The Connecticut Streamflow and Sustainable Water Use Estimator: A Decision-Support Tool to Estimate Streamflow and Water Availability at Ungaged, User-Defined Stream Locations in Connecticut

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The Connecticut streamflow and sustainable water use estimator is a decision-support tool that provides estimates of daily unaltered streamflow, water-use adjusted streamflow (for the portions of the state where water-use data are available), and water availability for ungaged, user-defined sites in Connecticut. 

USGS reference streamgages used for developing regression equations in Connecticut

Locations of 61 U.S. Geological Survey reference streamgages with drainage basins, including 36 streamgages used for developing regression equations in the Connecticut Streamflow and Sustainable Water Use Estimator, Connecticut and vicinity.

    Freshwater streams in Connecticut are subject to many competing demands, including public water supply; agricultural, commercial, and industrial water use; and ecosystem and habitat needs. In recent years, drought has further stressed Connecticut’s water resources. To sustainably allocate and manage water resources among these competing uses, Federal, State, and local water-resource managers require data and modeling tools to estimate the water availability at a variety of temporal and spatial scales for planning purposes. The Connecticut Streamflow and Sustainable Water Use Estimator (CT SSWUE), developed by the U.S. Geological Survey in cooperation with the Connecticut Department of Energy and Environmental Protection, is a decision-support tool for estimating daily unaltered streamflow and sustainable water use at ungaged sites in Connecticut.

    The CT SSWUE estimates unaltered daily mean streamflow and water-use-adjusted streamflow for the period from October 1, 1960, to September 30, 2015, and the monthly sustainable net withdrawal at ungaged sites in Connecticut. Unaltered streamflow is the estimated daily mean streamflow in a drainage basin in the absence of any water withdrawals or wastewater discharges and with minimal human development. Sustainable net withdrawal is the maximum net withdrawal (withdrawal minus wastewater discharges) that can be drawn from a basin without critically depleting the water available through natural streamflow patterns. Sustainable net withdrawal is defined for this study as the difference between the unaltered daily mean streamflow and a user-defined target minimum streamflow.

    Weighted least squares and Tobit regression techniques were used to develop equations for estimating streamflow at ungaged sites at 19 streamflow quantiles with exceedance probabilities ranging from 0.005 to 99.995 percent. Regressions were based on streamflow quantiles and basin characteristics from 36 reference streamgages in and around Connecticut. Four basin characteristics―drainage area, mean of the soil permeability, mean of the average annual precipitation, and ratio of the length of streams that overlay sand and gravel deposits to the total length of streams in the basin ―are used as explanatory variables in the equations. At an ungaged site, interpolation between the streamflow quantiles estimated from the regression equations produces a continuous flow-duration curve. A time series of daily mean streamflow at an ungaged site is then estimated by assuming that for each day, the streamflow quantile occurs on the same date at both a reference streamgage, and the ungaged site.

West Cornwall covered bridge spanning the Housatonic River near Cornwall Connecticut

West Cornwall covered bridge spanning the Housatonic River near Cornwall Connecticut

(Credit: Tracey Thayer. Photograph by Tracey Thayer, used with permission.)

    The CT SSWUE computes water-use-adjusted streamflow using spatially referenced water-use information provided by the Connecticut Department of Energy and Environmental Protection. Available water-use information included permitted and registered water withdrawals and permitted wastewater discharges during 1998 to 2015 for the Thames River Basin and central coastal drainage basins. Water-use information was incorporated into the U.S. Geological Survey StreamStats web application (https://www.usgs.gov/mission-areas/water-resources/science/streamstats?qt-science_center_objects=0#qt-science_center_objects) for Connecticut and can be used for computing water-use-adjusted streamflow and sustainable net withdrawal at selected points of interest. Altered daily streamflow is computed by applying average daily withdrawals and wastewater discharges to the water balance equation. Average daily surface water withdrawals and wastewater discharges are applied directly to the daily water balance equation. Time-lagged alterations on streamflow from groundwater withdrawals or wastewater discharges are estimated using a response coefficient method developed using results of previously published, calibrated groundwater models.

    This study was completed in 2018 with the publication of a Scientific Investigations Report, a software application, and a User Manual for the software.