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behavior may have evolved in response to the induced spring-rain climate (Weaver, 1980). Leaves produced in late summer would not be likely to pay back the carbohydrate investment required to produce them unless they had received improbable rains; it would have been better if the plants could store carbohydrates for leaf production in the following spring when soil water 'Mas likely to be available. If so, natural selection should have favored plants with this behavior. Heavy watering in the preceding fall (about 15 cm), spring (about 15 cln) or summer (25 mm/week) increased yields markedly in the first year of application and even more in later years (Weaver, 1981a, 1982a). The asymptotic increase in yields with increasing water in the first year suggests that yields are restricted in isolated wet years by plant population size. In later years the exponential increase in yield with increasing water, through at least 700 'mID per year, suggests that communities will rapidly thicken to completely use a limiting resource. Though further increases in yield would not be expected in the short term, long term irrigation would probably result in invasion of tall grass or crop species producing higher yields: Bouteloua might be replaced in turn by Agropyron and Sorhastrum-Andropogon. Fertilized corn and alfalfa yields would be similar to those of Agropyron and tall grass species. The fact that yields increase exponentially with increasing water (Weaver, 1982a) would suggest that societal benefits would be maximized if available water supplies were concentrated in space. If new resource supplies were diffused, they would have little or no effect. Water supplies may be concentrated by (1) clustering summer seeding in one locale, (2) seeding in winter to create irrigation reservoirs to be used on the best. lands, or (3) implementing mechanisms, such as range pitting, to collect locally available water in certain microsites. Yields per unit of water applied were higher when applied in the fall or spring than in the summer (Weaver, 1981a, 1982a). This fact can tentatively be attrib~ted both to relatively small evaporation losses and to relatively efficient use of the water available in the spring. Evaporation of water applied in the off season was low because of low air temperatures and because of rapid penetration of large applications below evaporation-susceptible surface layers. Plant water use efficiencies were believed to have been high in the spring because cool, moist conditions resulted in loss of few water molecules per C02 molecule absorbed during a specific period. These conclusions would encourage serious consideration of non-summer cloud seeding. Non-grazing season watering would only be beneficial where soil reservoirs go unfilled in the average year, so that deep, fine-textured soils would not commonly be filled to field capacity by late spring. 3. FORAGE QUALITY Because the value of range production depends on quality as well as quantity, forage samples were analyzed for digestibility, nitrogen and protein content, and phosphorus content. Digestibility was not affected by water regime (Welker, 1982). Nitrogen content was not affected by small supplenlents; it may have increased slightly with large additions. Phosphorus contents were unaffected. 48