Closure of "Regional Flow-Duration Curves for Ungauged Sites in Massachusetts"

A regional hydrologic model is developed for estimating flow-duration curves at ungauged and unregulated basins in Massachusetts. Flow-duration curves often exhibit complex shapes, requiring probability density functions with three or more parameters. This study approximates the lower half of daily flow-duration curves using a two-parameter lognormal probability density function. A conjugate gradient algorithm is employed to fit lognormal density functions to the lower half of observed flow-duration curves at 23 basins. Regional regression equations are developed to describe the lognormal model parameters in terms of easily measured basin characteristics. The resulting regional flow-duration model only requires estimates of the watershed area, and a basin relief parameter, both of which are easily obtained from U.S. Geological Survey 7.5-min quadrangle maps. In addition, confidence intervals are derived for flow-duration curves estimated at ungauged sites. A validation experiment reveals that the resulting regional hydrologic model can provide remarkably precise estimates of a flow-duration curve at an ungauged site, considering the simplicity of the model and its ease of application.

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