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Ou2558 management research, including major efforts on the assessment of the impqcts of increased growing season rainfall on agricultural and natural ecosystems, and collection and analysis of rainfall measurements. Montana and Reclamation personnel share facilities and equipment and coordinate field assignments at the Miles City, Montana HIPLEX site. During the first field season in 1975, Montana established and operated a recording rain gage network of more than 70 sites. The network was expanded to 121 sites for the 1976 and 1977 seasons. Rainfall data have been reduced to computer-compati b1e form in I5-minute intervals. By combining these data with other meteoro- logical observations, rainfall frequencies and amounts have been determined for the five major storm types affecting eastern Montana from May through July. An optimum rainfall monitoring network for future precipitation enhancement experiments has been developed. Comparisons between radar and rain gage estimates of rainfall were used to determine the feasibility of a combined measurement system. It appears that it may be most cost effective to use radar, calibrated by a somewhat sparse rain gage network, to monitor rainfall patterns over a large area. Sprinkler irrigation systems are being used to simulate six different rainfall situations with different combinations of seasonal timing and amount. These six treatments are applied to both predominantly coo1- and warm-season grasses. This ongoing study will determine and quantify effects of precipitation augmentation on the quality and quantity of forage production, relationships between plant growth and plant and soil water stress, invasion of either desirable range grasses or weedy species, and many other important ecosystems parameters. The first two seasons of this experiment produced several preliminary findings. For example, fungal-bacterial diseases were no more prevalent in wet plots than in dry plots. The season in which showers occur appears to significantly affect water-use efficiency of the plants. While rangeland grasses may be able to use May-June rainfall in excess of 25 mm/week efficiently, less response takes place later, with plant growth markedly slower after August 1, even when water is not a limiting factor. Some weed types seem to prefer relatively wet plots; however, other types prefer drier plots. The study of short- and mid-term impacts of a precipitation augmenta- tion program on carbon cycling and community dynamics, specifically on insect populations and cattle, correlates with the irrigation studies. This study is significant because grasshoppers, aphids, and III-ll