Reduction Target is calculated as:
\[ \mathrm{Percentage\ Reduction\ Target} = 100 \times \frac{Q_e-Q_r}{Q_n-Q_r} \]
The seasonal Reduction Needed is calculated as the positive difference between resilient and regulated flow volumes:
\[ \mathrm{Reduction\ Needed} = \max(Q_e-Q_r,0) \]
The result is expressed in million cubic meters. A value of 0 MCM indicates that the regulated flow already meets or exceeds the environmental flow requirement.
Monthly Environmental Flows
Monthly environmental flow volumes are calculated directly from the same representative daily hydrographs used to derive the seasonal environmental flows.
For each station, flow condition, and water-year type, representative daily flow volumes are summed within each calendar month:
\[ V_m = \sum_{i \in m} V_i^{\mathrm{daily}} \]
where \(V_i^{\mathrm{daily}}\) is the representative daily flow volume on day \(i\), and \(V_m\) is the total flow volume for month \(m\).
Monthly volumes are calculated separately for natural, regulated, and resilient flow conditions under dry, moderate, and wet water-year types.
Because the seasonal and monthly values are calculated from the same representative daily hydrographs, they are internally consistent. Monthly values provide a finer temporal resolution for evaluating how environmental flow conditions vary throughout the year.
The monthly Environmental Flow Deficit Ratio is calculated as:
\[ \mathrm{EFlow\ Deficit\ Ratio}_m = \frac{Q_{e,m}-Q_{r,m}} {Q_{n,m}-Q_{r,m}} \]
where \(Q_{e,m}\), \(Q_{r,m}\), and \(Q_{n,m}\) represent the monthly resilient, regulated, and natural flow volumes, respectively.
The monthly Percentage Reduction Target is calculated as:
\[ \mathrm{Percentage\ Reduction\ Target}_m = 100 \times \frac{Q_{e,m}-Q_{r,m}} {Q_{n,m}-Q_{r,m}} \]
The monthly Reduction Needed is calculated as:
\[ \mathrm{Reduction\ Needed}_m = \max(Q_{e,m}-Q_{r,m},0) \]
The result is reported in million cubic meters. A value of 0 MCM indicates that the regulated monthly flow already meets or exceeds the monthly environmental flow requirement.
References
Saiz-Rodríguez, R., Sandoval-Solis, S., Garza-Díaz, L., Richter, B. D., & Prunes, E. (2026). Resilient Flow Regimes in the Rio Grande–Río Bravo Basin. River Research and Applications. https://doi.org/10.1002/rra.70114
Interpretation Note
These environmental flow estimates are derived from basin-specific analyses using representative daily hydrographs constructed from naturalized and observed streamflow records. They are therefore more representative of environmental flow requirements in the Rio Grande–Rio Bravo Basin than global-scale environmental flow estimates.