Hydrologic Variability and Change in the New England Region, USA
Traditional water resources management aims to ensure the steady and reliable water supply for human uses and maximize the economic benefits by dampening natural flow variability. However, such management practices essentially changes the flow regime in many ways and ecological degradation is one of the obvious consequences of that. Natural functioning and productivity of the native species require enough water flow in the streams and lake levels with sufficient quality. Thus, to protect the natural ecosystem diversity, sustainable water allocation policies have been developed and employed by many societies around the world. "Chapter 587: In-stream Flow and Lake and Pond Water Levels" is an excellent example of proactive management and planning within a water allocation framework in achieving long-term sustainability of water resources in Maine. Success of this water policy largely depends on using a reasonable guide of ranges of hydrologic variability that may occur in the future, as well as updating the policy to reflect changes in water resources from human activities. A primary context for this work is Maine's newly established water allocation framework, Chapter 587. The focus of this study is twofold: (a) to analyze a multi-century tree-ring based record of droughts in Maine and a framework to estimate watershed-specific drought risk and (b) to understand the recent changes in the streamflow variability across the New England region, with a particular focus on the nature of surface runoff and baseflow relationships. We use the multi-century reconstructed PDSI record to understand the natural envelope of drought occurrence (severity and duration) in the state of Maine. This work is motivated by the need to augment the scientific basis to support the emerging water allocation framework in Maine, Chapter 587. Through a joint analysis of the reconstructed PDSI and historical streamflow record for twelve streams in the state of Maine, we find that: (a) the uncertainties around the current definition of natural drought in the Chapter 587 (based on the 20th century instrumental record) can be better understood within the context of the nature and severity of past droughts in this region, and (b) a drought index provides limited information regarding at-site hydrologic variations. To fill this knowledge gap, a drought index-based risk assessment methodology for streams across the state is developed. Considering the importance of baseflow in river and lake stability during the dry seasons, computing the baseflow from total streamflow is another goal of this study. Three different baseflow separation algorithms were applied to thirty-one stream gauges with natural flow systems in the New England region to calculate and compare long-term Baseflow Index (BFI). A new approach is developed to determine trends at different significance level in daily streamflow, baseflow and surface runoff and applied to the abovementioned stations. In addition, clustering analysis is performed based on seasonal BFI quantiles. This work is a potential tool to support the water managers in decisionmaking in different water sensitive sectors. An improved understanding of sensitivity and severity of changes in surface runoff and baseflow is certainly important to human and ecosystem use of streamflow. Future changes, if examined in this framework, are likely to allow a reassessment of policy, a great challenge in changing climate.