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T <br /> 1 <br /> 2 <br /> r <br /> Stream flow within a waterway generally consists of two component water <br /> sources - stormwater runoff and ground water discharge. As much as 50%of total <br /> annual streamflow is known-to result from ground water discharge. The ground <br /> water component of total stream flow is known as "baseflow."For the purposes of <br /> hydrological evaluation,the volume of this base flow is considered to be equal to <br /> the volume of the portion of precipitation which infiltrates into the land surface. <br /> The infiltration component of precipitation"recharges"the ground water <br /> storage/aquifer system. In this report,the determination of volume and rate of <br /> baseflow-and ground water recharge- for the County's river and stream basins <br /> are derived by applying an analysis technique known as "hydrograph separation" <br /> to stream flow records. Long-term historical records of streamflow data from 17 <br /> USGS gaging stations cover all but one (the 5.5 square mile South Hyco Creek <br /> basin)of the drainage basins in Orange County, were analyzed to develop <br /> baseflow and ground water recharge estimates for each basin. <br /> The purpose of the report is to provide a broad overview on a basin-by-basin <br /> approach toward a better understanding of ground water availability. The report <br /> does tip provide data that would allow for site-specific estimates of ground water <br /> quantity. Two examples are provided in the report that illustrate ways of using <br /> historical streamflow data and assumed site conditions for planning purposes,but <br /> neither example is designed for use in site-specific analysis. <br /> A copy of the report is being provided under separate cover, and a staff summary <br /> of the report is attached. The principal findings of the report are: <br /> • Most of the ground water in Orange County is found in a geologic zone of <br /> weathered bedrock and soil,termed the"regolith" <br /> • The recharge to the ground water system is highest along a north-south zone <br /> in the center of the County, and lowest in the western and southeastern <br /> (Triassic basin)portions of the County. The Haw River basin in western <br /> Orange has the lowest mean recharge with 311 gallons/day/acre, while the <br /> Morgan Creek basin upstream of Chapel Hill has the highest rate at 477 <br /> gallons/day/acre. <br /> • A significant portion of streamflow in Orange County streams is ground water <br /> "baseflow", ranging from 48%in Morgan Creek upstream from Chapel Hill to <br /> 32%in New Hope River(Bolin Creek and the areas draining to Jordan Lake). <br /> • Mean recharge rates and recharge duration statistics(the percentage of time <br /> during which ground water flow meets or exceeds a specified amount)can be <br /> used for ground water management planning. Two examples are provided that <br /> illustrate ways data could be used to estimate a required recharge area. <br /> • There is considerable ground water available in Orange County,to varying <br /> degrees in the different basins. These resources can be relied upon to provide <br />