Modeling naturalized vs regulated flow regimes for E-Flows: Bias-corrected VegET–mizuRoute–mizuLake modelling in the Lerma–Santiago Basin, Mexico

Environmental flow (e-flow) assessment requires robust characterization of both naturalized (baseline) and regulated (current) flow regimes at ecologically relevant spatial scales. The Lerma–Santiago River Basin (Mexico) is one of the most socio-economically important and hydrologically monitored basins in the country, yet translating point-based discharge observations into consistent, basin-wide flow information for environmental flow assessments remains challenging due to flow regulation, reservoirs, diversions, and spatial heterogeneity in climate forcing. Here we present a high-resolution hydrological modelling framework designed to generate spatially explicit discharge time series for environmental flow assessment and monitoring across the full basin.

The modelling system couples the VegET agro-hydrologic model for runoff generation with the vector-based mizuRoute routing model to simulate discharge along a MERIT-derived high-density river network and associated hydrologic response units (HRUs), enabling flow estimation across both major rivers and smaller tributaries. To quantify uncertainty linked to precipitation forcing, VegET is driven by multiple rainfall datasets (CHIRPS v2, CHIRPS v3, ERA5, and MSWEP), producing an ensemble of runoff and discharge simulations. Two flow regimes are generated: (i) a naturalized regime using VegET–mizuRoute to represent baseline hydrology without reservoir regulation, and (ii) a regulated regime integrating mizuLake, which explicitly accounts for lakes and reservoirs within routing to reproduce anthropogenically altered flow dynamics. Given the strong monitoring network in the basin, simulated discharge is additionally bias-corrected using observed streamflow, improving the realism of monthly hydrographs and low-flow/high-flow characteristics.

The resulting discharge products provide a consistent, reach-scale representation of flow variability and uncertainty, delivering the core hydrological inputs required to define, compare, and track e-flow targets across the Lerma–Santiago Basin under both naturalized and managed conditions.