Statistical Models to Predict and Assess Spatial and Temporal Low-Flow Variability in New England Rivers and Streams.

In the northern hemisphere, summer low flows are a key attribute defining both quantity and quality of aquatic habitat. I developed one set of models for New England streams/rivers predicting July/August median flows averaged across 1985 to 2015 as a function of weather, slope, % imperviousness, watershed storage, glacial geology and soils. These models performed better than most USGS models for summer flows developed at a statewide scale. I developed a second set of models predicting interannual differences in summer flows as a function of differences in air temperature, precipitation, the North Atlantic Oscillation Index (NAO), and lagged NAO. Use of difference equations eliminated the need for transformations and accounted for serial autocorrelations at lag 1. The models were used in sequence to estimate time series for monthly low flows and for two derived flow metrics (tenth percentile (Q10) and minimum 3-in-5 year average flows). The first metric is commonly used in assessing risk to low flow conditions over time while the second has been correlated with increased probability of localized extinctions for brook trout. The flow metrics showed increasing trends across most of New England for 1985-2015. However, application of summer flow models with average and extreme climate projections to the Taunton River, MA, a sensitive watershed undergoing rapid development, projected that low flow metrics will decrease over the next 50 years.