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<br />. <br /> <br />'i <br /> <br />Pleprints Second WHO Scientific Conference on Weather Nodificaticm, <br />Boulder, Colorado, August 2-0, 1970. Secretariat of the World <br />Meteorological Organization, Geneva. pp. 577-584 <br /> <br />ON THE USE OF HmlOCLHIES FOR ESTIMATING <br /> <br />ENVIRONMENTAL EFFECTS OF PRECIPITATION MANAGEMENT <br /> <br />by <br /> <br />Wallace E. Howell <br />Bureau of Reclamation <br /> <br />1. lntroduction <br /> <br />~~enever prolonged application of precipitation management is contemplated, its effects on <br />the environment must be considered. A problem analysis of this situation was made by Cooper <br />and .Jolly (1969) and more recently by Cooper (1974). Investigations based largely on the <br />former analysis, notably the San Juan Ecology Project (Steinhoff and Ives, 1976) and the <br />1.ledicine Bow Project (Knight et al., 1975), took a componential approach, attempting to <br />determine the impact of snow augmentation on selected environmental components and to incor- <br />porate these into an impact overview treated as an aggregation of impacts on components. <br /> <br />This approach encountered difficulties. Five years' work by a team bringing many talents <br />and taking a multidisciplinary approach yielded few satisfactory ans\~ers about the magnitude <br />and significance of impacts on specific environmental components from a given snowpack incre- <br />ment. 1101'; the tentative results should be formed into an overview of impact on the environ- <br />ment as a whole depends on assumptions about the strength and direction of intercomponent <br />effects about which not enough is known for more than a speculative exploration. As Cooper <br />(1973) I.oted, "ecosystems do not necessarily respond in ways that would be predicted from <br />a knowleJg~ of the properties of their individual components. * * * The complexity and var- <br />iability, in both space and time, of real ecosystems is such that it is probably impossible <br />to dcvise a gCI~erally agreed upon quantitativ(~ description against which future states of <br />the system can be judged." <br /> <br />An alternative approach is the integrative or holistic one of exam1n1ng environments as a <br />whole that have been subjected to precipitation changes similar to those expected from pre- <br />cipitation management. The Special Commission on Weather Modification of the National <br />Science Foundation (19~6) pointed out that among the avenues open to improved forecasting of <br />biological effects is "examination of areas biologically and climatically analogous to the <br />changed and unchanged situations." <br /> <br />A climatic analogue of this sort is called a homoclime. It is assumed that if a homoclime <br />can be found for the prospective altered state of the study area (the idioclime), it will <br />exhibit the environmental differences from the idioclime that would result from precip- <br />itation management. The gradients observed, whether of single components or of groups, <br />will be those that result from all the effects of precipitation, direct and indirect, on the <br />particular environment. Also revealed will be the differences that depend on things other <br />than precipitation, so that the basis is provided for judging the importance of precipita- <br />tion management impacts in comparison with other impacts, especially those arising from <br />other technologies, or in comparison with other physiographic and geographic differences. <br /> <br />2. Concepts Underlying the Selection of Homoclimes <br /> <br />Precipitation and temperature are the outstanding determinants of climate, though not the <br />only ones. Every major scheme of environmental classification rests on these two elements <br />