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0021 <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 a site-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 ns 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. From this information, seasonal and long- <br /> term recharge to the ground water system was estimated for 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 /> 14 <br />