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7 <br /> The H3&mgraph Separation Methodology <br /> The method used to estimate recharge rates in Orange County is termed hydrograph separation. <br /> It is not possible to measure the direct ground water recharge from precipitation.However,the <br /> amount of precipitation that is discharged from the ground into streams can be measured by <br /> splitting the years of historical streamflow data(hydrographs)from the 17 USGS gaging stations <br /> that cover Orange County.The hydrographs are separated into two components: ground water <br /> discharge, and overland runoff. Once this is done,the amount of flow determined to be ground <br /> water discharge is added up over the hydrograph period.If(as research indicates)there has been <br /> no long-term change in ground water storage, ground water discharge will be equal to ground <br /> water recharge. <br /> USGS examined several hydrograph separation methods in this study. The Pettyjohn-Henning <br /> "local-minimum"method,which estimates values of daily mean base flows,was selected.This <br /> method was selected because it provides the lowest(most conservative)estimate of daily mean <br /> baseflow. Baseflow estimates account for the loss of ground water to riparian evapotranspiration <br /> (i.e.,ground water lost to vegetation and evaporative losses on floodplains). Using this method <br /> of estimating recharge will help ensure that sufficient ground water is available for riparian <br /> vegetation. <br /> Ground Water Recharge in Orange o un j's 12 Drainage Basins <br /> Using the methodology described above,the USGS prepared recharge hydrographs and duration <br /> statistics for each of the 12 basins.The hydrographs illustrate the monthly levels of recharge <br /> along with median and mean recharge rates,over the years in which the gaging station was <br /> reporting data.The duration statistics show the percentage of time certain flows were met or <br /> equaled.These statistics allow for some comparison of base-flow in the different basins. <br /> The results of the recharge rate analysis,using 17 gaging stations over 12 basins, are summarized <br /> in the following chart: <br /> BASIN' Years a <br /> Haw River 67 98.4 sq.mi. 311 gal/day/acre <br /> Cane Creek 20 33:7 sq.mi. 361 gal/day/acre <br /> Morgan Creek-Upstream of CH 20 `37.6 sq.mi. 477 gal/day/acre <br /> Morgan Creek-Upstream of WC 6 8.35-sq.mi. 427 gal/day acre <br /> New Hope Creek 13 34.6 sq mi 339 gal/day/acre <br /> New Hope River 24 24.5 sq mi 324 gal/day/acre <br /> Upper Eno River 54 66 .0 sq mi 399 gal/day/acre <br /> Eno River-lower 18 56.3 sq mi 341 gal/day/acre <br /> 7-Mile Creek 8 14.1 sq mi 367 gal/day/acre <br /> Little River 34 63.6 sq mi 352 gal/day/acre <br /> Flat River 70 10.5 sq mi 347 gal/day/acre <br /> North Hyco Cr 31 4.1 sq mi 353 gat/day/acre <br /> 4 <br />