Hydrological methodologies are the most efficient approaches for environmental flow (eflow) assessments. This paper presents a hydrological methodology for determining eflows in rivers with scarce development to promote proactive environmental water allocations that limit flow alteration and unsustainable water use. The analysis includes the natural intra-annual and inter-annual ranges of flow variability. Eflows are determined based on four hydrological low-flow conditions and a flood regime. The main contribution is that the flow regime components are adjusted to a four-tiered environmental objective class system based on a novel “frequency-of-occurrence” approach. The method is applied in three rivers in western Mexico with highly variable flow regimes. The eflows are largely (96%) within the central range of previous implementations, and the outcomes reveal an overall good and acceptable level of the method’s performance (for 83% of the cases R ² ≥ 0.84, slope = 1 ± ≤ 0.2), consistent with supporting indices of flow variability.

Since the Brisbane Declaration in 2007, implementation of environmental flows in public policies has witnessed a steady increase around the globe. Environmental water reserves are an annual volume that is designated to remain in the ecosystem for the sustainable management of river basins. In Mexico, these reserves are determined on the basis of the Mexican Environmental Flows Norm and must be established at a river basin scale through a presidential decree for 50 years. In this manuscript, we present and discuss the implementation strategy of the norm developed for the National Water Reserves for the Environment Program, and its results in 25 reference sites based on environmental flow assessments conducted from 2012 to 2015 using hydrological and holistic methodologies. An analytical evaluation revealed an overall consistency between the Norm's environmental objectives (baseline) and the current ecological conditions on-site for the 80% of the cases (96% over high confidence rating). Furthermore, in 72% of the reference sites, the coefficient of variation among the reserve's was below the fourth quartile (<11%), whereas those remaining above that limit can be attributed to a difference in the methods' hydrologic scope. The recommended volumes for environmental allocation are feasible under the current water availability conditions in the 94% of the river basins. Although challenges have appeared in the process, to date, one reserve has been decreed on the basis of the strategic approach of setting sustainable limits of water allocation and being built an enriched flow-ecology relationships' understanding system, urgently needed to prevent ecosystems degradation and secure ecological processes.