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~ Water & Sewer Authority of Cabarrus County W°o~CC Water and Wastewater System Master Plan Safe Yield Update and Regional Drought Operations early response actions such as voluntary conservation. Subsequent triggers indicate an imminent water shortage, and eventually the need for strict water rationing. These triggers must provide sufficient warning for drought response by the region's water customers. Similarly, indicators are useful for determining the appropriate timing for lessening or discontinuing staged water use restrictions Since the bulk of the raw water supplies in Cabarms County are surface water storage reservoirs, the parameters evaluated are precipitation, stream flow, and reservoir volume. These parameters directly influence surface water sources, and they are easy to monitor. The mass-balance models, developed for computation, of safe yield, provided the tool to evaluate the effectiveness of drought indicators. The safe yield model simulates hypothetical reservoir operations, considering stream flow, precipitation, releases, and withdrawal rates to develop historical reservoir stage records. £.£. 1 Precipitation. Precipitation is the parameter used to determine meteorological drought, considering seasonal rainfall patterns, degree of dryness, and duration of the dry period. In agricultural applications, differences between actual and expected evapo- transpiration and topsoil moisture can define a drought. Sustained periods of departures from expected precipitation will eventually affect the groundwater and surface water base flows. Although the concept of accumulated precipitation deficit is a simple concept to grasp, it is not as bffective as an indicator or predictor of water supply drought. £.2.2 Stream Flow. The annual cycle of reservoir inflow generally peaks in the spring, and then slowly declines through the summer months with minimum flows typically observed in the fall. Conversely, the annual pattern for water demand peaks during the summer months with more modest demands in the spring and fall, and the lowest demands (reflecting minimum outdoor water use) in the winter. Winter and spring are typically the "refill" periods where inflow exceeds the moderate demand. Hydrological drought occffrs when a precipitation deficiency affects the surface or subsurface water supplies. It takes varying periods of time for precipitation deficiencies to affect the different parts of the hydrologic system, such as soil moisture, stream flow, groundwater, and reservoir levels. A few months of below-normal rainfall are not likely to affect the volume of water stored in the reservoirs; however, each reservoir will behave differently, based on its unique combination of drainage area, storage volume, and inflow factors. PN 096873,0800 13 January 23, 2004 BLACK & VEATCH ~______q..~ Int, rnatlonal Company