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EXECUTIVE SUMMARY The LOREP (Lake Oroville Runoff Enhancement Project) was intended to be a 5-year randomized winter cloud seeding demonstration project conducted by the California Department of Water Resources. The long-term goal of the project was to increase runoff to Oroville Reservoir, the main reservoir of the State Water Project located in northern California. Orographic cold-cloud seeding operations were conducted in 6-h randomized blocks during the 1991-92, 1992-93, and 1993-94 winter seasons. Ground-truth precipitation measurements (snowfall water equivalent) were obtained with a network of up to 11 automatic recording and transmitting precipitation gauges. Statistical analyses of the randomized cases were attempted, but the sample size obtained (87 cases: 56 seed, 31 no seed) during these three winter seasons proved to be too small for obtaining statistically significant results. Physical studies were conducted in the hope of documenting the magnitude of the increases possible in the seasonal snowpack obtained by seeding winter storms using liquid propane. The field studies conducted emphasized the transport and dispersion of ice crystals produced by the ground-based release of liquid propane from high altitude dispensers along the crest of the northern Sierra Nevada. These studies included a combination of raw ins on de upper- air observations, low altitude research aircraft data collection flights over the target area, a vertically pointing dual-channel microwave radiometer to measure the integrated liquid water and vapor within passing clouds, mountaintop weather stations to measure icing rate and other meteorological parameters, tracer gas release from two propane dispenser sites, both airborne and ground-based tracer sampling measurements, and a radar wind profiler. The balloon ascent rates for special rawinsonde launches made just downwind from the main Sierra Nevada crest showed a very well defined mountain lee (gravity) wave present during most precipitation events. Strong airflow descent to the lee of the Sierra will thus have a detrimental effect on the growth of ice crystal particles generated on the crest. The tracer SF 6 (sulphur hexaflouride) was used to simulate the transport and dispersion of propane generated ice crystals. Aircraft measurements of SF 6 indicated that at the normal flight altitudes of 2500 m over the downwind valley and 2800 m over the downwind ridge, the aircraft was flying near the top of the tracer gas plumes. When the aircraft was able to fly below cloud base near the release altitude of 2200 m, substantial SF6 was observed. The lower portion of the plume was also observed to descend into the valley about 700 m below the release altitude. A simple two-dimensional model (GUIDE) was used to determine the impact these lee waves have on ice crystal trajectories. GUIDE model output is presented for one well documented seeding case (February 17, 1993) to show how such models might be used operationally to predict particle trajectories downwind from the Sierra. The results indicate that the GUIDE model does a good job of predicting the location of seeding plumes downwind from the propane dispensers if the proper wind information can be input to the model. The physical studies point out the complexities associated with the transport and the successful targeting of seeding induced ice crystals initiated over complex terrain. Nearly continuous horizontal and vertical wind fields along with a vertical profile of temperature are needed to accurately predict particle growth and fallout. The 915-MHz wind protl1er with RASS (Radar Acoustic Sounding System) may provide the technology necessary to obtain nearly continuous measurements ofthe three-dimensional wind field and temperature profile, Xl