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<br />physically based models will prove a more satisfactory method of predicting runoff provided, <br />of course, that precipitation data or forecasts of suitable accuracy are available. However, <br />precipitation output from GCMs does not simulate properly the statistics of actual <br />precipitation events, in part because their coarse resolution does not properly represent the <br />mountain ranges that generate orographic precipitation. Therefore, Reclamation is adapting a <br />local-scale precipitation model for use in the GCCRP. <br /> <br />The local-scale precipitation model in question originated with Rhea (1978). It has been <br />modified in Reclamation and elsewhere in recent years. It predicts precipitation on the basis <br />of topography and fields of wind, temperature and humidity. The companion papers by <br />Matthews and Medina describe some of the work with the model on GCCRP to date, <br />including the use of regional model output from the National Center for Atmospheric <br />Research to set boundary conditions for it. Runs of the precipitation model for Arizona with <br />boundary conditions determined by output from the MM4 regional model have shown <br />statistically significant skill in depicting precipitation fields, thereby confmning the utility of <br />nested models in simulating daily precipitation. Eventually, output from doubled-C02 runs <br />from GCMs, or from regional models using GCM output to define their boundary conditions, <br />will be used as input to the modified precipitation model. As the model poses very modest <br />computer requirements, Reclamation plans to make enough runs over the next two or three <br />years to generate rather detailed precipitation climatologies for selected basins in the western <br />United States under several likely global climate change scenarios. <br /> <br />5. APPLICATION OF RESULTS <br /> <br />The precipitation climatologies to be developed will be a key element in Reclamation's <br />GCCRP, which includes many studies of options for management of western water resources <br />under changed climate conditions. The scenarios and precipitation climatologies win be made <br />available to interested groups in other agencies also, notably to the GS for use in the joint <br />Reclamation - GS study of potential impacts of climate change in selected western basins. <br /> <br />REFERENCES <br /> <br />Boer, G. J., N. McFarlane, J.-P. Blanchet, and M. Lazare, 1990: Greenhouse gas induced <br />climatic change simulated with the CCC second generation GCM. In Application of the <br />Canadian Climate Center General Circulation Model Output for Regional Climate Impact <br />Studies - Guidelines for Users, Version 1, 2-5. Atmospheric Environment Centre, <br />Downsview, Ontario. <br /> <br />Cayan, D. R., and D. H. Peterson, 1989: The influence of North Pacific atmospheric <br />circulation on streamflow in the West. In Aspects of Climate Variability in the Pacific and <br />the Western Americas (D. H. Peterson, Editor), Geophys. Monog. 55, Amer. Geophys. Union, <br />375-397. <br /> <br />Printed January 18, 1991 <br />