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'~ Water & Sewer Authority of Cabarrus County VVSACC Water and Wastewater System Master Plan Safe Yield Update and Regional Drought Operations -I Based on these results, a common (cfs/mi2) simulated stream flow record (Jan. 1900 - Nov. 2002) based on USGS stream flow gages was generated for all reservoirs. When Second Creek gage data was available, this gage was used without other gage data. For all other periods, a weighted average approach was taken for the previously selected gages. Reservoir inflows were obtained by multiplying the common (cfs/mi2) record by the corresponding reservoir's drainage area. 1.4 Reservoir Operating Assumptions For the purpose of updating the mass-balance model and safe yield values for Lake Howell, Kannapolis Lake, Lake Fisher, and Lake Concord, operating constraints were re- evaluated in light of the extreme drought. Table 2 summarizes the operating assumptions used in the updated safe yield models for the four reservoirs. Table 2 Reservoir Operating Constraints Total Usable Minimum Normal Minimum Water Source Volume Volume Basis Release (mg) (rog) Elev. (ft) Elev. (ft) (cfs) Lake Howell 6270.9 5296.3 650 630 Gravity flow to WTP 6.0 Kannapolis Lake 1262.2 941.1 726 712 Invert Intake Elev. 0.0 Lake Fisher 836.0 749.6 646 629 Elev. SluiCe Gate #5 0.0 Lake Concord 179.2 179.2 658 641 Bottom of Reservoir 0.0 1.5 Safe Yield Conclusions The safe yield of a water source is a measure of the capacity of the source and is defined as the allowable draft rate at which water can be continuously withdrawn during a low flow or drought event. It is a function of the stream flow, topogr, aphic conditions of the watershed, climatological conditions affecting evaporation from the reservoir, watershed development conditions affecting sedimentation of the stream, reservoir seepage, and usable storage capacity in the reservoir. 1.5.1 Significant Water Supply Droughts. The updated mass-balance models were used to simulate an approximately 100-year period o,f hypothetical operations for each reservoir (1900-2002). The most significant 8 to 10,periods of drodght were identified for each reservoir, and a yield was computed for each separate drought. The droughts are ranked based on the safe yield results. The lowest safe yields correspond to the most severe droughts for each source. The recent drought ranks as the drought-of-record for all reservoirs. PN 096873.0800 7 January 23, 2004 BLACK & VEATCH Intamatmnal Company