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002503 Recognizing the effects of cloud seeding will depend on extensive measurements taken as part of the pilot project. Present indica- tions are that convective elements of Sierra Nevada cloud systems have significant seeding potential. In some situations, carryover effects of seeding are possible. The design proposed by MBA includes measurement networks covering both the primary area and nearby regions which may be affected by cloud seeding. The question of long-term preseason forecasting of the precipitation or precipitation modification potential for a given season has received considerable attention over the years without noticeable results. In fact, the variability and uncertainty of precipitation occurrence, even in the case of large-scale winter orographic systems, is such that predictability of more than at most a few days in advance is not possible. Advanced forecasts of wet or dry years are still well beyond the state of scientific understanding. Since the risk of making a wrong decision, either way, can be very high and since excessivly wet and dry years occur with significant frequency, MBA said an alternative to historical data or yearly forecasts must be used to make weather modification decisions. Hydrologic Considerations: The variability of precipitation runoff in the Sierra Nevada is substantially reduced by the reservoir system. The American River drainage is dominated by the Folsom Reservoir, which was designed to accommodate the predicted 100-year flood level. The operating procedures used to control the spill rates and retained capacity of the reservoir are based on a combination of factors includ- ing existing snowpack, stream stage, and short-term meteorological forecasts. With completion of Auburn Dam, reservoir storage will have an even larger impact. Consequently, an alternative to long- range forecasts or statistical analyses of error probability and risk is to integrate the seeding decision rules into the existing reservoir operating system. This alternative was adopted in the MBA design. It uses the reservoir operating levels and spill rates in conjunction with existing snowpack and short-term forecasts, updated as the event approaches, to determine whether additional precipitation is expected to be advantageous or detrimental. The reservoirs serve many purposes, two of which are to provide flood control and to ameliorate the year-to-year precipitation variability. In the latter capacity, they hold excess runoff from one year to be used in the next. Thus, the effect of seeding to increase runoff in one year can be extended through the reservoir system to alleviate deficits in a subsequent dry year. Possible recharge of depleted ground water also may help in alleviating effects from a previous dry year. 28