Determining Mexican climate-adaptive environmental flows reference values for people and nature

Abstract

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.