Abstract
Environmental flows are critical to the recovery and conservation of freshwater ecosystems worldwide. However, estimating the flows needed to sustain ecosystem health across large, diverse landscapes is challenging. To advance protections of environmental flows for streams in California, United States, we developed a statewide modeling approach focused on functional components of the natural flow regime. Functional flow components in California streams—fall pulse flows, wet season peak flows and base flows, spring recession flows, and dry season baseflows—support essential physical and ecological processes in riverine ecosystems. These functional flow components can be represented by functional flow metrics (FFMs) and quantified by their magnitude, timing, frequency, duration, and rate-of-change from daily streamflow records. After calculating FFMs at reference-quality streamflow gages in California, we used machine-learning methods to estimate their natural range of values for all stream reaches in the state based on physical watershed characteristics, and climatic factors. We found that the models performed well in predicting FFMs in streams across a diversity of landscape and climate contexts, according to a suite of model performance criteria. Using the predicted FFM values, we established initial estimates of ecological flows that are expected to support critical ecosystem functions and be broadly protective of ecosystem health. Modeling functional flows at large regional scales offers a pathway for increasing the pace and scale of environmental flow protections in California and beyond.
Highlights
The protection of environmental flows—water needed to sustain biodiversity and the services that healthy freshwater ecosystems support—is essential to reversing worldwide trends in freshwater ecosystem degradation (Reid et al, 2019; Tickner et al, 2020)
The predictive performance of each model was evaluated by comparing predictions of functional flow metrics with observations at gages excluded from model training
Climate variables were generally the most influential predictors in the functional flow metrics (FFMs) models, physical catchment variables were important for some metrics (Table 2; Supplementary Table S3)
Summary
The protection of environmental flows—water needed to sustain biodiversity and the services that healthy freshwater ecosystems support—is essential to reversing worldwide trends in freshwater ecosystem degradation (Reid et al, 2019; Tickner et al, 2020). To address this need, river scientists have developed a broad suite of environmental flow assessment tools (Horne et al, 2017), and advanced policy agendas for environmental flows (Arthington et al, 2018). Environmental flow protections in the western US have primarily focused on major rivers supporting Pacific salmon and trout (Oncorhynchus spp.), and other threatened fish species listed under the federal Endangered Species Act (Gillilan and Brown, 1997; Obester et al, 2022). New environmental flow approaches are needed to broaden the pace, scope, and scale of flow protections across diverse river types and geographies
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