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CHAPTER III. ECOLOGICAL STUDIES A. EFFECTS OF WATER SUPPLEMENTS ON GREAT PLAINS GRASSLANDS 1. SYNTHESIS Knowledge about the response of grazing lands to water supplements made in different seasons and different quantities should be useful in guiding the development of a weather modification technology, evaluating it, and eventually applying it. In 1975-80 the Bureau of Reclamation compared the behavior of seeded and unseeded clouds in the high plains (Knudsen, et al., 1981; Kerr, 1982). Biologists with the Montana Department of Natural Resources and Conservation and Montana State University (Weaver et a1., 1981; Ne~..bauer et al., 1980) conducted concurrent experiments to estimate the effects of successful precipitation enhancement programs by increasing rainfall in the fall (about 15 cm), in the spring (about 15 cm) or in the sununer (single showers of various sizes or repeated showers increasing precipitation to 6 rom, 12 rom, or 25 rom per w'eek). Two grassland were studied, one dominated by Bouteloua gracilis and one dominated by Agropyron smithii" representative of the range types found in the region (Kuchler, 1964). Studies addressed how additional water would, in the short term, affE!ct yields, forage quality, nutrient availability, soil eros ion and pest attack rates. The longer term effects of water supplements and the effects of cessation of a successful water supplementation program were also briefly investigated. Conclusions were summarized below. 2. PRODUCTION Irrigation of small plots suggests that single showers, applied to either Agropyron or Bouteloua grasslands, will stimulate little additional forage production unless they add more than 25 mm to the soil water stores (Weaver and Johnson, 1982). This conclusion is reinforced by the failure of repeated light (6 - 12 rom) showers to significantly increase yields in experimental plots (Weaver, 1982a). Several hypothesis were advanced to explain the failure of light showers to stimulate above-ground growth. First, leaves and nearly root-free surface soils (Weaver, 1982c) may have intercepted most of the light rainshowers, preventing penetration to absorbing organs. The fact that plant water potentials (water contents) were increased little in the first to second (Weaver and Johnson, 1982) or third to fourth (Weaver, 1982a) days after irrigation supported this hypothesis. On sandier soils light showers mlay have greater effects (Sala and Lauenroth, 1982). Second, if water stresses were relieved by light showers the period of relief was likely to be short because water in surface soils was 'subject to rapid evaporation. On HIPLEX sites 10 mm rains usually dried in 1 to 1.5 days (Weaver and McClelland, 1981, 1982); on sandier sites where water penetrated more deeply it may have dried more slowly (Parton, et al., 1981). Third, relief of drought may not result in above-ground production either because photosynthetic systems were damaged or because photosynthates were redirected to reproductive functions like seed production in wheat, or below-ground carbohydrate storage in Agropyron smithii (Weaver and Harvey, 1982). Fourth, since in the absence of drought, plant production may stop in early August, it was suspected that still other stimulii may slow or prevent late season production (Weaver and Forcella, 1982). This 47