There are more than a dozen wintertime cloud seeding programs in the western U.S. whose primary goal is to increase snowfall over specific drainage basins in order to subsequently increase stream runoff in the spring and summer months. To date the documentation of seeding effects has focused mainly on in-cloud microphysical changes, snowfall characteristics and precipitation rate changes at the surface, and evidence of the seeding material in snow layers in seeding target areas. Some operational projects have compared stream runoff from seeded and unseeded basins as part of their evaluation, but predicting the additional runoff due to seeding efforts has not been routinely attempted.

Within Nevada’s Weather Damage Modification Program (WDMP), a cooperative research effort with the U.S. Bureau of Reclamation, hydrologic modeling has been incorporated into the research in an attempt to predict how changes in the snowpack due to cloud seeding will alter runoff in the affected streams of the targeted basins. In addition to runoff the model also predicts how water is distributed into soil moisture and ground water, and what is lost to evapotranspiration. To accomplish this task two basins within the Nevada seeding program target area have been partitioned into specific Hydrologic Response Units (HRU’s) which have similarities in terms of terrain slope and aspect, soil type and vegetation. Data from the HRU’s are used as input to the Modular Modeling System (MMS)and the Precipitation Runoff Modeling System (PRMS) of the U. S. Geological Survey.

This paper presents results from the MMS-PRMS for the Walker River Basin whose headwater region is on the eastern (mainly downwind) side of the Sierra Nevada just north of Yosemite National Park. During the winter of 2003-04 this headwater region was targeted by four to six ground seeding generators and occasionally by an aircraft. Initially model predictions are made based on the total precipitation input (from SNOTEL data) for the winter. Sensitivity to the impact of seeding is then assessed by removing a certain percentage of the precipitation from HRU’s that are the most likely to be affected by ground or aircraft seeding. The changes in all water budget terms are tabulated. The first test yielded a similar percentage change in runoff that was applied to precipitation. Trace chemistry results from WDMP snow sampling efforts will be used to better define the HRU’s affected by cloud seeding and additional sensitivity test results will be described.