Use of a moisture stress index for examining climate trends and corn yields in iowa

Shaw, R.H.

Iowa State Journal of Research 51(3): 249-254

1977


Accession: 006875870

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Abstract
A weighted stress index was calculated for 3 stations, 1 in western, 1 in central and 1 in E Iowa [USA] for the years 1933-36 and for 1947. A comparison of these indexes with those calculated for 1954-75 shows that 1934, 1936 and 1947 were more severe drought years than 1974 and 1975. The most severe drought year was 1936. Groups of stress years occurred about every 20 yr, about the same length period as the 20-yr sunspot cycle. An examination of the yearly pattern of the stress index in 1954-1975, weighted by acres of corn grown in the counties where soil-moisture sites were located, shows the most severe index occurred in 1955-56. The index for 1974 and 1975 was considerably less. The comparison between the actual corn yield in the counties where soil-moisture sites were located and the yield calculated from the stress index and adjusted for technology showed a good relationship. Yields in 1970 were lower than predicted. How much of this was due to the corn blight in that year could not be estimated. Yields in 1974 were predicted close to the actual yield when adjusted for effects of late planting and of the late summer and early fall freezes. The pattern of stress in 1975 showed no reduction due to stress by July 1, a 314 kg/ha reduction by Aug. 1, and a 753 kg/ha reduction by Sept. 1.