Methods for estimating low-flow frequency characteristics at ungaged sites were developed for two areas in northern Florida. In the Yellow, Blackwater, Escambia, and Perdido River Basins study area (northwestern Florida), regional regression equations were developed for estimating the 7- and 30-day, 2- and 10-year low-flow characteristic (Q7,2, Q7,10, Q30,2, and Q30,10) by determining values of basin characteristics from digital Geographical Information System (GIS) coverages or hardcopy maps. A GIS, ARC-INFO, was used to quantify basin characteristics that were used in regression equations. Sources of digital data used in this analysis are elevation data, from a digital elevation model, stream length and location data from a digital hydrography coverage, and watershed boundaries digitized from topographic maps. The most accurate regression equations employed a basin characteristic that was based on a simple conceptual model of one- dimensional ground-water flow using Darcy's law. Slightly less accurate equations were obtained using drainage area as the only explanatory variable. The standard error of prediction for the Darcy and drainage area equations of Q7,2 was 65 and 74 percent, respectively; Q7,10, 58 and 62 percent, respectively; Q30,2, 51 and, 54 percent, respectively; and Q30,10, 44 and 51 percent, respectively. In the Santa Fe River Basin study area (northeastern Florida), a flow-routing method was used to estimate low-flow characteristics at ungaged sites from low stream- flow analyses based on records at gaged sites. The use of the flow-routing method is suggested for areas where regression analysis proves unsuccessful, where low-flow characteristics have been defined at a significant number of sites, and where information about the basin characteristics has been thoroughly researched. Low-flow frequency characteristics determined at 40 sites and measurements made during five synoptic runs in 1989-91 were used to develop a flow-routing method. Low-flow frequency characteristics and drainage areas were used to define river profiles for major streams within the Santa Fe River Basin. These river profiles serve as indicators of changes in a stream's low-flow characteristics with respect to change in drainage area. Unit low flows were also determined for each site where low-flow characteristics were determined. Areas of zero flow were defined for Q7,2 and Q7,10 conditions based on measurements made during synoptic runs and from low-flow frequency analyses. The flow-routing method uses the drainage areas to interpolate low-flow values between or near gaged sites on the same stream. Low-flow values are transferred from a gaged site, either upstream or downstream, to the ungaged site. A step-by-step process for flow routing must be made when tributary or other inflow enter a stream. The strength of the flow-routing method is that the values at gaged sites reflect the overall basin characteristics in the vicinity of the gaged sites. However, the accuracy of low-flow estimates may be less in areas of decreasing and increasing flow if sufficient data are not available to assess changing hydraulic and hydrologic conditions.
Additional publication details
USGS Numbered Series
Methods for estimating low-flow characteristics of ungaged streams in selected areas, northern Florida
Water-Resources Investigations Report
U.S. Geological Survey ;
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