Environmental flow (eflow) reference values play a key role in environmental water science and practice. In Mexico, eflow assessments are set by a norm in which the frequency of occurrence is the managing factor to integrate inter-annual and seasonal flow variability components into environmental water reserves. However, the frequency parameters have been used indistinctively between streamflow types. In this study, flow variability contributions in 40 rivers were evaluated based on hydrology, climate, and geography. Multivariate assessments were conducted based on a standardized contribution index for the river types grouping (principal components) and significant differences (one-way PERMANOVA). Eflow requirements for water allocation were calculated for different management objectives according to the frequency-of-occurrence baseline and an adjustment to reflect the differences between river types. Results reveal that there are significant differences in the flow variability between hydrological conditions and streamflow types (p-values < 0.05). The performance assessment reveals that the new frequency of occurrence delivers climate-smart reference values at least at an acceptable level (for 85–87% of the cases, r2 ≥ 0.8, slope ≤ 3.1), strengthening eflow assessments and implementations. @en

Almost a decade ago, the Mexican government targeted to establish environmental water reserves (EWR)—a volume of water allocated for ecological protection based on the Environmental Flow Mexican Norm (eflows, NMX-AA-159-SCFI-2012, ratified in 2017)—in strategic low-pressured for water use and high conservation importance river basins throughout the country. To date, 12 EWRs have been declared for up to 50 years, which encompass 295 river basins and ~55% of the national mean annual runoff (MAR). In this article, we conducted a quality evaluation of the EWRs established. First, the EWR level was analyzed against the MAR and according to wider hydrological conditions. The EWR fulfillment was evaluated by comparing the volumes enacted against the theoretical (Norm implementation). Our findings revealed that independently of individual and regional water use and conservation merits context, ~75% of the EWRs met theoretical volumes at least at an acceptable level, of which medians ranged from 24% to 73% MAR (natural parametrization and A–D environmental objectives). These outcomes prove the usefulness and consistency of the Mexican strategic hierarchical approach for eflow assessments. We aim for them to be considered as the baseline for future on-site eflow implementation and environmental water policy assessments, to show the nationwide potential benefits for protecting free-flowing rivers and to encourage a regional escalation of the strategy.@en

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.

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Environmental flows (e-flows) science has significantly advanced in the last decades. In Mexico, a standard for e-flow assessments was recently published as a regulatory instrument to support water planning and management. However, the appropriateness of the technical procedure in a climate change context has not been investigated. Do the e-flows cope with the non-stationary challenge of the flow regime and the water availability shifts in the long term? This thesis aimed to determine Mexican climate-adaptive e-flows reference values for people and nature. The research was based on state-of-the-art environmental water science and practice, and on the current national standard for conducting desktop and on-site assessments (Chapter 2). A novel frequency-of-occurrence approach for assessing e-flows and integrating regimes into volumes for water allocation was developed for perennial rivers (Chapter 3), and adjusted for intermittent and ephemeral streams. This was based on the magnitude of the contribution of hydrological wet, average, dry and very dry low flow conditions (inter-annual and seasonal variability), as well as a flood regime per stream type (Chapters 4 and 5). River discharge, basin rainfall trends, and e-flow regimes were examined in a set of 40 study cases selected according to climate, geography and hydrology representativeness. Hydrology-based likely environmental reserve volumes for preventive water allocation, expressed as a percentage of the mean annual runoff, were obtained based on a central range distribution approach. The performance assessment of these reference values demonstrated that the impact on water availability for allocating such volumes is no different from the current method (baseline) though significantly improved for avoiding under and over-estimations.@en

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.

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The study of flow-ecology and flow alteration-ecological response relationships in environmental water science is key in environmental flow assessments (EFA). Nowadays, the more advanced hydrologic-based methods in EFA focus on building those relationships for the inter-annual and seasonal flow regime variability components. In Mexico, the most detailed hydrological method is set by a norm, applicable nation-wide, in which the natural frequency of occurrence is used to integrate these variability components to the environmental flow proposals. However, to date it hasn’t been investigated the magnitude of the hydrological contributions of wet, average, dry and very dry conditions from the flow variability components and if is it differentiated on rivers and streams by climatic-influence or their geographic location. The primary goal of this work is to fill this gap. Forty-one streams were selected based on three criteria: available information to fulfill hydrological requirements, their variability and geographic representation. First, the hydrological requirements were flow records at daily scale for at least 20 consecutive years (36 with gauging stations and 5 from rainfall-runoff models). Second, the nature of current: ephemeral (19), intermittent (7) or perennial (15). Third, their geographical location in two separate subselections: a) with regard the Tropic of Cancer to cover the incidence of climatic extreme events such droughts (northern = 13) and floods (southern = 28); and b) according to exorreic (Atlantic or Pacific [13 and 21, respectively]) or endorreic (7) drainage. The inter-annual and seasonal (dry and wet) variability of wet, average, dry and very dry conditions were obtained in the form of discharge volume (m3/sec) and their relative contribution to the maximum historical of the full set of flow records. Preliminary results of inter-annual and seasonal variability show that there is a greater contribution in wet condition for ephemeral streams followed by intermittent and perennial while the contribution of average, dry and very dry conditions is inverted, all in general with the same proportions. The streams located with regard the Tropic of Cancer show the same pattern of contributions (northern is greater than southern in wet condition but inverted in average, dry and very dry conditions). Finally, although drainage zones classification presents also this same pattern (endorreic larger than exorreic in wet condition but reversed for the rest), streams that drain to the Pacific Ocean have almost the same magnitude of contribution in all conditions than the median from all streams. Furthermore, the streams that drain to the Atlantic and Pacific Oceans have practically the same level of contribution in average and dry conditions for the inter-annual and wet season variability, and average for the inter-annual. These results suggest that the nature of current appears to be the most comprehensive classification that better reflects climatic-influence. Future research will focus on (1) conducting a variance hypothesis test; (2) if there is a meaningful variance, what does this contribution means in terms of ecological and biological processes?; and (3) assessing whether the frequency of occurrence of dry or wet conditions are increasing or decreasing. @en