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Last modified
7/28/2009 2:41:01 PM
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4/24/2008 2:56:59 PM
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Weather Modification
Title
Precipitation Enhancement Effects of Sorghum, Using a Daily Growth Model
Date
4/1/1982
Weather Modification - Doc Type
Report
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<br />. <br /> <br />function is so constructed that water stress does not become appreciable <br />until the ratio between available soil water and maximum soil wAter decreases <br />to a value of about 0.5. Stress then increases rapidly as the ~atio decreases <br />further. The temperature stress function is such that if the a~erage daily <br />temperature is below 5 oC, temperature stress is at a maximum and no photo- <br />synthate is produced. From 5 to 25 oC, temperature stress is g~adually <br />reduced, and no stress is present between 25 and 40 oc. From 4Q to 45 oc, <br />stress gradually increases again, and at temperatures above 45 IC, stress is <br />again at a maximum and photosynthate production halts. Since a~ average <br />daily temperature of 40 Oc (104 OF) is rarely encountered at the five sites <br />used, temperature stress is almost always due to low temperaturJs. After the <br />photosynthate reduction caused by these stresses is taken into dccount, the <br />net photosynthate remaining is partitioned to the various parts lof the plant <br />according to a procedure developed by Vanderlip and Arkin (1977). <br /> <br />In this study, six time periods (stages) determining the growth and develop- <br />ment of the plant are considered: (1) a preplanting period of about 2 <br />months, included so that the effect of pregrowing season precipitation on <br />yield could be assessed; (2) planting to emergence; (3) emergende to differen- <br />tiation (floral initiation); (4) differentiation to end of leaflgrowth; (5) <br />end of leaf growth to anthesis (blooming); and (6) anthesis to Rhysiological <br />maturity. Grain filling begins several days after anthesis, and the model <br />stops on the day of physiological maturity. Physiological matu~ity is <br />determined primarily by growing-season temperatures, and it occurs on the <br />order of 100 days after emergence. Partitioning of the photosy~thate occurs <br />during the four stages preceding maturity. The model is descri~ed in detail <br />in the Users Guide (Maas and Arkin, 1978).. I <br /> <br />The output of the model is a daily record of the growth and dev~lopment of <br />the modeled sorghum plant. Also provided in the output are daily values of <br />soil water, precipitation, and other environmental variables. <br /> <br />~ <br /> <br />Model Sensitivity <br /> <br />The developers of the sorghum model compared modeled and measured grain yield <br />for 24 data sets from five states (Vanderlip and Arkin, 1977). IThey found <br />that the model did not consistently over- or underestimate the ~easured yield <br />(the mean difference was 0.3 g/plant). The precision of the modeled yield, <br />however, was not high (standard deviation of the difference was118.4 g/plant). <br />Although the sorghum model used in this paper includes some imp~ovements over <br />the version tested in 1977, there are several limitations to the model. It <br />assumes no nutrient limitation, no weed competition, and no ins~ct or disease <br />problems. The model ignores water surplus (water in excess of ihat needed to <br />saturate the soil) and, therefore, cannot consider potential damage from <br />excess moisture. The model is known to underestimate high evapo~ranspira- <br />tion values and overestimate low evapotranspiration values (Vanderlip and <br />Schmidt, 1981). These values are based on the calculation of p~tential <br />evapotranspiration by the model. Because potential evapotranspiration <br />depends only on temperature and this study does not alter temperratures, the <br />overestimation and underestimation of evapotranspiration are not affected by <br />changes in precipitation. This study is directed toward the ch~nge in yield <br />I <br />I <br />I <br /> <br />3 <br />
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