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This approach suggested that April through July total runoff would increase at least 10 gct in most watersheds for a 10 pet increase in the mean snowpack. However, some basins indicated increased on only 6-8 pet. Closer examination suggested that snow courses in or near some watersheds, though well correlated with runoff, poorly represented actual snowpack conditic:ms throughout the watershed. These snow courses tended to be low elevation courses for which the calculated linear regression equations predicted substantial flow even with zero SWE on April 1. In general, the higher the snow course elevation, the greater was the prediction of percentage increase in runoff. About 20 pet runoff enhancements were predicted for 2 of the basins in Table 2 and for I small experimental watershed with representative SWE observations and limited slope. Two of these 3 basins are known to have relatively impervious base material. Only 8 pct runoff enhancement was predicted for a second small experimental watershed with very permeable base material. It seems probable that subsurface flow reduced the measured runoff in that drainage. Other important factors believed to influence the snowpack-runoff relationships are those that cause variability in sublimation and evapotranspiration losses (slope, aspect, vegetation). All five drainages listed in Tables 1 and 2 with predicted runoff increases less than 10 pet had steep south-facing or southwest-facing slopes which would maximize such losses. The geology of at least two of these drainages could reduce runoff. A given percentage snowpack increase at higher elevations probably will result in a larger runoff increment than the same percentage increase in the lower elevation snowpack of the basin. The evidence in this paper supports that concept, and higher elevations usually have greater SWE, which suggests that a cloud seeding projects should emphasize targeting of the higher elevations in mountain watersheds. However, slopes exposed to strong solar radiation and/or sustained winds may lose much of their windfall. As the technology of winter weather modification improves, more attention should be given to targeting areas which maximize runoff. ,-" The conversion of snowpack to runoff is a complex, nonlinear process. Observations from well- instrumented watersheds, coupled with physically-based numerical models, could be used to provide accurate and reliable estimates of runoff responses to enhanced snowpack. In the meantime, the estimates made by this simpler approach are probably reasonable, conservative first approximations. These estimates indicated that in most mountain locations in the Upper Colorado River Basin, a 10 pet increase in SWE should result in at least a 10 pet increase in spring and summer runoff. Runoff increases up to 20 pet may be expected from some drainages. Conversely, drainages with high evapotranspiration losses or very permeable soils may produce less than 10 pet runoff increases. 1994 articles and papers: 10. Heimbach, J. A., Jr., and w: D. Hall, 1994: Applications of the Clark model to winter storms over the Wasatch Plateau. J. Weather M()dification,26, 1-11. ( ABSTRACT The configuration of the Clark mesoscale model to a field experiment conducted over the central Utah Wasatch Plateau experimental area is briefly described. Its application is demonstrated using one case from the early winter 1991 UtahlNOAA Cooperativt;: Atmospheric Modification Research Program. Observations of sulfur hexaflouride and ice nuclei were used to test the model. The results were in 50