Multi-index summer flow regime characterization to inform environmental flow contexts: A New England case study

Abstract

Abstract Summer flow regime is an important determinant of the health and integrity of riverine ecosystems. In particular, summertime low flows and the range of variability impact the extent and connectivity of habitat, production in and composition of ecosystems, as well as water quality. Four streamflow-based metrics are used in the analysis and assessment of summertime hydrologic variability and trends: Lane Index, UK Institute of Hydrology Index, Base Flow Index, and 7-day Minimum Flow Index. In this study, historical streamflow records at 33 locations in New England (USA) were analyzed to understand the nature of trends as well as the extent of changes, individually and collectively, for the four indices. The main contribution of this paper is to develop a comprehensive characterization of the regional flow regime and trends therein. Two complementary statistical analyses applied in this study are: (a) quantile regression-based estimates of the spatial and temporal patterns of changes in flow indices variability over the past six decades and (b) interrelationship between flow indices based on non-parametric probability density estimation approach. The multi-index regional analysis revealed that flow variability (based on the Lane Index) within the summer season has increased significantly, with 25 out of the 33 streamgages showing increases, of which 12 are highly significant. Similar trends are mirrored in the UK Institute of Hydrology Index based analysis. Base Flow Index trends show decreases across 30 of the 33 streamgages at the median level, and 19 are highly significant. Trends are most pronounced at the upper quartile level, with 21 highly significant. Regional analysis of 7-day Minimum Flow Index reveal broad patterns towards increases at the three selected quantile levels ( τ = 0.25 , 0.5 , 0.75 ) with 29, 22, and 19 streamgages showing increases, of which 13, 10, and 6 are highly significant respectively. There is also diversity in trends across quantiles, wherein 17 locations show increases at all quantile levels, while 22 stations showed increases at the median level. The analysis of interrelationships between the four indices revealed inverse variations between Lane Index and 7-day Minimum Flow, and also a diversity of relationships ranging from weakly to strongly nonlinear. As such, our analysis underscores the importance of the use of multiple indices to fully assess variability and change of relevance to management and policy concerns, such as Maine’s water allocation rulemaking.