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21 <br />IV. What We Have Learned: Water Quantity <br />The availability of ground water supplies -now and into the future -was the <br />initial impetus for the investigation of ground water resources and the Water <br />Resources project. As such, substantial time and energy has been spent in <br />determining the following components: <br />1. What is the sustainable yield of the ground water system in the County <br />(i.e., the amount of ground. water that can be removed from the ground <br />water system without exceeding recharge and/or depleting long-term <br />storage)? <br />2. What type of ground water yield can be expected in the County, and <br />where might the highest-yielding wells be located? <br />3. Using the well data and statistical analysis,. develop a general prediction of <br />areas in the County that are more likely to produce higher-yield wells. <br />The two reports on ground water produced by USGS (in conjunction with the <br />County) offer the first major research into this resource in the County, and will <br />provide useful information for many years to come. While we may. never be able <br />to predict on asite-specific basis the ground water yield for a piece of land, we <br />can now begin to speak knowledgeably about groundwater availability at sub- <br />watershed level in the County. <br />A. 1996 Recharge Rate Study <br />Published in 1996 (please see page 4), this study looked at the rate of recharge <br />to the County's ground water supplies through investigating data for 12 <br />watershed basins in the County. Ground water is taken out of storage -into <br />springs, streams and lakes, or is pumped from wells. Recharge may be defined <br />as the replenishment of that ground water. <br />A technique known as "hydrograph separation" allowed USGS to use computer <br />models to isolate the ground water component of stream flow at each of, the <br />USGS gaging stations in these basins. Fram this information, seasonal and long- <br />term recharge to the ground water system was estimated far each basin. <br />Mean annual recharge to the system ranges from 4.15 to 6.4 inches per year, <br />with a mean of 4.9 inches per year for the 12 basins. The inches per year rates <br />can be converted to gallons per day per acre to assist in water supply planning <br />purposes. The higher the recharge rate, the more available ground water for use <br />-- the lower the recharge, the less available ground water. <br />i it <br />