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Methods for estimating flow-duration and annual mean-flow statistics for ungaged streams in Oklahoma
| Content Provider | Semantic Scholar |
|---|---|
| Author | Esralew, Rachel A. Smith, Steven J. |
| Copyright Year | 2010 |
| Abstract | Flow statistics can be used to provide decision makers with surface-water information needed for activities such as water-supply permitting, flow regulation, and other water rights issues. Flow statistics could be needed at any location along a stream. Most often, streamflow statistics are needed at ungaged sites, where no flow data are available to compute the statistics. Methods are presented in this report for estimating flow-duration and annual mean-flow statistics for ungaged streams in Oklahoma. Flow statistics included the (1) annual (period of record), (2) seasonal (summer-autumn and winter-spring), and (3) 12 monthly duration statistics, including the 20th, 50th, 80th, 90th, and 95th percentile flow exceedances, and the annual mean-flow (mean of daily flows for the period of record). Flow statistics were calculated from daily streamflow information collected from 235 streamflow-gaging stations throughout Oklahoma and areas in adjacent states. A drainage-area ratio method is the preferred method for estimating flow statistics at an ungaged location that is on a stream near a gage. The method generally is reliable only if the drainage-area ratio of the two sites is between 0.5 and 1.5. Regression equations that relate flow statistics to drainage-basin characteristics were developed for the purpose of estimating selected flow-duration and annual mean-flow statistics for ungaged streams that are not near gaging stations on the same stream. Regression equations were developed from flow statistics and drainage-basin characteristics for 113 unregulated gaging stations. Separate regression equations were developed by using U.S. Geological Survey streamflow-gaging stations in regions with similar drainage-basin characteristics. These equations can increase the accuracy of regression equations used for estimating flow-duration and annual mean-flow statistics at ungaged stream locations in Oklahoma. Streamflow-gaging stations were grouped by selected drainage-basin characteristics by using a k-means cluster analysis. Three regions were identified for Oklahoma on the basis of the clustering of gaging stations and a manual delineation of distinguishable hydrologic and geologic boundaries: Region 1 (western Oklahoma excluding the Oklahoma and Texas Panhandles), Region 2 (northand south-central Oklahoma), and Region 3 (eastern and central Oklahoma). A total of 228 regression equations (225 flow-duration regressions and three annual mean-flow regressions) were developed using ordinary least-squares and left-censored (Tobit) multiple-regression techniques. These equations can be used to estimate 75 flow-duration statistics and annual mean-flow for ungaged streams in the three regions. Drainage-basin characteristics that were statistically significant independent variables in the regression analyses were (1) contributing drainage area; (2) station elevation; (3) mean drainage-basin elevation; (4) channel slope; (5) percentage of forested canopy; (6) mean drainage-basin hillslope; (7) soil permeability; and (8) mean annual, seasonal, and monthly precipitation. The accuracy of flow-duration regression equations generally decreased from high-flow exceedance (low-exceedance probability) to low-flow exceedance (high-exceedance probability) . This decrease may have happened because a greater uncertainty exists for low-flow estimates and low-flow is largely affected by localized geology that was not quantified by the drainage-basin characteristics selected. The standard errors of estimate of regression equations for Region 1 (western Oklahoma) were substantially larger than those standard errors for other regions, especially for low-flow exceedances. These errors may be a result of greater variability in low flow because of increased irrigation activities in this region. Regression equations may not be reliable for sites where the drainage-basin characteristics are outside the range of values of independent variables used to develop the equations. The equations are not accurate for streams that are regulated by water-supply reservoirs, streams that are affected by local irrigation activity, or streams that are affected by urban change. Methods for Estimating Flow-Duration and Annual MeanFlow Statistics for Ungaged Streams in Oklahoma By Rachel A. Esralew and S. Jerrod Smith 2 Methods for Estimating Flow-Duration and Annual Mean-Flow Statistics for Ungaged Streams in Oklahoma Introduction Successful management of reservoir operations, permitting for stream withdrawals, and maintenance of water-quality standards are supported by knowledge of the magnitude of flow that can be expected in a stream at any given time. Flow-duration statistics that describe the percentage of time a stream discharge is equaled or exceeded, and annual meanflow statistics (mean of annual daily flow), commonly are used by State agencies and other water-resources managers to help effectively manage Oklahoma surface-water resources (R.S. Fabian, Oklahoma Water Resources Board, oral and written communication, June 2007). The historic continuous daily streamflow information collected at U.S. Geological Survey (USGS) streamflow-gaging stations provides a valuable resource for estimating flow statistics during a range of temporal circumstances, including annual, monthly, and seasonal time periods. This information can be calculated at streamflow-gaging stations where long-term daily streamflow record is available, generally 10 or more years of record, and is the criteria used in previous studies in Oklahoma (Heimann and Tortorelli, 1987; Tortorelli, 2002; Lewis and Esralew, 2009). The Interagency Advisory Committee on Water Data also recommends at least 10 years of data for computation of long-term streamflow statistics (Interagency Advisory Committee on Water Data, 1982). The USGS has collected daily streamflow at 235 gaging stations in the study area for 10 or more years (fig. 1). The number and location of long-term gaging stations is limited in any given drainage-basin by user priorities and geographical constraints. The operation of a long-term gaging station at every location where streamflow is needed is not feasible. Because of the need for estimating flow-duration and annual mean-flow statistics for ungaged stream locations in Oklahoma, the USGS, in cooperation with the Oklahoma Water Resources Board (OWRB), investigated methods for estimating long-term streamflow statistics at ungaged stream locations. Streamflow information can be extrapolated for an ungaged stream location near a gaging station on the same stream reach, from the gaging-station record, to estimate the flow statistics. The drainage-area ratio method is the preferred method for estimating flow statistics (Ries and Friezs, 2000). However, alternative methods are needed in Oklahoma to estimate flow statistics at ungaged stream locations that are not near gaging stations. Statistics calculated at gaging stations with long-term daily mean-streamflow record are highly variable at in any given drainage basin. Streamflow statistics are strongly related to physical drainage-basin characteristics such as size, topography, geology, land-cover, and climate (Ries and others, 2008). Regression equations that describe the relation of physical drainage-basin characteristics to streamflow can be developed to estimate streamflow statistics at ungaged stream locations for flow-duration and annual mean-flow statistics. One of the challenges involved with developing regression equations that accurately describe flow is selection of the appropriate independent variables. Drainage-basin characteristics that influence streamflow variability such as contributing drainage area, drainage-basin hillslope and main-channel slope, soil permeability, forested area, and mean annual, seasonal, and monthly precipitation, vary across Oklahoma. The influence that these variables have on streamflow also may vary across Oklahoma. For example, streamflow may be affected by the amount of forested area in some drainage basins in eastern Oklahoma that are heavily forested, but forested area may not have a strong influence on streamflow in most drainage basins in less forested western Oklahoma. Selection of independent variables for regression equations that account for the variation in streamflow statistics for the entire state may be difficult and may result in regression estimates that have high error. Regression equations can be developed for smaller, more homogeneous regions to minimize the error of streamflow estimates (Haan, 1977). Statistical methods can be used to group gaging stations with similar physical drainage-basin characteristics, as has been done for other studies (Hortness and Berenbrock, 2001; Risley and others, 2008). Statistical stream groupings can be defined by grouping streams with similar drainage-basin characteristics through statistical analysis and visual inspection to define regression regions for Oklahoma. Comparison of regional regression equations, in terms of differences in independent variable selection and regression accuracy, can be used to evaluate and provide context for differences in streamflow variability in Oklahoma as a function of these drainage-basin characteristics. Purpose and Scope The purpose of this report is to document the methods and results of a study designed to estimate flow-duration statistics (annual, seasonal, and monthly) and annual mean flow for ungaged streams in Oklahoma. This report includes (1) a description of the study area and how variability of physical characteristics and anthropogenic activities may affect streamflow, (2) a summary of calculated flow-duration and annual mean-flow statistics at gaging stations with long-term (10 or more years) daily streamflow data, (3) a summary of available methods for estimating flow statistics at ungaged sites, (4) a description of the data and methods used to develop regression equations for estimating flow-duration |
| File Format | PDF HTM / HTML |
| DOI | 10.3133/sir20095267 |
| Alternate Webpage(s) | https://pubs.usgs.gov/sir/2009/5267/pdf/SIR09-5267.pdf |
| Alternate Webpage(s) | https://doi.org/10.3133/sir20095267 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
