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<br />I NTRODUCTI ON <br /> <br />.. <br /> <br />Application of a precipitation enhancement technology to provide additional <br />water directly on crops and rangeland raises questions about th~ effects on <br />crop yield of the amount and timing of additional precipitation.1 This paper <br />presents the results of using a model simulating the daily growth and develop- <br />ment of a grain sorghum plant to investigate these effects. Da~a from five <br />sites located in the central and southern High Plains were used. The model <br />was exercised with observed precipitation and temperature and with 15 precipi- <br />tation enhancement scenarios. These scenarios were selected to provide a <br />range of precipitation conditions that could reasonably be expeoted from a <br />weather modification program. , . I <br /> <br />Much of the previous work related to amount and timing of preciRitation <br />involved regression equations that consider meteoro16gical char~cteristic5 <br />averaged or summed over weeks, months, or even seasons. Changndn and Huff <br />(1971) studied the impact of more water on corn and soybean yie~ds in . <br />Illinois. Bark (1978) examined the effects of precipitation ennance- <br />ment for a number of crops in Kansas, and Ross et al. (1973) fodused on South <br />Dakota. These three investigations used monthly models. Caprid and Williams <br />(1973) used a weekly model for their Montana studies, while Alldway et al. <br />(1975) used a combination monthly-seasonal model for their studX of Texas <br />agriculture. All investigators agree that yield improves with increasing <br />precipitation. J <br /> <br />More recently, dynamic growth models of important crops, using aily values <br />of meteorological and other environmental variables, have become available <br />(Arkin et al., 1977). By their nature, models that use daily vdlues are more <br />sensitive to the effects of timing than those models that use weekly or <br />monthly values. We used a grain sorghum model because it is reasonably <br />well developed and because grain sorghum is the world's third mdst important <br />human food grain (after rice and wheat), though most grain sorg~um grown in <br />the United States is used for animal feed. <br /> <br />. <br /> <br />A preliminary study (James, 1980) using the grain sorghum model and daily <br />data from northwest Kansas demonstrated the utility of these models for <br />investigating the effects of timing and amount of precipitationJ The sorghum <br />model was exercised with 1976 data for Goodland, Kansas, .first ~ith the <br />observed distribution of precipitation to give a base yield, th~n with all <br />precipitation assigned to the first of the month, and finally with all <br />precipitation assigned to the 15th of the month. The latter twd distributions <br />showed yield increases of 12 and 15 percent, respectively. A mdnthly modE~l, <br />which cannot recognize intramonth rainfall distributions, gives an unvary'ing <br />yield when the precipitation schedule is changed. <br /> <br />The present study indicates that the total amount of precipitation is usually <br />the most important meteorological factor in the development andlyield of a <br />crop. Also, the timing of precipitation can significantly alte~ the general <br />relationship between amount and yield. An increment of water aJ;lplied near <br />the start of grain filling (a period when evapotranspiration isloften at a <br />maximum) is usually more effective in increasing yield than the same amount <br />applied at an earlier time. On the other hand, sufficient additional water <br />applied prior to planting or early in the growing season can build soil water <br /> <br />.. <br /> <br />1 <br />