Quantifying and predicting the benefits of environmental flows: Combining large‐scale monitoring data and expert knowledge within hierarchical Bayesian models

Despite large investments of public funds into environmental flows programs, we have little ability to make quantitative predictions of the ecological benefits of restored flow regimes. Rather, ecological predictions in environmental flow assessments typically have been qualitative and based largely upon expert opinion. Widely applicable, quantitative models would help to justify existing flow programs and to inform future planning. Here, we used a hierarchical Bayesian analysis of monitoring data coupled with expert‐derived prior distributions, to develop such a model. We quantified the relationship between the duration and frequency of inundation, and encroachment of terrestrial vegetation into regulated river channels. The analysis was informed by data from 27 sites on seven rivers. We found that longer inundation durations reduce terrestrial vegetation encroachment. For example, a 50‐day continuous inundation during winter reduced predicted vegetation cover to a median of 11% (95% CI: 7%–35%) of cover predicted under non‐inundated conditions. This effect varied among sites and rivers, and was moderated by the frequency of inundation events. The hierarchical structure improved precision of model predictions relative to simpler analysis structures. Informative prior distributions also improved precision relative to minimally informative priors. The hierarchical Bayesian analysis allows us to make quantitative predictions of ecological response under the full range of flow conditions, allowing us to assess the benefits of planned or delivered environmental flows. It can be used to make estimates of ecological effects at sites that have not been sampled, and also to scale up site‐level results to catchment and regional scales. Quantitative predictions of ecological effects provide a more objective risk‐based approach, allowing improved planning of environmental flows and building public confidence in these major investments of public funds.