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Database (WHD) with a watershed area of 150 square miles or less. Data are displayed for two commonly used low flow statistics, 7Q2 and 7Q10, which is the minimum seven-day mean streamflow (cfs) that occurs every two, and ten years, respectively. USGS gauges in Wisconsin were found by downloading the “USGS Gage Stations” layer available on the WDNR Surface Water Data Viewer. For each USGS gage daily streamflow data were download using the dataRetrieval (v2.7.4) R package on July 7th, 2018. Data are available for each metric if the minimum required data exists (i.e. two years of record for 7Q2 and ten years for 7Q10). A full year of record was limited to years were at least 250 daily streamflow measurements were recorded.
Low flow statistics for most of Wisconsin, excluding the Chippewa River and Fox-Wolf River basins, were estimated using published regression equations from USGS Reports (1977-1981). Each report provides two equations to calculate 7Q2 and 7Q10 for ungauged sites using the single base-flow measurement method. This method consists of predicting the low flow statistics from watershed area (miles-2) and a baseflow index (Bf). Bf is calculated by using an estimate of annual baseflow, usually from a one-time grab sample, divided by the watershed area (USGS Reports define baseflow as the 90% exceedance flow for gaged stations). To calculate low-flow statistics at all stream reaches this layer substituted a single baseflow grab sample measurement with the modeled 90% exceedence flow from the WHD-Plus. The WHD-Plus contains modeled values for many flow and water temperature statistics for nearly all the stream reaches in the WHD. If quality field data on the observed baseflow (i.e. multiple samples over obvious baseflow conditions) are available, a user may input that streamflow value into the Bf to independently calculate 7Q2 or 7Q10 using the following equation, and the look up table:
7Q2 = C2 * DA^A2 * (baseflow/DA)^B2
7Q10 = C10 * DA^A10 * (baseflow/DA)^B10
Where, 7Q2 and 7Q10 low flow metrics (cfs), DA = drainage area (miles-2), baseflow = measured or modeled estimate of 90% exceedance flow (cfs), C2, A2, B2, C10, A10 & B10 = regression coefficients from lookup table depending on major river basin. To create to create low flow metrics for the Chippewa River and Flow-Wolf River basins methods presented in “Low-Flow Characteristics of Streams in the Menominee-Oconto-Peshtigo River Basin, Wisconsin” (USGS Report 80-749) were followed as closely as possible. Empirically derived 7Q2 and 7Q10 values were created for all USGS gauges in each watershed with sufficient data (two years of record for 7Q2 and ten years of record for 7Q10). For gauges with fewer than three times the reoccurrence interval (two or ten years) a Log-Pearson Type III distribution built with the annual seven-day minimum stream flows and appropriate percentiles were calculated to estimate flow statistics for 7Q2 and 7Q10. This method is the standard recommended method for flood frequency analysis and the procedure used by USGS (Riggs 1972, U.S. Water Advisory Committee on Water Data 1982). For any stream gauge with more than three times the reoccurrence interval, the empirically derived low-flow statistic was used to calculate 7Q2 and 7Q10 instead of using the Log-Pearson Type III distribution (approximating methods in “Low-Flow Characteristics of Streams…” USGS Water Resource Investigations). Multiple regression was used to estimate coefficients and intercepts for each low-flow statistic to recreate the equations for the river basins in Wisconsin in missing USG reports. Following the USGS recommendations, these equations are not applicable to any stream with a drainage area of greater than 150 mi-2. |