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I afternoon thunderstorm acti~ity. Evapotranspiration is quite large during July, IAugust, and September. Average daily summer precipitation over the watershed of the Animas River, as measured at Silverton, Electra Lake, Tacoma, and Durango, needs to be,greater than '6.4 mm to produce a response in stream flow. The net yield in runoff from summer rainfall is quite small compared with the net ,yield from winter precipi- tation. Mean annual specific yield of water runoff from the major portion of the study area is about 500 mm, with spring (April-July): runoff 3.5 to 4.5 times greater than runoff during the re~linder of the year. There'is a sharp gradient between the geographic areas which collect enough precipitation to produce an annual runoff yield of 254 mm per unit area and the areas from which less than ~5 mm of runoff occur. Runoff characteristics of the area were investigated by Morel-Seytoux (1968) and ,Grant (1969). The latter report includes detailed information on snowpack run- off, streamflow trends for the past 60 years, and in- vestigation of flood potential from snowmelt. Vegetation The variety in climate, soils, and physiography has produced a spectrum of vegetation types. In the upper elevations of the alpine tundra, vegetation is often intruded by bedrock outcrops. Below timberline, spruce-fir stands and aspen 'stands are intermingled with open meadows and Gambell oak communities. The lower elevations are primar:i!ly timbered with exten- sive stands of ponderosa pi~e and Gambel oak. U.S. Forest Service Township Map~ (1961--1963) provide some information on habitat, whi~e Krebs (this vol., p. 81) has produced detailed maps ~or much of the area. I Three study areas were located within the spruce-fir forests and two in the uppe~ elevations of the alpine tundra (Fig. 1, p. 3). Th~ detailed studies of the tundra ecosystem under vary:i!ng snow conditions were concentrated in the Williams Lakes and Eldorado Lake basins. Forest ecosystem research projects were pri- marily conducted on Missionary Ridge, with additional investigation on Wolf Creek :Pass and at Rico. For a detailed description of each study area see the Intro- ductions to the tundra and forest ecosystems. , BACKGROUND OF CLOUD SEEDING 'FOR THE SAN JUAN ECOLOGY PROJECT1J (W. Howell, U. S.: Bureau of Reclamation) When winter snowstorms sweep over the San Juan Moun- tains, not all of the moisture that condenses in the form of clouds above the mountains falls as snow. Much of it remains in parti~les too small to fall. Carried beyond the mountains by the wind to where the airflow sinks once more toward the plains, these par- ticles re-evaporate. The rate at which snow reaches the ground, expressed as a proportion of the rate at which moisture condenses in 'the cloud, is called the efficiency of precipitation.' It has been found that in sqme storms, especially those having relatively dee~ cloud systems with cloud- top temperatures below about -27 C, the precipitation efficiency tends to be relatively high, and in such situations there is little that the current knowledge of weather modification could do to increase the snow-' fall. In weak storms that sondense very little mois- ture, there is likewise lit~le potential for stimula- tion. However, it has been found that when the clouds are deep enough and active enough to condense relatively large amounts of moisture but the cloud top temperature is warmer than about -26 C, seeding of the clouds with artificial ice nuclei often raises the precipitation efficiency from a rather low value to one typical of the colder clouds. Under these particular conditions, cloud seeding has the poten- tial of substantially increasing the rate of precipi- tation, probably by as much as a hundred percent. The Colorado River Basin Pilot Project was designed as a statistical test of the capability of the cloud seeding technology of 1971 to bring about precipita- tion increases. When the weather forecasters expected stormcloud conditions considered favorable for seeding within a twenty-four hour period beginning at 11 A.M. (and if established safety criteria were met), an "experimental day" was declared. A randomized deci- sion then was made whether the experimental day would be seeded or left' unseeded as a control. The experi- ment was intended to run for four consecutive winters and accumulate 160 experimental days about equally divided between seeded and unseeded. It actually ran for five winters and accumulated 71 seeded and 76 unseeded days. It was thought that the snowfall on seeded days might ,exceed that on unseeded experimental days by about 15 percent and possibly up to 30 percent, and that ex- perimental days might account for as much as half of the season's snowfall. This would have corresponded to a maximum precipitation increase of about 7.5 per- cent for the season as a whole. Although evaluation is not yet complete, preliminary results indicate that no such sizable overall increase was realized. There appear to have been many "unseeded" days imme- diately following seeded days when silver iodide smoke, trapped in the valleys upwind of the mountain range, affected the clouds and effectively caused these days to be seeded. On other days strong winds carried the seeding effect over the mountain crest and outside the study area. On still other days, designated as "seeded," the silver iodide failed for one reason or another to reach the clouds. On still other occasions the forecast of favorable conditions was unfulfilled or else fulfilled for only a portion of the 24-hour period, so that any seeding effect was greatly diluted. Nevertheless there exists an identifiable subset of experimental days, free from these disturbing influ- ences, for which substantial snowfall increases could be identified with a high degree of confidence. The tentative conclusions from the project character- ize it as a limited success. It revealed serious weaknesses of the 1971 technology in the methods used to identify "seedable" occasions and to place the seeding material in the desired place at the desired time. It failed also to give a reasonably accurate measure of the potential of cloud seeding for in- creasing the seasonal precipitation. On the other hand, it furnished additional evidence that the under- lying principles are sound and that present weakness has to do mainly with the practical difficulties of effective application. During the five winter seasons of the Pilot Project, 320 kg of silver iodide, containing 147 kg of silver, 1/ The San Juan Ecology Proj;ect did not include studies of the techniques of cloud seeding, but to set the stage for the remainder of this' volume, we asked the Bureau of Reclamation to provide an overview of the cloud seeding activities, which: they considerately provided. 5