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caused by a change in precipitation; therefore, a relative change in yield rather than the absolute accuracy of the model is the point of interest for this paper. Maas and Arkin (1980) undertook a sensitivity analysis of the grain sorghum model. Their results showed that the modeled grain weight was extremely sensitive to changes in soil water, considerably sensitive to changes in temperature and plant population, and less sensitive to changes is solar radiation. . Because the model is imperfect and because the input data can contain varying degrees of uncertainty, the simulated yield contains noise. After conversa- tions with one of the model developers, 1/ it was assumed, somewhat arbi- trarily, that changes in yield of less than 6 percent were within the noise. SITE SELECTION AND DATA AVAILABILITY Three basic criteria governed site selection: (1) grain sorghum had to be grown in the area, (2) a reasonable range of planting dates, climates, and soil types had to be included, and (3) daily observations of precipitation and maximum and minimum temperatures (U.S. Department of Commerce, 1970) and either observed daily radiation (U.S. Department of Commerce, 1979) or mean daily radiation for each month had to be available. Daily radiation observa- tions are available from only a few sites, but these data have been used to prepare maps of mean daily solar radiation for each month of t~e year (U.S. Department of Commerce, 1968). Because the sorghum model is relatively insensitive to radiation, these mean values are considered adequate for the purpose of this paper. Based on the three criteria, five study sites were selected: Concordia and Goodland, Kansas; Oklahoma City, Oklahoma; and Lubbock and San Angelo, Texas. Table 1 presents pertinent data for each site. The data in table 1 were obtained from local agricultural experiment - stations and county agents. EFFECTS OF TEMPERATURE STRESS AND WATER STRESS ON YIELD In the grain sorghum model, both temperature stress and water stress reduce potential photosynthate production. To examine the separate effects of these stresses, as well as their combined effects, the model was exercised under actual conditions, under conditions where no water stress was permitted to occur (i.e., only temperature stress acts to affect yield), under conditions ~ where nG temperature stress was permitted (i.e., only wate~ stress affects yield), and where neither stress was permitted. If a given no-temperature- stress yield is greater than the associated no-water-stress yield, then temperature stress is the dominant factor in reducing yield. Conversely, the major factor in reducing yield is water stress when the no-water-stress yield exceeds the no-temperature-stress yield. 1/ Dr. Gerald F. Arkin, Texas Agricultural Experiment Station, Temple, Texas, personal communication. 4