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Calibration studies to evaluate the HIPLEX irrigation approach (Weaver and McClelland, 1981, 1982) would be useful if application would be considered by others. 8. LONGER TERM EFFECTS HIPLEX experiments have dealt with short term (1-5 year) changes in ecosystem composition, structure and function induced by supplemental watel~. For the long term communities expected response to supplemental water was to thicken (Weaver and Newbauer, in prep.) or give way to more productive equilibrium communites over periods as long as 100-200 years. The equiliblrium communities should be similar to those of grass or croplands characteristic of regions to the east where similar temperature regimes occur with more rainfall. If grasslands were converted to croplands, it would be assumed that the transition period would be short and smooth because appropriate plant varieties, pest control measures, and agricultural methods would be available for import from moister regions. If grasslands were maintained as a produ1ctive alternative on shallow soil sites or possibly even on deep soil sites (Weaver, 1982a), maximizing the speed of conversion by seeding of tall grass species before or as water supplementation begins would be recommended. This proc1edure should minimize the changes that intermediate stages dominated by undesirable coarse herbs such as thistle, sunflowers, mustards, clovers or euphorbs might produce. B. EFFECT OF DISCONTINUANCE Range managers would be concerned about the effects of discontinuing a succp.ssful cloud-seeding program. Some relevant observations would be derived from observations of community responses to discontinuance of experimental irrigation. After two years no new weeds have been seen. Both desirable tall grasses (Andropo~on, Sor~hastrum, and panicum) and weeds (thistle, mustards, and sunflowers) were disappearing with cessation of irrigation. After two years the vigor of grasses, as indicated by seed production, was still hi~her on previously irrigated plots than on previously unirrigated plots. Yields in some previously irrigated plots, especially those receiving fall water, were significantly greater than previously unirrigated plots. Documentation of this phenomenon would suggest that limited water supplies, already concentrated on limited areas to take advantage of the exponential yield-water relationship, might be allocated to different areas in different years or biennia (Weaver, 1982b). The changes reported above relate to ecosystems whose communiteswere little changed because water enrichment was slight or, if large, for a time period too short to result in large changes. Discontinuance of a water supplementation program sufficiently strong and long to result in major species composition changes would probably be followed by one or more weedy successional stages. C. RECOMMENDATIONS - WEATHER MODIFICATION STRATEGY Our studies would suggest the following procedures: 1) Make water available in the spring when plant water use efficiencies are likely to be highest. 50